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search.xml
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<?xml version="1.0" encoding="utf-8"?>
<search>
<entry>
<title>Archlinux装机笔记</title>
<url>/2022/10/24/Archlinux%E8%A3%85%E6%9C%BA%E7%AC%94%E8%AE%B0/</url>
<content><![CDATA[<h2 id="前言"><a href="#前言" class="headerlink" title="前言"></a>前言</h2><p><strong>重要数据·务必备份</strong>！固态硬盘报废，连带 Linux 无了，只能重装。恐将来也会频繁装机，故写一篇笔记，希望一劳永逸。</p>
<span id="more"></span>
<h2 id="进入桌面前"><a href="#进入桌面前" class="headerlink" title="进入桌面前"></a>进入桌面前</h2><h3 id="准备装机盘"><a href="#准备装机盘" class="headerlink" title="准备装机盘"></a>准备装机盘</h3><ol>
<li>在 <a href="https://archlinux.org/download/">archlinux</a> 找到镜像源，下载 ISO。</li>
<li>用 <a href="https://rufus.ie/zh/">Rufus</a> 之类的刷 U 盘。<ul>
<li>MBR 分区</li>
<li>ISO 模式</li>
</ul>
</li>
</ol>
<h3 id="引导进装机盘"><a href="#引导进装机盘" class="headerlink" title="引导进装机盘"></a>引导进装机盘</h3><p>华硕主板的快捷键是 <code>F11</code>。如果黑屏，将 DP 线换成 HDMI。</p>
<h3 id="archinstall"><a href="#archinstall" class="headerlink" title="archinstall"></a>archinstall</h3><ol>
<li>进临时操作系统之后，连网线，运行 <code>archinstall</code>。</li>
<li>选镜像源（mirror region）</li>
<li>选择格式化硬盘</li>
<li>创建 superuser。</li>
<li>其他选项都检查一下。archinstall 可能会有大幅的更新。</li>
<li>install! </li>
<li>最后提示 chroot，在里面 <code>systemctl enable sddm</code>。</li>
</ol>
<h3 id="卡-wayland"><a href="#卡-wayland" class="headerlink" title="卡 wayland"></a>卡 wayland</h3><p>N 卡 + nvidia 专有驱动 + wayland = 黑屏</p>
<p>在 <code>~/.local/share/sddm/wayland-session.log</code> 可查看 log。</p>
<ol>
<li>先用 X11 登陆，把环境都配置好再回来。</li>
<li>添加内核参数：<code>nvidia-drm.modeset=1</code>。这是 wayland 要求的。<ul>
<li>适用于 systemd-boot：<code>kate /boot/loader/entries/xxxx.conf</code>。追加参数。</li>
</ul>
</li>
<li>已经能用了，但是 BUG 很多又卡顿，Fuck Nvidia。遂放弃，或者<a href="https://forum.manjaro.org/t/installing-video-nvidia-causes-kde-sddm-to-fail-to-load/110395/6">参考</a></li>
</ol>
<h2 id="进入桌面后"><a href="#进入桌面后" class="headerlink" title="进入桌面后"></a>进入桌面后</h2><h3 id="添加-sudo-免密"><a href="#添加-sudo-免密" class="headerlink" title="添加 sudo 免密"></a>添加 sudo 免密</h3><figure class="highlight bash"><table><tr><td class="code"><pre><span class="line">su</span><br><span class="line">vim /etc/sudoers.d/00_codesire</span><br><span class="line"><span class="comment"># 内容如下</span></span><br><span class="line">codesire ALL=(ALL:ALL) NOPASSWD: ALL</span><br></pre></td></tr></table></figure>
<h3 id="调整快捷键"><a href="#调整快捷键" class="headerlink" title="调整快捷键"></a>调整快捷键</h3><ol>
<li>Konsole</li>
<li>System Settings</li>
<li>KRunner</li>
<li>（后面安装的）Flameshot: <code>/usr/bin/flameshot gui</code></li>
</ol>
<h3 id="安装-yay"><a href="#安装-yay" class="headerlink" title="安装 yay"></a>安装 yay</h3><p>利用 git 下载预编译版安装。</p>
<figure class="highlight bash"><table><tr><td class="code"><pre><span class="line">sudo pacman -S git</span><br><span class="line"><span class="built_in">cd</span> ~ && <span class="built_in">mkdir</span> -p .<span class="built_in">local</span>/opt && <span class="built_in">cd</span> .<span class="built_in">local</span>/opt</span><br><span class="line">git <span class="built_in">clone</span> https://aur.archlinux.org/yay-bin.git</span><br><span class="line"><span class="built_in">cd</span> yay-bin && makepkg -si</span><br></pre></td></tr></table></figure>
<h3 id="安装中文字体"><a href="#安装中文字体" class="headerlink" title="安装中文字体"></a>安装中文字体</h3><figure class="highlight bash"><table><tr><td class="code"><pre><span class="line">sudo pacman -S noto-fonts noto-fonts-cjk noto-fonts-emoji</span><br><span class="line"><span class="built_in">cd</span> ~ && <span class="built_in">mkdir</span> -p .config/fontconfig</span><br><span class="line">kate .config/fontconfig/fonts.conf</span><br></pre></td></tr></table></figure>
<p>配置如下：</p>
<figure class="highlight xml"><table><tr><td class="code"><pre><span class="line"><span class="meta"><?xml version='1.0' encoding='UTF-8'?></span></span><br><span class="line"><span class="meta"><!DOCTYPE <span class="keyword">fontconfig</span> <span class="keyword">SYSTEM</span> <span class="string">'urn:fontconfig:fonts.dtd'</span>></span></span><br><span class="line"><span class="comment"><!-- ${XDG_CONFIG_HOME}/fontconfig/fonts.conf</span></span><br><span class="line"><span class="comment"> - vim:ft=xml:fenc=utf-8:noet:ts=3:sw=3:</span></span><br><span class="line"><span class="comment"> --></span></span><br><span class="line"><span class="tag"><<span class="name">fontconfig</span>></span></span><br><span class="line"> <span class="tag"><<span class="name">alias</span>></span></span><br><span class="line"> <span class="tag"><<span class="name">family</span>></span>serif<span class="tag"></<span class="name">family</span>></span></span><br><span class="line"> <span class="tag"><<span class="name">prefer</span>></span></span><br><span class="line"> <span class="tag"><<span class="name">family</span>></span>Noto Serif<span class="tag"></<span class="name">family</span>></span></span><br><span class="line"> <span class="tag"><<span class="name">family</span>></span>Noto Color Emoji<span class="tag"></<span class="name">family</span>></span></span><br><span class="line"> <span class="tag"><<span class="name">family</span>></span>Noto Sans CJK SC<span class="tag"></<span class="name">family</span>></span></span><br><span class="line"> <span class="tag"><<span class="name">family</span>></span>Noto Sans CJK TC<span class="tag"></<span class="name">family</span>></span></span><br><span class="line"> <span class="tag"><<span class="name">family</span>></span>Noto Sans CJK JP<span class="tag"></<span class="name">family</span>></span></span><br><span class="line"> <span class="tag"></<span class="name">prefer</span>></span></span><br><span class="line"> <span class="tag"></<span class="name">alias</span>></span></span><br><span class="line"> <span class="tag"><<span class="name">alias</span>></span></span><br><span class="line"> <span class="tag"><<span class="name">family</span>></span>sans-serif<span class="tag"></<span class="name">family</span>></span></span><br><span class="line"> <span class="tag"><<span class="name">prefer</span>></span></span><br><span class="line"> <span class="tag"><<span class="name">family</span>></span>Noto Sans<span class="tag"></<span class="name">family</span>></span></span><br><span class="line"> <span class="tag"><<span class="name">family</span>></span>Noto Color Emoji<span class="tag"></<span class="name">family</span>></span></span><br><span class="line"> <span class="tag"><<span class="name">family</span>></span>Noto Sans CJK SC<span class="tag"></<span class="name">family</span>></span></span><br><span class="line"> <span class="tag"><<span class="name">family</span>></span>Noto Sans CJK TC<span class="tag"></<span class="name">family</span>></span></span><br><span class="line"> <span class="tag"><<span class="name">family</span>></span>Noto Sans CJK JP<span class="tag"></<span class="name">family</span>></span></span><br><span class="line"> <span class="tag"></<span class="name">prefer</span>></span></span><br><span class="line"> <span class="tag"></<span class="name">alias</span>></span></span><br><span class="line"> <span class="tag"><<span class="name">alias</span>></span></span><br><span class="line"> <span class="tag"><<span class="name">family</span>></span>monospace<span class="tag"></<span class="name">family</span>></span></span><br><span class="line"> <span class="tag"><<span class="name">prefer</span>></span></span><br><span class="line"> <span class="tag"><<span class="name">family</span>></span>Noto Sans Mono<span class="tag"></<span class="name">family</span>></span></span><br><span class="line"> <span class="tag"><<span class="name">family</span>></span>Noto Color Emoji<span class="tag"></<span class="name">family</span>></span></span><br><span class="line"> <span class="tag"><<span class="name">family</span>></span>Noto Sans Mono CJK SC<span class="tag"></<span class="name">family</span>></span></span><br><span class="line"> <span class="tag"><<span class="name">family</span>></span>Noto Sans Mono CJK TC<span class="tag"></<span class="name">family</span>></span></span><br><span class="line"> <span class="tag"><<span class="name">family</span>></span>Noto Sans Mono CJK JP<span class="tag"></<span class="name">family</span>></span></span><br><span class="line"> <span class="tag"></<span class="name">prefer</span>></span></span><br><span class="line"> <span class="tag"></<span class="name">alias</span>></span></span><br><span class="line"> <span class="tag"><<span class="name">match</span> <span class="attr">target</span>=<span class="string">"font"</span>></span></span><br><span class="line"> <span class="tag"><<span class="name">edit</span> <span class="attr">mode</span>=<span class="string">"assign"</span> <span class="attr">name</span>=<span class="string">"antialias"</span>></span></span><br><span class="line"> <span class="tag"><<span class="name">bool</span>></span>true<span class="tag"></<span class="name">bool</span>></span></span><br><span class="line"> <span class="tag"></<span class="name">edit</span>></span></span><br><span class="line"> <span class="tag"><<span class="name">edit</span> <span class="attr">mode</span>=<span class="string">"assign"</span> <span class="attr">name</span>=<span class="string">"autohint"</span>></span></span><br><span class="line"> <span class="tag"><<span class="name">bool</span>></span>true<span class="tag"></<span class="name">bool</span>></span></span><br><span class="line"> <span class="tag"></<span class="name">edit</span>></span></span><br><span class="line"> <span class="tag"><<span class="name">edit</span> <span class="attr">mode</span>=<span class="string">"assign"</span> <span class="attr">name</span>=<span class="string">"dpi"</span>></span></span><br><span class="line"> <span class="tag"><<span class="name">double</span>></span>96<span class="tag"></<span class="name">double</span>></span></span><br><span class="line"> <span class="tag"></<span class="name">edit</span>></span></span><br><span class="line"> <span class="tag"><<span class="name">edit</span> <span class="attr">mode</span>=<span class="string">"assign"</span> <span class="attr">name</span>=<span class="string">"hinting"</span>></span></span><br><span class="line"> <span class="tag"><<span class="name">bool</span>></span>true<span class="tag"></<span class="name">bool</span>></span></span><br><span class="line"> <span class="tag"></<span class="name">edit</span>></span></span><br><span class="line"> <span class="tag"><<span class="name">edit</span> <span class="attr">mode</span>=<span class="string">"assign"</span> <span class="attr">name</span>=<span class="string">"hintstyle"</span>></span></span><br><span class="line"> <span class="tag"><<span class="name">const</span>></span>hintslight<span class="tag"></<span class="name">const</span>></span></span><br><span class="line"> <span class="tag"></<span class="name">edit</span>></span></span><br><span class="line"> <span class="tag"><<span class="name">edit</span> <span class="attr">mode</span>=<span class="string">"assign"</span> <span class="attr">name</span>=<span class="string">"lcdfilter"</span>></span></span><br><span class="line"> <span class="tag"><<span class="name">const</span>></span>lcdlight<span class="tag"></<span class="name">const</span>></span></span><br><span class="line"> <span class="tag"></<span class="name">edit</span>></span></span><br><span class="line"> <span class="tag"><<span class="name">edit</span> <span class="attr">mode</span>=<span class="string">"assign"</span> <span class="attr">name</span>=<span class="string">"rgba"</span>></span></span><br><span class="line"> <span class="tag"><<span class="name">const</span>></span>rgb<span class="tag"></<span class="name">const</span>></span></span><br><span class="line"> <span class="tag"></<span class="name">edit</span>></span></span><br><span class="line"> <span class="tag"><<span class="name">edit</span> <span class="attr">mode</span>=<span class="string">"assign"</span> <span class="attr">name</span>=<span class="string">"size"</span>></span></span><br><span class="line"> <span class="tag"><<span class="name">int</span>></span>15<span class="tag"></<span class="name">int</span>></span></span><br><span class="line"> <span class="tag"></<span class="name">edit</span>></span></span><br><span class="line"> <span class="tag"></<span class="name">match</span>></span></span><br><span class="line"> <span class="tag"><<span class="name">dir</span>></span>~/.fonts<span class="tag"></<span class="name">dir</span>></span></span><br><span class="line"><span class="tag"></<span class="name">fontconfig</span>></span></span><br></pre></td></tr></table></figure>
<p>刷新字体缓存：</p>
<figure class="highlight bash"><table><tr><td class="code"><pre><span class="line">fc-cache -fv</span><br></pre></td></tr></table></figure>
<p>可能需要重启生效，但是先不重启。</p>
<h3 id="安装常用软件"><a href="#安装常用软件" class="headerlink" title="安装常用软件"></a>安装常用软件</h3><p>谜之 CPU 软件包，不要全装：</p>
<figure class="highlight bash"><table><tr><td class="code"><pre><span class="line">sudo pacman -S amd-ucode</span><br><span class="line">sudo pacman -S intel-ucode</span><br></pre></td></tr></table></figure>
<p>man，浏览器，输入法，vscode，openssh，firacode，flameshot：</p>
<figure class="highlight bash"><table><tr><td class="code"><pre><span class="line">yay -S man-db man-pages google-chrome-stable fcitx5-im fcitx5-qt fcitx5-gtk fcitx5-chinese-addons fcitx5-pinyin-zhwiki visual-studio-code-bin openssh ttf-fira-code nerd-fonts-fira-code flameshot</span><br></pre></td></tr></table></figure>
<p>在 System Settings 添加 Pinyin 输入。修改切换输入法的快捷键。</p>
<p>如果需要开启启动 sshd：<code>systemctl enable --now sshd</code></p>
<p>网络自由，Fuck Timeout：</p>
<figure class="highlight bash"><table><tr><td class="code"><pre><span class="line">curl -Ls https://mirrors.v2raya.org/go.sh | sudo bash</span><br><span class="line">sudo systemctl <span class="built_in">disable</span> v2ray --now</span><br><span class="line">yay -S v2raya-bin</span><br><span class="line">sudo systemctl start v2raya.service</span><br><span class="line">sudo systemctl <span class="built_in">enable</span> v2raya.service</span><br></pre></td></tr></table></figure>
<p>进浏览器管理 <code>127.0.0.1:2017</code>。</p>
<h3 id="还原-zsh"><a href="#还原-zsh" class="headerlink" title="还原 zsh"></a>还原 zsh</h3><p>参考 <a href="https://github.com/Codesire-Deng/rc">Codesire-Deng/rc</a></p>
<figure class="highlight bash"><table><tr><td class="code"><pre><span class="line">yay -S zsh</span><br><span class="line">chsh -s $(<span class="built_in">which</span> zsh)</span><br><span class="line">sh -c <span class="string">"<span class="subst">$(curl -fsSL https://raw.githubusercontent.com/ohmyzsh/ohmyzsh/master/tools/install.sh)</span>"</span></span><br><span class="line">yay -S zsh-syntax-highlighting-git</span><br></pre></td></tr></table></figure>
<ul>
<li>覆盖拷贝 <a href="https://github.com/Codesire-Deng/rc">Codesire-Deng/rc</a> 的配置文件。</li>
<li>安装完后，调整 konsole 和 vscode 的默认终端。</li>
</ul>
<h3 id="开发环境"><a href="#开发环境" class="headerlink" title="开发环境"></a>开发环境</h3><h4 id="C-相关"><a href="#C-相关" class="headerlink" title="C++ 相关"></a>C++ 相关</h4><figure class="highlight bash"><table><tr><td class="code"><pre><span class="line">yay -S linux-headers cmake clang mimalloc boost cloc </span><br></pre></td></tr></table></figure>
<h4 id="perf-相关"><a href="#perf-相关" class="headerlink" title="perf 相关"></a>perf 相关</h4><figure class="highlight bash"><table><tr><td class="code"><pre><span class="line">yay -S perf</span><br><span class="line"><span class="built_in">cd</span> ~/Downloads && git <span class="built_in">clone</span> https://github.com/brendangregg/FlameGraph.git</span><br><span class="line">sudo <span class="built_in">ln</span> -s ~/Downloads/FlameGraph/stackcollapse-perf.pl /usr/bin/stackcollapse-perf.pl</span><br><span class="line">sudo <span class="built_in">ln</span> -s ~/Downloads/FlameGraph/flamegraph.pl /usr/bin/flamegraph.pl</span><br></pre></td></tr></table></figure>
<h4 id="Node-相关"><a href="#Node-相关" class="headerlink" title="Node 相关"></a>Node 相关</h4><figure class="highlight bash"><table><tr><td class="code"><pre><span class="line">curl -L https://raw.githubusercontent.com/tj/n/master/bin/n -o n</span><br><span class="line">sudo bash n lts</span><br><span class="line">sudo npm install -g n</span><br><span class="line"><span class="built_in">rm</span> n</span><br><span class="line">sudo npm install -g cnpm --registry=https://registry.npmmirror.com</span><br><span class="line">sudo npm install -g hexo-cli</span><br></pre></td></tr></table></figure>
<h3 id="编译-Clang-套件"><a href="#编译-Clang-套件" class="headerlink" title="编译 Clang 套件"></a>编译 Clang 套件</h3><blockquote>
<p>若已有 <code>yay clang-git</code> 则可忽略此步。但是 <code>clang-git</code> 的质量堪忧。</p>
</blockquote>
<p>参考 <a href="https://clang.llvm.org/get_started.html">Clang Docs</a></p>
<figure class="highlight bash"><table><tr><td class="code"><pre><span class="line">git <span class="built_in">clone</span> --depth=1 https://github.com/llvm/llvm-project.git</span><br><span class="line"><span class="built_in">cd</span> llvm-project</span><br><span class="line">cmake -B build -DLLVM_ENABLE_PROJECTS=<span class="string">"clang;clang-tools-extra;compiler-rt"</span> -DCMAKE_BUILD_TYPE=Release -G <span class="string">"Unix Makefiles"</span> ./llvm</span><br><span class="line">cmake --build build -j 14 <span class="comment"># 不要使用全部的核心！UI会崩溃</span></span><br><span class="line">cmake --install build</span><br><span class="line"><span class="built_in">which</span> clang-format</span><br><span class="line">clang-format --version</span><br></pre></td></tr></table></figure>
<p>可在 llvm-project/llvm/CMakeLists.txt 中搜索 <code>LLVM_ALL_PROJECTS</code> 查看所有项目。</p>
<h3 id="生成-SSH-key"><a href="#生成-SSH-key" class="headerlink" title="生成 SSH key"></a>生成 SSH key</h3><p>参考 <a href="https://docs.github.com/cn/authentication/connecting-to-github-with-ssh/generating-a-new-ssh-key-and-adding-it-to-the-ssh-agent">Github Docs</a></p>
<figure class="highlight bash"><table><tr><td class="code"><pre><span class="line">ssh-keygen -t ed25519 -C <span class="string">"[email protected]"</span></span><br><span class="line"><span class="built_in">eval</span> <span class="string">"<span class="subst">$(ssh-agent -s)</span>"</span></span><br><span class="line">ssh-add ~/.ssh/id_ed25519</span><br></pre></td></tr></table></figure>
<p>将公钥添加到 <a href="https://github.com/settings/keys">Github SSH keys</a>。</p>
<h3 id="systemd-boot-追加-Windows"><a href="#systemd-boot-追加-Windows" class="headerlink" title="systemd-boot 追加 Windows"></a>systemd-boot 追加 Windows</h3><ol>
<li>找到 Windows EFI 分区：<ul>
<li><code>lsblk -o NAME,FSTYPE,SIZE,MOUNTPOINT</code></li>
</ul>
</li>
<li>挂载这个分区：<ul>
<li><code>sudo mkdir /mnt/win-efi</code></li>
<li><code>sudo mount /dev/sdb1xxx /mnt/win-efi</code></li>
</ul>
</li>
<li>拷贝 Windows EFI 到当前硬盘的 EFI（<code>/boot/EFI</code>在不同系统下可能不同，例如可能是<code>/boot/efi/EFI</code>）<ul>
<li><code>sudo cp -r /mnt/win-efi/EFI/Microsoft /boot/EFI</code></li>
</ul>
</li>
<li>修改 bootloader 的配置，例如倒计时<ul>
<li>参考 <a href="https://wiki.archlinux.org/title/Systemd-boot">Wiki: Systemd-boot</a></li>
<li><code>kate /boot/loader/loader.conf</code></li>
</ul>
</li>
<li>重启即可。在选择界面按 <code>d</code> 即可指定默认项。</li>
</ol>
<h3 id="开机挂载硬盘"><a href="#开机挂载硬盘" class="headerlink" title="开机挂载硬盘"></a>开机挂载硬盘</h3><ol>
<li>确认设备文件名<ul>
<li><code>sudo fdisk -l</code></li>
</ul>
</li>
<li>找到 UUID 和文件系统类型<ul>
<li><code>blkid</code></li>
</ul>
</li>
<li>条目追加至 <code>/etc/fstab</code><ul>
<li>例如：<code>UUID=3A848F25848EE32D /run/media/codesire/3A848F25848EE32D ntfs defaults 0 0</code></li>
</ul>
</li>
</ol>
<h3 id="附：Windows-改为-UTC-时间"><a href="#附：Windows-改为-UTC-时间" class="headerlink" title="附：Windows 改为 UTC 时间"></a>附：Windows 改为 UTC 时间</h3><p>新建<code>UTC.reg</code>，内容如下：</p>
<figure class="highlight plaintext"><table><tr><td class="code"><pre><span class="line">Windows Registry Editor Version 5.00</span><br><span class="line"></span><br><span class="line">[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\TimeZoneInformation]</span><br><span class="line">"RealTimeIsUniversal"=dword:00000001</span><br></pre></td></tr></table></figure>
<p>用管理员运行之即可。</p>
<h2 id="结语"><a href="#结语" class="headerlink" title="结语"></a>结语</h2><p>珍爱生命，珍惜时间。</p>
]]></content>
<tags>
<tag>减少脱发指南</tag>
</tags>
</entry>
<entry>
<title>C++20 Coroutine</title>
<url>/2021/11/18/C-20-Coroutine/</url>
<content><![CDATA[<blockquote>
<p><strong>协程</strong>(coroutine)是能够暂停和恢复的函数。</p>
</blockquote>
<p>协程是<strong>线程阻塞造成性能下降</strong>的最佳解决方案，尤其是应用在静态线程池中。</p>
<span id="more"></span>
<small>
<h2 id="吐槽"><a href="#吐槽" class="headerlink" title="吐槽"></a><del>吐槽</del></h2><p><del>GoLang 设计的 goroutine 简单好用，大名鼎鼎；虚拟机语言（例如 C#，Javascript，Java）的协程更是逆天改命，强势占领高性能并发的高地；机器队这边简直神速，而我们的人工队 C++ 在干什么呢，不会还没做完 STL network 吧，不会连 <code>format</code> 的编译器支持都没有吧，不会还没推广 <code>import <module></code> 吧 🥵🥵🥵……</del></p>
<h2 id="C-20-协程永远的神"><a href="#C-20-协程永远的神" class="headerlink" title="C++20 协程永远的神"></a>C++20 协程永远的神</h2></small>
<h2 id="C-的协程"><a href="#C-的协程" class="headerlink" title="C++的协程"></a>C++的协程</h2><p>C++的协程是：</p>
<ol>
<li>对称的。一个协程暂停后，可返回 caller 或恢复任意协程。</li>
<li>语言级特性。编译器知道你在使用协程。然而不比库强到哪里去。</li>
<li>无栈(Stackless) 的。没有独立运行时栈，无惧爆栈，调度成本低。</li>
</ol>
<p>一个协程在被命令「暂停」时，会保证将数据和当前运行位置保存在堆内存（以便恢复现场），然后转移运行权。</p>
<p>协程允许程序员更美观地编写异步代码，也使懒惰求值的算法成为可能。</p>
<p>当一个函数出现以下三种关键字之一，它就是协程：</p>
<ol>
<li><code>co_await</code> 暂停（直到被命令「恢复」）。</li>
<li><code>co_yield</code> 暂停同时返回一个值。</li>
<li><code>co_return</code> 结束整个协程并返回一个值。</li>
</ol>
<h2 id="使用协程的理由"><a href="#使用协程的理由" class="headerlink" title="使用协程的理由"></a>使用协程的理由</h2><ol>
<li>相比于回调和 sender/receiver，协程的使用<strong>成本更低</strong>，<strong>性能下限更高</strong>。</li>
<li>降低使用者的心智负担和阅历要求，催化高质量工程，资本宠儿。<ul>
<li>例如可以摆脱 asio 里常见的 <code>std::shared_ptr</code>。</li>
</ul>
</li>
</ol>
<h2 id="C-协程的弱点"><a href="#C-协程的弱点" class="headerlink" title="C++协程的弱点"></a>C++协程的弱点</h2><ol>
<li>除非编译器优化，每个协程都需要通过 <code>operator new</code> 来分配 frame：<ul>
<li>动态内存分配可能引发性能问题；</li>
<li>在嵌入式或异构（例如 GPU）环境下，缺乏动态内存分配能力，难以工作。</li>
</ul>
</li>
<li>除非编译器优化，协程的可定制点太多，需要大量间接调用/跳转（而不是内联），同样引起性能问题。<ul>
<li>目前，编译器通常难以内联协程；</li>
<li>HALO 优化理论：<a href="http://open-std.org/JTC1/SC22/WG21/docs/papers/2021/p2300r3.html#biblio-p0981r0">P0981R0</a>。</li>
</ul>
</li>
<li><strong>动态分配</strong>和<strong>间接调用</strong>的存在，导致协程暂时无法成为异步框架的最优方法。</li>
<li>Debug 的体验风评不佳。</li>
</ol>
<h2 id="协程的限制"><a href="#协程的限制" class="headerlink" title="协程的限制"></a>协程的限制</h2><ol>
<li>不能使用可变参数（variadic arguments），但可以使用变参模板</li>
<li>不能使用 <code>return</code></li>
<li>不能使用占位符返回类型 （<code>auto</code> 或者 Concept）</li>
<li>不能是 constexpr 函数</li>
<li>不能是构造函数或者析构函数</li>
<li>不能是 <code>main</code> 函数</li>
</ol>
<h2 id="协程的运行过程"><a href="#协程的运行过程" class="headerlink" title="协程的运行过程"></a>协程的运行过程</h2><p>所有协程必须关联着几个对象：</p>
<ol>
<li><strong>promise object</strong>，在协程内部进行操作，协程向其写入结果或者异常。</li>
<li><strong>coroutine handle</strong>，在协程外部进行操作，用于恢复协程或者销毁协程帧(frame)。</li>
<li><strong>coroutine state</strong>，保存协程的信息，分配于堆内存上（除非被优化），对程序员不可见。具体保存着：<ol>
<li>promise object</li>
<li>所有协程参数（按值复制或移动）</li>
<li>记录暂停点的状态机</li>
<li>局部变量和临时变量</li>
</ol>
</li>
</ol>
<hr>
<p>当协程「开始」时，它会：</p>
<ol>
<li>使用 <code>operater new</code> 来构造 coroutine state。</li>
<li>将所有协程参数拷贝或移动到 coroutine state。<em>小心发生「垂悬引用」，尤其在协程恢复后。</em></li>
<li>构造 promise object。优先调用接受所有协程参数的构造函数，否则调用默认构造函数。</li>
<li>调用 <code>promise.get_return_object()</code>，其结果保存为本地变量。此结果在第一次暂停时会返回给 caller。包括这一步和以前的步骤，所有异常都会抛给 caller，而不是放入 promise object。</li>
<li>调用 <code>promise.initial_suspend()</code>，紧接着 <code>co_await</code> 它。<em>常见的返回值是 <code>suspend_always</code> 用于懒汉协程，或者 <code>suspend_never</code> 用于饿汉协程</em></li>
<li>当 <code>co_await promise.initial_suspend()</code> 恢复后，协程开始运行其函数体。</li>
</ol>
<hr>
<p>当协程到达暂停点，它会：</p>
<ol>
<li>将 return object 返回给执行权所有者，类型应为协程的返回类型，允许发生隐式转换。</li>
</ol>
<hr>
<p>当协程到达 <code>co_return [expr]</code> 语句，它会：</p>
<ol>
<li>调用 <code>promise.return_void()</code>，条件是：<ol>
<li><code>co_return;</code></li>
<li><code>co_return expr</code> 而 expr 的类型是 void</li>
<li>函数体结束</li>
</ol>
</li>
<li>或者调用 <code>promise.return_value(expr)</code>。</li>
<li>析构所有自动变量。</li>
<li>调用 <code>promise.final_suspend()</code>，紧接着 <code>co_await</code> 它。</li>
</ol>
<hr>
<p>当协程因未捕获异常而结束，它会：</p>
<ol>
<li>捕获这个异常，并在 <code>catch</code> 块中调用 <code>promise.unhandled_exception()</code></li>
<li>调用 <code>promise.final_suspend()</code>，紧接着 <code>co_await</code> 它。此时恢复另一个协程是 UB。</li>
</ol>
<hr>
<p>当 coroutine state 被析构（要么遇到 <code>co_return</code>，要么未捕获异常，在要么被 handle 销毁）时，它会：</p>
<ol>
<li>析构 promise object。</li>
<li>析构所有协程参数。</li>
<li>使用 <code>operator delete</code> 来释放coroutine state。</li>
<li>转移执行权。</li>
</ol>
<h2 id="关于堆分配"><a href="#关于堆分配" class="headerlink" title="关于堆分配"></a>关于堆分配</h2><p>动态内存分配可能成为<strong>严重性能瓶颈</strong>！</p>
<p>程序员可通过自定义 <code>operator new</code> 来控制 coroutine state 的分配。这部分暂时忽略不讲。</p>
<h2 id="Promise类型推导"><a href="#Promise类型推导" class="headerlink" title="Promise类型推导"></a>Promise类型推导</h2><p>这部分暂时忽略不讲。</p>
<h2 id="co-await"><a href="#co-await" class="headerlink" title="co_await"></a>co_await</h2><p>一元运算符 <code>co_await</code> 会暂停协程并将转移执行权。其操作数必须定义 <code>operator co_await</code>，或者能通过当前协程的 <code>Promise::await_transform</code> 转换成这种类型。</p>
<figure class="highlight cpp"><table><tr><td class="code"><pre><span class="line"><span class="keyword">co_await</span> expr</span><br></pre></td></tr></table></figure>
<p><img src="/2021/11/18/C-20-Coroutine/co_await.svg" alt="co_await"></p>
<p>首先，<code>expr</code> 要被转换为 awaitable，规则如下：</p>
<ol>
<li>如果 <code>expr</code> 是由 initial suspend point，final suspend point 或者 yield expression 生成的，那么 awaitable 就是 <code>expr</code> 本身。</li>
<li>否则，如果当前协程有定义 <code>Promise::await_transform</code>，那么 awaitable 就是 <code>promise.await_transform(expr)</code>。</li>
<li>否则，awaitable 就是 <code>expr</code> 本身。</li>
</ol>
<p>然后生成一个 awaiter object，规则如下：</p>
<ol>
<li>根据重载解析的结果调用 <code>opeartor co_await(awaitable)</code> 或者 <code>awaitable.operator co_await()</code></li>
<li>如果重载解析找不到函数，那么 awaiter 就是 awaitable 本身。</li>
<li>如果重载解析有歧义，那么程序是 ill-formed。</li>
</ol>
<p>然后，调用 <code>awaiter.await_ready()</code>并判断，决定是否暂停协程：</p>
<ul>
<li>若 <code>false</code>，协程暂停，必要的信息存放于 coroutine state。然后调用 <code>awaiter.await_suspend(handle)</code>。在这个函数中，通过 <code>handle</code> 可以访问 coroutine state，也是这个函数有责任安排协程在某个 executor 上恢复（甚至立即就地恢复），或者干脆销毁协程：<ul>
<li>若 <code>awaiter.await_suspend(handle)</code> 返回 void，执行权立即转移（给 caller/resumer）。</li>
<li>否则若返回 bool，<ul>
<li>若 <code>true</code> 则转移执行权（给 caller/resumer）。</li>
<li>若 <code>false</code> 则恢复当前协程。</li>
</ul>
</li>
<li>否则返回一个 coroutine handle（对应其他协程），调用这个 <code>other_handle.resume()</code>。注意链式调用可能最终恢复当前协程。</li>
<li>若 <code>awaiter.await_suspend()</code> 抛出一个异常，则立即恢复协程，并由该协程接收这个异常。</li>
<li>当前协程恢复后，返回 <code>awaiter.await_resume()</code> 作为 <code>co_await expr</code> 的结果。</li>
</ul>
</li>
<li>若 <code>true</code>，协程直接返回 <code>awaiter.await_resume()</code> 作为 <code>co_await expr</code> 的结果。</li>
</ul>
<p>注意在进入 <code>awaiter.await_suspend(handle)</code> 之前，当前协程已经完全暂停，此时当前 handle 可以在线程之间自由传递，并由其他调度者恢复。在这种情况下，协程可能已经被恢复，awaiter 随之已经被析构，所以 <code>await_suspend()</code> 不应再访问 <code>*this</code>。</p>
<h2 id="co-yield"><a href="#co-yield" class="headerlink" title="co_yield"></a>co_yield</h2><figure class="highlight cpp"><table><tr><td class="code"><pre><span class="line"><span class="keyword">co_yield</span> expr</span><br><span class="line"><span class="keyword">co_yield</span> braced-init-list</span><br></pre></td></tr></table></figure>
<p>等价于</p>
<figure class="highlight cpp"><table><tr><td class="code"><pre><span class="line"><span class="keyword">co_await</span> promise.<span class="built_in">yield_value</span>(expr)</span><br></pre></td></tr></table></figure>
]]></content>
<tags>
<tag>C++</tag>
<tag>Concurrency</tag>
<tag>语法</tag>
<tag>C++协程</tag>
</tags>
</entry>
<entry>
<title>C++ Smart Pointer</title>
<url>/2021/12/15/C-smart-pointer/</url>
<content><![CDATA[<p>本文意在阐述 C++ 智能指针的实现原理，应用场景和潜在的坑。</p>
<span id="more"></span>
<h2 id="shared-ptr"><a href="#shared-ptr" class="headerlink" title="shared_ptr"></a>shared_ptr</h2><p>基于引用计数的智能指针。在构造和析构时修改引用计数。当引用计数为 0 时，析构资源。</p>
<p>单个引用计数器是线程安全的。但是，shared_ptr 并非线程安全，至少因为两个原因：</p>
<ol>
<li>「改写资源指针」与「改写引用计数」，这两步并非原子化。</li>
<li>「改写共享引用计数」与「改写弱引用计数」，这两步也非原子化。</li>
</ol>
<p>陈硕的书和<a href="https://www.cnblogs.com/Solstice/archive/2013/01/28/2879366.html">博客</a>给出了一个竞争条件的例子。如果要从多个线程读写同一个 shared_ptr 对象，是<strong>需要加锁</strong>的。</p>
<h3 id="shared-ptr-的实现原理"><a href="#shared-ptr-的实现原理" class="headerlink" title="shared_ptr 的实现原理"></a>shared_ptr 的实现原理</h3><ol>
<li>一个 <code>shared_ptr</code> 持有两个裸指针，分别指向资源和「控制块」 (control block)</li>
<li>控制块主要用于实现引用计数。具体的内容有：<ol>
<li>指向所管理资源的指针，或者是资源本身</li>
<li>deleter（已擦除类型）</li>
<li>allocator（已擦除类型）</li>
<li><code>shared_ptr</code> 共享引用计数器</li>
<li><code>weak_ptr</code> 弱引用计数器 （主流实现中 <code>shared_ptr</code> 亦计入弱引用计数）</li>
</ol>
</li>
<li>若使用 <code>make_shared</code> 或 <code>allocate_shared</code> 来构造指针，则资源和控制块一同参与内存申请。<em>这是潜在坑点。</em></li>
<li>若弱引用计数器不清零，则控制块占用的内存不会被回收。因此若控制块很大（直接存放了资源），则空间性能可能变差。</li>
</ol>
<figure class="highlight cpp"><table><tr><td class="code"><pre><span class="line"><span class="comment">// 别名构造函数（aliasing constructor）</span></span><br><span class="line"><span class="function"><span class="keyword">template</span>< <span class="keyword">class</span> Y ></span></span><br><span class="line"><span class="function"><span class="title">shared_ptr</span><span class="params">( <span class="type">const</span> shared_ptr<Y>& r, element_type* ptr )</span> <span class="keyword">noexcept</span></span>;</span><br><span class="line"><span class="comment">/*</span></span><br><span class="line"><span class="comment">新智能指针共享与 r 的控制块，但是 this->get() 总是返回 ptr，这可以与 r.get() 不相同！</span></span><br><span class="line"><span class="comment"></span></span><br><span class="line"><span class="comment">场景：ptr 是 *r 的数据成员，若 r 存活，则 ptr 必定存活。</span></span><br><span class="line"><span class="comment">*/</span></span><br></pre></td></tr></table></figure>
<h3 id="shared-ptr-的潜在坑点"><a href="#shared-ptr-的潜在坑点" class="headerlink" title="shared_ptr 的潜在坑点"></a>shared_ptr 的潜在坑点</h3><ol>
<li>环形引用导致内存泄漏：使用一个 <code>weak_ptr</code> 来打破环形引用。</li>
<li>不支持 <code>shared_ptr<T[]></code>：使用 <code>vector</code> 或 <code>array</code> 作容器。</li>
<li>容许基类析构函数不是虚函数（似乎是因为构造是泛型的）：小心。</li>
<li>退化成链状的数据结构在析构时<strong>栈溢出</strong>：反思数据结构的退化，以及是否滥用了 <code>shared_ptr</code>。</li>
<li>重复使用一个裸指针构造两个 <code>shared_ptr</code>，导致双重释放：<ol>
<li>裸指针用完就扔。</li>
<li>不要使用 <code>this</code> 来构造智能指针，而是使用 <code>shared_from_this</code>。<em>这是潜在坑点。</em></li>
</ol>
</li>
<li>若没有任何 <code>shared_ptr</code> 持有对象，此时调用 <code>shared_from_this</code> 就会出问题：<ol>
<li>C++17 之前，这是 UB。</li>
<li>C++17 以后，程序抛出 <code>std::bad_weak_ptr</code> 异常。</li>
<li>最佳实践是禁止访问构造函数，而提供工厂函数返回智能指针。</li>
</ol>
</li>
<li>将 <code>shared_ptr</code> 复制给未知的函数，可能导致环形引用。<ul>
<li>好比不要在上锁后执行为未知用户代码</li>
<li>GC 语言同样有回调地狱导致内存泄漏的问题</li>
</ul>
</li>
</ol>
<figure class="highlight cpp"><table><tr><td class="code"><pre><span class="line"><span class="comment">// enable_shared_from_this 最佳实践</span></span><br><span class="line"><span class="keyword">struct</span> <span class="title class_">Best</span> : std::enable_shared_from_this<Best> <span class="comment">// 公有继承</span></span><br><span class="line">{</span><br><span class="line"> <span class="function">std::shared_ptr<Best> <span class="title">getptr</span><span class="params">()</span> </span>{</span><br><span class="line"> <span class="keyword">return</span> <span class="built_in">shared_from_this</span>();</span><br><span class="line"> }</span><br><span class="line"> <span class="comment">// 工厂函数返回智能指针</span></span><br><span class="line"> <span class="comment">// nodiscard 防止暴毙</span></span><br><span class="line"> [[nodiscard]] <span class="function"><span class="type">static</span> std::shared_ptr<Best> <span class="title">create</span><span class="params">()</span> </span>{</span><br><span class="line"> <span class="comment">// 不能使用 make_shared 因为构造函数是私有的，不能转发</span></span><br><span class="line"> <span class="keyword">return</span> std::<span class="built_in">shared_ptr</span><Best>(<span class="keyword">new</span> <span class="built_in">Best</span>());</span><br><span class="line"> }</span><br><span class="line"><span class="keyword">private</span>:</span><br><span class="line"> <span class="built_in">Best</span>() = <span class="keyword">default</span>;</span><br><span class="line">};</span><br></pre></td></tr></table></figure>
<h3 id="shared-ptr-的应用场景-x2F-条件"><a href="#shared-ptr-的应用场景-x2F-条件" class="headerlink" title="shared_ptr 的应用场景/条件"></a>shared_ptr 的应用场景/条件</h3><ol>
<li>多线程/异步协作，难以确定资源生命周期。</li>
<li>主从关系明确，不存在环形引用：树、链表、DAG</li>
<li>可以在构造函数参数中提供自定义析构器：<code>[](Y* rc) { delect rc; }</code>。</li>
<li>原本使用 <code>unique_ptr</code> 的数据结构，需要对外提供强引用，则可以改用 <code>shared_ptr</code>。</li>
</ol>
<h2 id="unique-ptr"><a href="#unique-ptr" class="headerlink" title="unique_ptr"></a>unique_ptr</h2><p>独占所有权的智能指针。当持有资源且退出作用域时，会析构资源。所有权只能移动或引用，不能拷贝。</p>
<h3 id="unique-ptr-的潜在坑点"><a href="#unique-ptr-的潜在坑点" class="headerlink" title="unique_ptr 的潜在坑点"></a>unique_ptr 的潜在坑点</h3><ol>
<li>退化成链的数据结构在析构时<strong>栈溢出</strong>：反思数据结构；重写数据结构的析构函数。</li>
</ol>
<h3 id="unique-ptr-的应用场景"><a href="#unique-ptr-的应用场景" class="headerlink" title="unique_ptr 的应用场景"></a>unique_ptr 的应用场景</h3><ol>
<li>提供动态对象的 RAII，保证发生异常时对象仍被析构。</li>
<li>表示对堆对象的独占关系。适合用作类的数据成员，例如表示树的儿子。<ul>
<li>父亲指针永远只用裸指针，因为儿子存活时父亲一定存活</li>
<li>单/双向链表是树的特例，也适用 <code>unique_ptr</code> + 裸指针。</li>
</ul>
</li>
<li>在接口中明确表达「传递所有权」的语义。</li>
<li>作为具有移动语义的单个容器使用。</li>
<li>管理运行时确定长度的定长数组（<code>unique_ptr<T[]></code> 正确使用 <code>new []</code> <code>delete []</code>）</li>
<li>用于 Pimpl 编译隔离</li>
</ol>
<figure class="highlight cpp"><table><tr><td class="code"><pre><span class="line"><span class="comment">// Pimpl 抽象</span></span><br><span class="line"><span class="keyword">template</span><<span class="keyword">class</span> <span class="title class_">T</span>></span><br><span class="line"><span class="keyword">using</span> Pimpl = <span class="type">const</span> unique_ptr<T>;</span><br><span class="line"></span><br><span class="line"><span class="keyword">class</span> <span class="title class_">MyClass</span> {</span><br><span class="line"> <span class="keyword">class</span> <span class="title class_">Impl</span>; <span class="comment">// 定义于 *.cpp</span></span><br><span class="line"> Pimpl<Impl> pimpl;</span><br><span class="line"> <span class="comment">/*...*/</span></span><br><span class="line">};</span><br><span class="line"></span><br><span class="line"><span class="comment">// 运行时定长数组</span></span><br><span class="line"><span class="keyword">class</span> <span class="title class_">MyClass</span> {</span><br><span class="line"> <span class="type">const</span> unique_ptr<Data[]> array;</span><br><span class="line"> <span class="type">size_t</span> arr_size;</span><br><span class="line"></span><br><span class="line"> <span class="built_in">MyClass</span>(<span class="type">size_t</span> size): <span class="built_in">arr_size</span>(<span class="built_in">make_unique</span><Data[]>(size)) {}</span><br><span class="line"> <span class="comment">/*...*/</span></span><br><span class="line">};</span><br></pre></td></tr></table></figure>
<h2 id="weak-ptr"><a href="#weak-ptr" class="headerlink" title="weak_ptr"></a>weak_ptr</h2><p>从 <code>shared_ptr</code> 构造而来，但不参与引用计数。</p>
<h3 id="weak-ptr-的潜在坑点"><a href="#weak-ptr-的潜在坑点" class="headerlink" title="weak_ptr 的潜在坑点"></a>weak_ptr 的潜在坑点</h3><p>良好的设计使得 <code>weak_ptr</code> 坑点较少，只需记得弱引用的存在会使得控制块无法释放即可。</p>
<h3 id="weak-ptr-的应用场景"><a href="#weak-ptr-的应用场景" class="headerlink" title="weak_ptr 的应用场景"></a>weak_ptr 的应用场景</h3><ol>
<li>打破 <code>shared_ptr</code> 的循环引用。常用于环形链表、回调函数。</li>
<li>作为「可空资源的观察者」。根据资源是否为空表现不同的行为。</li>
</ol>
<figure class="highlight cpp"><table><tr><td class="code"><pre><span class="line"><span class="comment">// weak_ptr 做回调</span></span><br><span class="line"><span class="function"><span class="type">void</span> <span class="title">bad</span><span class="params">(<span class="type">const</span> shared_ptr<X>& x)</span> </span>{</span><br><span class="line"> obj.<span class="built_in">on_draw</span>([=]{ x-><span class="built_in">extra_work</span>(); }); <span class="comment">// x 已经泄露</span></span><br><span class="line">}</span><br><span class="line"></span><br><span class="line"><span class="function"><span class="type">void</span> <span class="title">good</span><span class="params">(<span class="type">const</span> shared_ptr<X>& x)</span> </span>{</span><br><span class="line"> obj.<span class="built_in">on_draw</span>([w=<span class="built_in">weak_ptr</span><X>(x)] {</span><br><span class="line"> <span class="keyword">if</span> (<span class="keyword">auto</span> x = w.<span class="built_in">lock</span>()) x-><span class="built_in">extra_work</span>(); <span class="comment">// x 有机会析构</span></span><br><span class="line"> });</span><br><span class="line">}</span><br><span class="line"></span><br><span class="line"></span><br><span class="line"><span class="comment">// weak_ptr 做 cache</span></span><br><span class="line"><span class="function">shared_ptr<Payload> <span class="title">create</span><span class="params">(<span class="type">int</span> id)</span> </span>{</span><br><span class="line"> <span class="type">static</span> map<<span class="type">int</span>, weak_ptr<Payload>> cache;</span><br><span class="line"> <span class="type">static</span> mutex mut_cache;</span><br><span class="line"> <span class="function">lock_guard<mutex> <span class="title">hold</span><span class="params">(mut_cache)</span></span>;</span><br><span class="line"> <span class="keyword">auto</span> sp = cache[id].<span class="built_in">lock</span>();</span><br><span class="line"> <span class="keyword">if</span> (!sp) cache[id] = sp = <span class="built_in">make_shared</span><Payload>();</span><br><span class="line"> <span class="keyword">return</span> sp;</span><br><span class="line">}</span><br><span class="line"></span><br></pre></td></tr></table></figure>
<h2 id="参考资料"><a href="#参考资料" class="headerlink" title="参考资料"></a>参考资料</h2><ol>
<li><a href="https://en.cppreference.com/w/cpp/memory">cppreference</a></li>
<li><a href="https://www.youtube.com/watch?v=JfmTagWcqoE">CppCon 2016: Herb Sutter “Leak-Freedom in C++... By Default.”</a></li>
<li><a href="http://senlinzhan.github.io/2015/04/24/%E6%B7%B1%E5%85%A5shared-ptr/">Senlin《谈谈 shared_ptr 的那些坑》</a></li>
</ol>
]]></content>
<tags>
<tag>C++</tag>
</tags>
</entry>
<entry>
<title>C++ 多态</title>
<url>/2022/01/28/C-%E5%A4%9A%E6%80%81/</url>
<content><![CDATA[<h1 id="Preface"><a href="#Preface" class="headerlink" title="Preface"></a>Preface</h1><p>「多态」是一个很宽泛的理念，对应繁多实现。本文将记录我所看到的多态的各种形态。作者见识短浅，还请读者多多指教。</p>
<ul>
<li>本文已计划的内容：<ul>
<li><input checked="" disabled="" type="checkbox"> 基于 <code>std::visit</code><ul>
<li><input checked="" disabled="" type="checkbox"> 实现运行时多态的优化</li>
<li><input checked="" disabled="" type="checkbox"> 实现<em>重载模式</em></li>
</ul>
</li>
<li><input checked="" disabled="" type="checkbox"> CRTP 实现静态多态<ul>
<li><input checked="" disabled="" type="checkbox"> 静态多态</li>
<li><input checked="" disabled="" type="checkbox"> 静态接口</li>
<li><input checked="" disabled="" type="checkbox"> CRTP 的一些分析和 trick</li>
</ul>
</li>
<li><input disabled="" type="checkbox"> C++20 前基于 SFINAE 的模板偏特化</li>
<li><input disabled="" type="checkbox"> C++20 后基于 <code>concepts</code> 的模板特化</li>
<li><input disabled="" type="checkbox"> Hack 虚函数表，用于优化<span id="more"></span></li>
</ul>
</li>
<li>本文<strong>不会</strong>出现的内容：<ul>
<li><code>virtual</code> 虚函数的基础用法</li>
<li>函数指针模拟虚函数</li>
<li>运行时反射</li>
</ul>
</li>
</ul>
<h1 id="std-visit"><a href="#std-visit" class="headerlink" title="std::visit"></a>std::visit</h1><h2 id="Runtime-Branching"><a href="#Runtime-Branching" class="headerlink" title="Runtime Branching"></a>Runtime Branching</h2><p>假如你要对一个矢量做线性变换 $\vec{x} = k \cdot \vec{x} + b$，又想针对 $k=1$ 或 $b=0$ 的情况做优化，应该怎么办？</p>
<p>这是最简单却很慢的代码：</p>
<figure class="highlight cpp"><table><tr><td class="code"><pre><span class="line"><span class="keyword">for</span> (<span class="type">int</span> i=<span class="number">0</span>; i<n; ++i) {</span><br><span class="line"> <span class="keyword">if</span> (k != <span class="number">1</span>)</span><br><span class="line"> x[i] *= k;</span><br><span class="line"> <span class="keyword">if</span> (b != <span class="number">0</span>)</span><br><span class="line"> x[i] += b;</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<p>这段代码满足速度要求，但是太啰嗦：</p>
<figure class="highlight cpp"><table><tr><td class="code"><pre><span class="line"><span class="keyword">if</span> (k == <span class="number">1</span>) {</span><br><span class="line"> <span class="keyword">if</span> (b == <span class="number">0</span>) {</span><br><span class="line"> <span class="comment">// ...</span></span><br><span class="line"> } <span class="keyword">else</span> {</span><br><span class="line"> <span class="comment">// ...</span></span><br><span class="line"> }</span><br><span class="line">} <span class="keyword">else</span> {</span><br><span class="line"> <span class="keyword">if</span> (b == <span class="number">0</span>) {</span><br><span class="line"> <span class="comment">// ...</span></span><br><span class="line"> } <span class="keyword">else</span> {</span><br><span class="line"> <span class="comment">// ...</span></span><br><span class="line"> }</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<p>想两全其美怎么办？可以利用 <code>variant</code> 搭配 <code>visit</code> 使用：</p>
<figure class="highlight cpp"><table><tr><td class="code"><pre><span class="line"><span class="comment">// since C++17</span></span><br><span class="line"><span class="keyword">using</span> bool_variant = std::variant<std::true_type, std::false_type>;</span><br><span class="line"></span><br><span class="line"><span class="function"><span class="keyword">constexpr</span> bool_variant <span class="title">to_variant</span><span class="params">(<span class="type">bool</span> x)</span> </span>{</span><br><span class="line"> <span class="keyword">if</span> (x)</span><br><span class="line"> <span class="keyword">return</span> std::true_type{};</span><br><span class="line"> <span class="keyword">else</span></span><br><span class="line"> <span class="keyword">return</span> std::false_type{};</span><br><span class="line">}</span><br><span class="line"></span><br><span class="line"><span class="function"><span class="type">void</span> <span class="title">linear</span><span class="params">(std::vector<<span class="type">int</span>> &x, <span class="type">int</span> k, <span class="type">int</span> b)</span> </span>{</span><br><span class="line"> bool_variant has_k = <span class="built_in">to_variant</span>(k != <span class="number">1</span>);</span><br><span class="line"> bool_variant has_b = <span class="built_in">to_variant</span>(b != <span class="number">0</span>);</span><br><span class="line"></span><br><span class="line"> <span class="keyword">auto</span> transform = [=, &x](<span class="keyword">auto</span> has_k, <span class="keyword">auto</span> has_b) {</span><br><span class="line"> <span class="keyword">for</span> (<span class="keyword">auto</span> &xi : x) {</span><br><span class="line"> <span class="keyword">if</span> <span class="built_in">constexpr</span> (has_k)</span><br><span class="line"> xi *= k;</span><br><span class="line"> <span class="function"><span class="keyword">if</span> <span class="title">constexpr</span> <span class="params">(has_b)</span></span></span><br><span class="line"><span class="function"> xi +</span>= b;</span><br><span class="line"> }</span><br><span class="line"> };</span><br><span class="line"></span><br><span class="line"> std::<span class="built_in">visit</span>(transform, has_k, has_b);</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<p>此时编译器为 <code>transform</code> 生成四个特化版本，正是我们想要的东西。<code>std::visit()</code> 会在运行时判断 <code>variant</code> 内储存的是哪个 type，然后挑选正确的重载分支。</p>
<p><code>variant</code> + <code>constexpr if</code> 并不是唯一的玩法，我们来看看 <a href="https://en.cppreference.com/w/cpp/utility/variant/visit">cppreference</a> 还有什么花活。</p>
<h2 id="The-Overload-Pattern"><a href="#The-Overload-Pattern" class="headerlink" title="The Overload Pattern"></a>The <em>Overload Pattern</em></h2><p>假设你要 log 一个 <code>variant<int, float, string></code>，你刚刚学到了 <code>constexpr if</code>，于是你想到这一段代码：</p>
<figure class="highlight cpp"><table><tr><td class="code"><pre><span class="line"><span class="keyword">template</span><<span class="keyword">class</span>> <span class="keyword">inline</span> <span class="keyword">constexpr</span> <span class="type">bool</span> always_false_v = <span class="literal">false</span>;</span><br><span class="line"></span><br><span class="line"><span class="keyword">using</span> ifs_variant = std::variant<<span class="type">int</span>, <span class="type">float</span>, std::string>;</span><br><span class="line"></span><br><span class="line"><span class="function"><span class="type">void</span> <span class="title">log</span><span class="params">(<span class="type">const</span> ifs_variant &arg)</span> </span>{</span><br><span class="line"> <span class="keyword">auto</span> logger = [](<span class="type">const</span> <span class="keyword">auto</span> &arg) {</span><br><span class="line"> <span class="keyword">using</span> T = std::<span class="type">decay_t</span><<span class="keyword">decltype</span>(arg)>;</span><br><span class="line"> <span class="function"><span class="keyword">if</span> <span class="title">constexpr</span> <span class="params">(std::is_same_v<T, <span class="type">int</span>>)</span> </span>{</span><br><span class="line"> std::cout << <span class="string">"int: "</span> << arg << <span class="string">"\n"</span>;</span><br><span class="line"> } <span class="keyword">else</span> <span class="keyword">if</span> <span class="built_in">constexpr</span> (std::is_same_v<T, <span class="type">float</span>>) {</span><br><span class="line"> std::cout << <span class="string">"float: "</span> << arg << <span class="string">"\n"</span>;</span><br><span class="line"> } <span class="keyword">else</span> <span class="keyword">if</span> <span class="built_in">constexpr</span> (std::is_same_v<T, std::string>) {</span><br><span class="line"> std::cout << <span class="string">"string: "</span> << arg << <span class="string">"\n"</span>;</span><br><span class="line"> } <span class="keyword">else</span> {</span><br><span class="line"> <span class="built_in">static_assert</span>(always_false_v<T>, <span class="string">"non-exhaustive visitor!"</span>);</span><br><span class="line"> }</span><br><span class="line"> };</span><br><span class="line"> </span><br><span class="line"> std::<span class="built_in">visit</span>(logger, arg);</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<p>三秒之后你觉得这段代码太丑，太多的 <code>if constexpr</code> 和 <code>is_same_v</code> 语句影响阅读，你也不想用 <code>always_false_v<T></code> 这样的孤儿代码。于是你想到了重载 <code>opeartor()</code>：</p>
<figure class="highlight cpp"><table><tr><td class="code"><pre><span class="line"><span class="function"><span class="type">void</span> <span class="title">log</span><span class="params">(<span class="type">const</span> ifs_variant &arg)</span> </span>{</span><br><span class="line"> <span class="keyword">struct</span> {</span><br><span class="line"> <span class="function"><span class="type">void</span> <span class="title">operator</span><span class="params">()</span><span class="params">(<span class="type">int</span> i)</span> </span>{</span><br><span class="line"> std::cout << <span class="string">"int: "</span> << i << <span class="string">"\n"</span>;</span><br><span class="line"> }</span><br><span class="line"> <span class="function"><span class="type">void</span> <span class="title">operator</span><span class="params">()</span><span class="params">(<span class="type">float</span> f)</span> </span>{</span><br><span class="line"> std::cout << <span class="string">"float: "</span> << f << <span class="string">"\n"</span>;</span><br><span class="line"> }</span><br><span class="line"> <span class="function"><span class="type">void</span> <span class="title">operator</span><span class="params">()</span><span class="params">(<span class="type">const</span> std::string &s)</span> </span>{</span><br><span class="line"> std::cout << <span class="string">"string: "</span> << s << <span class="string">"\n"</span>;</span><br><span class="line"> }</span><br><span class="line"> } logger;</span><br><span class="line"> </span><br><span class="line"> std::<span class="built_in">visit</span>(logger, arg);</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<p>这段代码会在缺少某个重载时报错（模板巨型报错），长度也合理，你的血压下降了一些，但仍未到安全区，因为 <code>void operator()</code> 的重复依然很丑。你想用模板元编程自动化这些函数重载。你的目标是：</p>
<figure class="highlight cpp"><table><tr><td class="code"><pre><span class="line"><span class="keyword">template</span> <<span class="keyword">typename</span>... Lambdas></span><br><span class="line"><span class="keyword">struct</span> <span class="title class_">Overloaded</span> : Lambdas... {</span><br><span class="line"> <span class="comment">// Import all function whatever the parameter is.</span></span><br><span class="line"> <span class="function"><span class="keyword">using</span> <span class="title">Lambdas::operator</span><span class="params">()</span>...</span>; </span><br><span class="line">};</span><br></pre></td></tr></table></figure>
<p>这样当你用 lambda 的类型做模板参数，就能自动导入 lambda 的 <code>opeartor()</code>。问题来了，你怎么传入 lambda 的类型？你根本没有办法用尖括号 <code><></code> 来指定 lambda 类型！而自动类型推导至少需要一个函数调用，你想到了构造函数！可以在构造函数中传入 lambda，这样就能推导出 <code>Lambdas</code> 的具体类型！</p>
<figure class="highlight cpp"><table><tr><td class="code"><pre><span class="line"><span class="keyword">template</span> <<span class="keyword">typename</span>... Lambdas></span><br><span class="line"><span class="keyword">struct</span> <span class="title class_">Overloaded</span> : Lambdas... {</span><br><span class="line"> <span class="function"><span class="keyword">using</span> <span class="title">Lambdas::operator</span><span class="params">()</span>...</span>;</span><br><span class="line"></span><br><span class="line"> <span class="built_in">Overloaded</span>(Lambdas&&...) {} <span class="comment">// Bang!</span></span><br><span class="line">};</span><br><span class="line"></span><br><span class="line"><span class="function"><span class="type">void</span> <span class="title">log</span><span class="params">(<span class="type">const</span> ifs_variant &arg)</span> </span>{</span><br><span class="line"> <span class="keyword">auto</span> logger = Overloaded{</span><br><span class="line"> [](<span class="type">int</span> i) { std::cout << <span class="string">"int: "</span> << i << <span class="string">"\n"</span>; },</span><br><span class="line"> [](<span class="type">float</span> f) { std::cout << <span class="string">"float: "</span> << f << <span class="string">"\n"</span>; },</span><br><span class="line"> [](<span class="type">const</span> std::string &s) { std::cout << <span class="string">"string: "</span> << s << <span class="string">"\n"</span>; },</span><br><span class="line"> };</span><br><span class="line"> </span><br><span class="line"> std::<span class="built_in">visit</span>(logger, arg);</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<p>Bang! 第五行编译报错，编译器说，你企图使用 Lambda 的默认构造函数，但这个函数是 <code>deleted</code>！</p>
<p>想了半天，你顿悟过来，Overloaded 的构造函数一定要显式调用 Lambdas 的构造函数才行，于是你写出：</p>
<figure class="highlight cpp"><table><tr><td class="code"><pre><span class="line"><span class="keyword">template</span> <<span class="keyword">typename</span>... Lambdas></span><br><span class="line"><span class="keyword">struct</span> <span class="title class_">Overloaded</span> : Lambdas... {</span><br><span class="line"> <span class="function"><span class="keyword">using</span> <span class="title">Lambdas::operator</span><span class="params">()</span>...</span>;</span><br><span class="line"></span><br><span class="line"> <span class="built_in">Overloaded</span>(Lambdas &&...t) : Lambdas{std::forward<Lambdas>(t)}... {};</span><br><span class="line">};</span><br></pre></td></tr></table></figure>
<p>编译终于过了！你还擅长举一反三，联想到 <code>make_tuple()</code> 可以在没有尖括号 <code><></code> 的情况下工作，你可以模仿它，于是你写出：</p>
<figure class="highlight cpp"><table><tr><td class="code"><pre><span class="line"><span class="keyword">template</span> <<span class="keyword">typename</span>... Lambdas></span><br><span class="line"><span class="keyword">struct</span> <span class="title class_">Overloaded</span> : Lambdas... {</span><br><span class="line"> <span class="function"><span class="keyword">using</span> <span class="title">Lambdas::operator</span><span class="params">()</span>...</span>;</span><br><span class="line"> <span class="comment">// no more explicit ctor</span></span><br><span class="line">};</span><br><span class="line"></span><br><span class="line"><span class="keyword">template</span> <<span class="keyword">typename</span>... Lambdas></span><br><span class="line"><span class="function"><span class="keyword">constexpr</span> <span class="keyword">auto</span> <span class="title">make_Overloaded</span><span class="params">(Lambdas &&...t)</span> </span>{</span><br><span class="line"> <span class="keyword">return</span> Overloaded<Lambdas...>{std::forward<Lambdas>(t)...};</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<p>这段代码很秀也很正确。可惜好景不长，一个保洁阿姨路过了你，说，小伙子你写的不行，来看我秀一手：</p>
<figure class="highlight cpp"><table><tr><td class="code"><pre><span class="line"><span class="keyword">template</span><<span class="keyword">class</span>... Ts> <span class="keyword">struct</span> <span class="title class_">Overloaded</span> : Ts... { <span class="function"><span class="keyword">using</span> <span class="title">Ts::operator</span><span class="params">()</span>...</span>; };</span><br><span class="line"><span class="function"><span class="keyword">template</span><<span class="keyword">class</span>... Ts> <span class="title">Overloaded</span><span class="params">(Ts...)</span> -> Overloaded<Ts...></span>;</span><br></pre></td></tr></table></figure>
<p>编译通过！阿姨的第二行模板恰好替代了 <code>make_Overloaded</code>，正确推导出所有模板参数！这是 C++17 的新功能 *<a href="https://en.cppreference.com/w/cpp/language/class_template_argument_deduction">Custom Template Argument Deduction Rules</a>*。这一功能在 C++20 中更加智能，所以你可以删掉那行显式推导规则。</p>
<p>现在，你进化成了 <code>std::visit</code> 不用 <code>Overloaded{}</code> 就不舒服星人。</p>
<h3 id="Demo"><a href="#Demo" class="headerlink" title="Demo"></a>Demo</h3><figure class="highlight cpp"><table><tr><td class="code"><pre><span class="line"><span class="meta">#<span class="keyword">include</span> <span class="string"><iostream></span></span></span><br><span class="line"><span class="meta">#<span class="keyword">include</span> <span class="string"><string></span></span></span><br><span class="line"><span class="meta">#<span class="keyword">include</span> <span class="string"><variant></span> <span class="comment">// C++17</span></span></span><br><span class="line"></span><br><span class="line"><span class="keyword">using</span> ifs_variant = std::variant<<span class="type">int</span>, <span class="type">float</span>, std::string>;</span><br><span class="line"></span><br><span class="line"><span class="keyword">template</span><<span class="keyword">class</span>... Ts> <span class="keyword">struct</span> <span class="title class_">Overloaded</span> : Ts... { <span class="function"><span class="keyword">using</span> <span class="title">Ts::operator</span><span class="params">()</span>...</span>; };</span><br><span class="line"><span class="function"><span class="keyword">template</span><<span class="keyword">class</span>... Ts> <span class="title">Overloaded</span><span class="params">(Ts...)</span> -> Overloaded<Ts...></span>;</span><br><span class="line"></span><br><span class="line"><span class="function"><span class="type">void</span> <span class="title">log</span><span class="params">(<span class="type">const</span> ifs_variant &arg)</span> </span>{</span><br><span class="line"> std::<span class="built_in">visit</span>(Overloaded{</span><br><span class="line"> [](<span class="type">int</span> i) { std::cout << <span class="string">"int: "</span> << i << <span class="string">"\n"</span>; },</span><br><span class="line"> [](<span class="type">float</span> f) { std::cout << <span class="string">"float: "</span> << f << <span class="string">"\n"</span>; },</span><br><span class="line"> [](<span class="type">const</span> std::string &s) { std::cout << <span class="string">"string: "</span> << s << <span class="string">"\n"</span>; },</span><br><span class="line"> }, arg);</span><br><span class="line">}</span><br><span class="line"></span><br><span class="line"><span class="function"><span class="type">int</span> <span class="title">main</span><span class="params">()</span> </span>{</span><br><span class="line"> ifs_variant i{<span class="number">10</span>}, <span class="built_in">f</span>(<span class="number">0.7f</span>), s{<span class="string">"string"</span>};</span><br><span class="line"> <span class="built_in">log</span>(i);</span><br><span class="line"> <span class="built_in">log</span>(f);</span><br><span class="line"> <span class="built_in">log</span>(s);</span><br><span class="line"> <span class="keyword">return</span> <span class="number">0</span>;</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<h1 id="The-CRTP"><a href="#The-CRTP" class="headerlink" title="The CRTP"></a>The <em>CRTP</em></h1><p>Curiously recurring template pattern (CRTP) 是指一个类的父类的模板参数含有自己。例如：</p>
<figure class="highlight cpp"><table><tr><td class="code"><pre><span class="line"><span class="keyword">template</span><<span class="keyword">typename</span> T></span><br><span class="line"><span class="keyword">class</span> <span class="title class_">Counter</span> {</span><br><span class="line"> <span class="comment">//...</span></span><br><span class="line">};</span><br><span class="line"></span><br><span class="line"><span class="keyword">class</span> <span class="title class_">Foo</span> : Counter<Foo> {</span><br><span class="line"> <span class="comment">//...</span></span><br><span class="line">};</span><br></pre></td></tr></table></figure>
<p>如此一来，父类在编译期就知道自己的派生类型，可以利用这一点做很多事，比如模板化的对象计数器、编译期多态、非退化的链式调用、模板化接口实现等等。网上很多声音说 CRTP 主要是用来消除动态绑定，但我认为 CRTP 主要是用来约束和简化代码的。我们来看看 CRTP 到底有什么用。</p>
<h2 id="Static-Polymorphism"><a href="#Static-Polymorphism" class="headerlink" title="Static Polymorphism"></a>Static Polymorphism</h2><p>由于父类知道派生类型，我们可以用 <code>static_cast<T*>(this)</code> 一步到位，将自己向下转型。还可以调用 <code>T::static_func</code> 实现静态分发。</p>
<h3 id="Demo-1"><a href="#Demo-1" class="headerlink" title="Demo"></a>Demo</h3><figure class="highlight cpp"><table><tr><td class="code"><pre><span class="line"><span class="meta">#<span class="keyword">include</span> <span class="string"><iostream></span></span></span><br><span class="line"></span><br><span class="line"><span class="keyword">template</span> <<span class="keyword">typename</span> T></span><br><span class="line"><span class="keyword">struct</span> <span class="title class_">Base</span> {</span><br><span class="line"> <span class="function"><span class="type">void</span> <span class="title">member_func</span><span class="params">()</span> </span>{ <span class="built_in">static_cast</span><T *>(<span class="keyword">this</span>)-><span class="built_in">member_func_impl</span>(); }</span><br><span class="line"> <span class="function"><span class="type">static</span> <span class="type">void</span> <span class="title">static_func</span><span class="params">()</span> </span>{ T::<span class="built_in">static_func_impl</span>(); }</span><br><span class="line">};</span><br><span class="line"></span><br><span class="line"><span class="keyword">struct</span> <span class="title class_">Obj</span> : Base<Obj> {</span><br><span class="line"> <span class="function"><span class="type">void</span> <span class="title">member_func_impl</span><span class="params">()</span> </span>{ std::cout << <span class="string">"Obj::member_func_impl\n"</span>; }</span><br><span class="line"> <span class="function"><span class="type">static</span> <span class="type">void</span> <span class="title">static_func_impl</span><span class="params">()</span> </span>{ std::cout << <span class="string">"Obj::static_func_impl\n"</span>; }</span><br><span class="line">};</span><br><span class="line"></span><br><span class="line"><span class="function"><span class="type">int</span> <span class="title">main</span><span class="params">()</span> </span>{</span><br><span class="line"> Obj a;</span><br><span class="line"> Base<Obj> &ref = a;</span><br><span class="line"> ref.<span class="built_in">member_func</span>();</span><br><span class="line"> Base<Obj>::<span class="built_in">static_func</span>();</span><br><span class="line"> <span class="keyword">return</span> <span class="number">0</span>;</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<p>会不会觉得有点脱裤子放屁？这样写有什么优势？用了 CRTP 之后，就宣告丧失了动态分发的能力，那为什么不干脆把 <code>Base</code> 类删掉呢？</p>
<p>有人说，<code>Base</code> 类描述了子类应该实现的几个函数，而且没有运行时开销。我认为最好使用 C++20 concept 来实现这种编译期约束。用 CRTP 是可以的，但是请注意标识符命名，我们来看一些 Bugs。</p>
<h3 id="Bugs"><a href="#Bugs" class="headerlink" title="Bugs"></a>Bugs</h3><figure class="highlight cpp"><table><tr><td class="code"><pre><span class="line"><span class="keyword">template</span> <<span class="keyword">typename</span> T></span><br><span class="line"><span class="keyword">struct</span> <span class="title class_">Base</span> {</span><br><span class="line"> <span class="comment">// Notice the "T::member_func()"</span></span><br><span class="line"> <span class="function"><span class="type">void</span> <span class="title">member_func</span><span class="params">()</span> </span>{ <span class="built_in">static_cast</span><T *>(<span class="keyword">this</span>)->T::<span class="built_in">member_func</span>(); }</span><br><span class="line"> <span class="function"><span class="type">static</span> <span class="type">void</span> <span class="title">static_func</span><span class="params">()</span> </span>{ T::<span class="built_in">static_func</span>(); }</span><br><span class="line">};</span><br><span class="line"></span><br><span class="line"><span class="keyword">struct</span> <span class="title class_">Obj</span> : Base<Obj> {</span><br><span class="line"> <span class="comment">// empty!</span></span><br><span class="line">};</span><br></pre></td></tr></table></figure>
<p>编译没有任何报错，而程序在运行时会因为无穷递归而陷入栈溢出。所以千万<strong>不要为了美观，让子类重复基类的函数名</strong>。</p>
<p>我们再看一个人为制造的错误：</p>
<figure class="highlight cpp"><table><tr><td class="code"><pre><span class="line"><span class="keyword">struct</span> <span class="title class_">Obj</span> : Base<Obj2> { <span class="comment">// a bug compiles</span></span><br><span class="line"> <span class="comment">// implementation which is unused</span></span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<p>编译器不会给你任何提醒，而你，<code>Obj</code>，沦为 <code>Obj2</code> 的替身。所幸的是，这个 bug 是可以修复的！只要将 <code>Base</code> 的构造函数设为私有，再加一个友元即可：</p>
<figure class="highlight cpp"><table><tr><td class="code"><pre><span class="line"><span class="keyword">template</span> <<span class="keyword">typename</span> T></span><br><span class="line"><span class="keyword">struct</span> <span class="title class_">Base</span> {</span><br><span class="line"><span class="keyword">private</span>:</span><br><span class="line"> <span class="built_in">Base</span>() = <span class="keyword">default</span>;</span><br><span class="line"> <span class="keyword">friend</span> T;</span><br><span class="line">};</span><br><span class="line"></span><br><span class="line"><span class="keyword">struct</span> <span class="title class_">Obj2</span> {};</span><br><span class="line"></span><br><span class="line"><span class="keyword">struct</span> <span class="title class_">Obj</span> : Base<Obj2> { <span class="comment">// Instantiating Obj does not compile</span></span><br><span class="line">};</span><br></pre></td></tr></table></figure>
<p>现在，<code>Obj</code> 无权访问 <code>Base</code> 的构造函数，如果企图构造 <code>Obj</code> 对象，编译器会报错，然而你还是要检查半天才能发现，原来是 CRTP 写错了！更惨的是，如果你只调用静态成员，那么编译通过！天王老子来了都不报错！慢慢找 Bug 吧你就。</p>
<h2 id="Template-Interface-Implementation"><a href="#Template-Interface-Implementation" class="headerlink" title="Template Interface Implementation"></a>Template Interface Implementation</h2><p>目前为止 CRTP 被我批评得太多了，实际上 CRTP 还是有点用的，它可以模板化接口实现，而这一点在 C++20 concept 中我暂时没发现对应的东西。</p>
<p>假设你要设计一个 <code>Scaleable</code> 接口，对于每一个具体类，它的逻辑都是：</p>
<figure class="highlight cpp"><table><tr><td class="code"><pre><span class="line"><span class="keyword">struct</span> <span class="title class_">Obj</span> : Scaleable {</span><br><span class="line"> <span class="comment">// getter, setter ...</span></span><br><span class="line"></span><br><span class="line"> <span class="function"><span class="keyword">override</span> <span class="type">void</span> <span class="title">scale</span><span class="params">(<span class="type">double</span> k)</span> </span>{</span><br><span class="line"> <span class="built_in">setValue</span>(<span class="built_in">getValue</span>() * k);</span><br><span class="line"> }</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<p>你立即想到 </p>
<ol>
<li>你不需要 <code>Scaleable</code> 的动态绑定 </li>
<li>每个类都复制粘贴一样的代码十分愚蠢 </li>
<li>C++20 concept 不能提供默认成员实现（作者以为的。如果可以请告诉我）</li>
</ol>
<p>你知道可以用 CRTP！于是你设计出：</p>
<figure class="highlight cpp"><table><tr><td class="code"><pre><span class="line"><span class="keyword">template</span> <<span class="keyword">typename</span> T></span><br><span class="line"><span class="keyword">struct</span> <span class="title class_">Scaleable</span> {</span><br><span class="line"> <span class="keyword">template</span> <<span class="keyword">typename</span> K></span><br><span class="line"> <span class="function">T &<span class="title">scale</span><span class="params">(K &&multiplicator)</span> </span>{</span><br><span class="line"> T &self = <span class="built_in">static_cast</span><T &>(*<span class="keyword">this</span>);</span><br><span class="line"> self.<span class="built_in">setValue</span>(self.<span class="built_in">getValue</span>() * multiplicator);</span><br><span class="line"> <span class="keyword">return</span> self;</span><br><span class="line"> }</span><br><span class="line">};</span><br><span class="line"></span><br><span class="line"><span class="keyword">struct</span> <span class="title class_">Obj</span> : Scaleable<Obj> {</span><br><span class="line"> <span class="type">double</span> value;</span><br><span class="line"> <span class="function"><span class="type">void</span> <span class="title">setValue</span><span class="params">(<span class="type">double</span> val)</span> </span>{ value = val; }</span><br><span class="line"> <span class="function"><span class="type">double</span> <span class="title">getValue</span><span class="params">()</span> </span>{ <span class="keyword">return</span> value; }</span><br><span class="line">};</span><br></pre></td></tr></table></figure>
<p>这段代码很漂亮，还支持非退化的链式调用。最重要的是，它在 <code>Obj</code> 定义时就表明了 Scaleable 的语义。如果使用 C++20 concept 或者其他非成员函数去实现，<code>scale</code> 的实现就可能藏在某个 header 里，你不能一眼看出 <code>Obj</code> 是否 Scaleable，语义的表达就会被割裂。</p>
<p>聪明的你又想到了接口方法还可以是类外的方法：</p>
<figure class="highlight cpp"><table><tr><td class="code"><pre><span class="line"><span class="keyword">template</span> <<span class="keyword">typename</span> T></span><br><span class="line"><span class="function">T &<span class="title">scale</span><span class="params">(Scaleable<T> &x, <span class="type">double</span> k)</span> </span>{</span><br><span class="line"> T &self = <span class="built_in">static_cast</span><T &>(x);</span><br><span class="line"> self.<span class="built_in">setValue</span>(self.<span class="built_in">getValue</span>() * k);</span><br><span class="line"> <span class="keyword">return</span> self;</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<p>是不是很像 C++20 concept？我觉得很像！</p>
<h2 id="A-CRTP-Helper"><a href="#A-CRTP-Helper" class="headerlink" title="A CRTP Helper"></a>A CRTP Helper</h2><p>每次实现 CRTP 接口的时候，有很多重复工作要做，比如千篇一律的 <code>static_cast</code> 要写，还有 private ctor + friend class 定式，太繁琐。<br>本来使用 CRTP 的初衷就是简化代码，如今岂不是南辕北辙？</p>
<p>你想到可以抽象出所有的 CRTP 接口的公共部分，做成一个基类：</p>
<figure class="highlight cpp"><table><tr><td class="code"><pre><span class="line"><span class="keyword">template</span> <<span class="keyword">typename</span> T></span><br><span class="line"><span class="keyword">struct</span> <span class="title class_">CRTP</span> {</span><br><span class="line"> <span class="function">T &<span class="title">self</span><span class="params">()</span> </span>{ <span class="keyword">return</span> <span class="built_in">static_cast</span><T &>(*<span class="keyword">this</span>); }</span><br><span class="line"> <span class="function"><span class="type">const</span> T &<span class="title">self</span><span class="params">()</span> <span class="type">const</span> </span>{ <span class="keyword">return</span> <span class="built_in">static_cast</span><<span class="type">const</span> T &>(*<span class="keyword">this</span>); }</span><br><span class="line"><span class="keyword">private</span>:</span><br><span class="line"> <span class="built_in">CRTP</span>() = <span class="keyword">default</span>;</span><br><span class="line"> <span class="keyword">friend</span> T;</span><br><span class="line">};</span><br><span class="line"></span><br><span class="line"><span class="keyword">template</span> <<span class="keyword">typename</span> T></span><br><span class="line"><span class="keyword">struct</span> <span class="title class_">Scaleable</span> : CRTP<T> { <span class="comment">/* ... */</span> };</span><br></pre></td></tr></table></figure>
<p>这看起来很美好，可是很快你又发现了问题：多个接口造成菱形继承：</p>
<figure class="highlight cpp"><table><tr><td class="code"><pre><span class="line"><span class="comment">// diamond inheritance: CRTP<T></span></span><br><span class="line"><span class="keyword">struct</span> <span class="title class_">Obj</span> : Scaleable<Obj>, Lengthable<Obj> { </span><br><span class="line"> <span class="comment">// ...</span></span><br><span class="line">};</span><br></pre></td></tr></table></figure>
<p>这里使用虚继承无助于解决问题：被虚继承的基类是不能用 <code>static_cast</code> 向下转型的。</p>
<p>试过很多方案都不能解决菱形继承问题（如果有请告诉我），只能硬避开菱形继承了：</p>
<h3 id="Demo-2"><a href="#Demo-2" class="headerlink" title="Demo"></a>Demo</h3><figure class="highlight cpp"><table><tr><td class="code"><pre><span class="line"><span class="keyword">template</span> <<span class="keyword">typename</span> T, <span class="keyword">template</span><<span class="keyword">typename</span>> <span class="keyword">class</span> <span class="title class_">Interface</span>></span><br><span class="line"><span class="keyword">struct</span> <span class="title class_">CRTP</span> {</span><br><span class="line"> <span class="function">T &<span class="title">self</span><span class="params">()</span> </span>{ <span class="keyword">return</span> <span class="built_in">static_cast</span><T &>(*<span class="keyword">this</span>); }</span><br><span class="line"> <span class="function"><span class="type">const</span> T &<span class="title">self</span><span class="params">()</span> <span class="type">const</span> </span>{ <span class="keyword">return</span> <span class="built_in">static_cast</span><<span class="type">const</span> T &>(*<span class="keyword">this</span>); }</span><br><span class="line"><span class="keyword">private</span>:</span><br><span class="line"> <span class="built_in">CRTP</span>() = <span class="keyword">default</span>;</span><br><span class="line"> <span class="keyword">friend</span> Interface<T>;</span><br><span class="line">};</span><br><span class="line"></span><br><span class="line"><span class="keyword">template</span> <<span class="keyword">typename</span> T></span><br><span class="line"><span class="keyword">struct</span> <span class="title class_">Scaleable</span> : CRTP<T, Scaleable> { <span class="comment">/* ... */</span> };</span><br><span class="line"></span><br><span class="line"></span><br><span class="line"><span class="keyword">template</span> <<span class="keyword">typename</span> T></span><br><span class="line"><span class="keyword">struct</span> <span class="title class_">Lengthable</span> : CRTP<T, Lengthable> { <span class="comment">/* ... */</span> };</span><br></pre></td></tr></table></figure>
<p>这样一来，<code>Scaleable</code> 和 <code>Lengthable</code> 的基类是两个不同的类，同时保持了零开销抽象，完美解决问题。</p>
<p>仔细观察你会发现，CRTP 避开菱形继承的手段恰好是 CRTP！</p>
<p>顺便提醒一句，调用 <code>self()</code> 的时候必须写 <code>this->self()</code>，否则编译器不知道正确的名字空间。</p>
<h2 id="Pros-amp-Cons"><a href="#Pros-amp-Cons" class="headerlink" title="Pros & Cons"></a>Pros & Cons</h2><p>你很可能已经感觉到把 CRTP 当作接口用有什么好处： </p>
<ol>
<li>接口表达了清晰、明确的语义； </li>
<li>既可以重写实现，又可以提供默认实现，十分灵活； </li>
<li>编译期内联能力，零开销抽象； </li>
<li>链式调用时类型不会退化。<br>但 CRTP 也有不足之处： </li>
<li>本质上是语言表达能力不足的妥协（还是推荐 concept）； </li>
<li>要求类继承接口，不利于库设计； </li>
<li>难以表达两种接口的交集； </li>
<li>容易写 Bug，编译报错信息不友好； </li>
<li>宣告放弃动态绑定，需要谨慎设计。</li>
</ol>
<h1 id="References"><a href="#References" class="headerlink" title="References"></a>References</h1><ul>
<li><a href="https://www.bilibili.com/video/BV1pq4y1y7oN">双笙子佯谬 - 聊一聊C++设计模式、函数式编程等</a></li>
<li><a href="https://en.cppreference.com/w/cpp/utility/variant/visit">std::visit - cppreference.com</a></li>
<li><a href="https://www.cppstories.com/2019/02/2lines3featuresoverload.html/">2 Lines Of Code and 3 C++17 Features - The overload Pattern - C++ Stories</a></li>
<li><a href="https://en.wikipedia.org/wiki/Curiously_recurring_template_pattern">Curiously recurring template pattern - Wikipedia</a></li>
<li><a href="https://www.fluentcpp.com/2017/05/16/what-the-crtp-brings-to-code/">What the Curiously Recurring Template Pattern can bring to your code - Fluent C++</a></li>
<li><a href="https://www.fluentcpp.com/2017/05/19/crtp-helper/">An Implementation Helper For The Curiously Recurring Template Pattern - Fluent C++</a></li>
<li><a href="http://baiy.cn/doc/cpp/inside_rtti.htm">白杨 - RTTI、虚函数和虚基类的实现方式、开销分析及使用指导</a></li>
</ul>
]]></content>
<tags>
<tag>C++</tag>
</tags>
</entry>
<entry>
<title>C++ 标准笔记</title>
<url>/2021/05/03/C/</url>
<content><![CDATA[<p>本文始于 2018/6/27，用于汇聚关于 C++ 的实用知识和碰到的坑，温故而知新。</p>
<blockquote>
<p>2022/1/2 【更改】标题改为《C++ 标准笔记》<br>2022/1/2 【新增】StandardLayout<br>2022/1/2 【新增】bit field</p>
<p>更新预告：</p>
<ol>
<li>CRTP 原理，目的和应用</li>
<li>CRTP 应用之表达式模板</li>
</ol>
</blockquote>
<span id="more"></span>
<h2 id="union-共用体"><a href="#union-共用体" class="headerlink" title="union 共用体"></a>union 共用体</h2><ul>
<li>union可匿名，常在结构定义中。</li>
<li>用于节省内存，尤其在嵌入式系统中。</li>
<li>union的定义形式与struct相同。</li>
</ul>
<hr>
<h2 id="enum-枚举"><a href="#enum-枚举" class="headerlink" title="enum 枚举"></a>enum 枚举</h2><ul>
<li>enum A {a,b,c}; 首项默认为0，默认后项比前项大一。<br>A被看做一种类型，甚至可以省略；a,c,b被看做常量。<br>a,b,c可以自动提升为int, 但int不能自动转换为枚举类型，除非强制转换A（1）。<br>每个enum根据其常量的最大最小值确定其上下限。</li>
<li>enum创建的常量是静态的, 可以用作静态类成员常量. 运行时所有对象不会包含枚举</li>
<li><strong>更强的安全性--类作用域内枚举(C++11)</strong><ul>
<li>enum class name : type {...};</li>
<li>enum struct name {...};</li>
<li>class或struct二选一, <code>:type</code>可选.</li>
<li>作用域内枚举不允许隐式地转换为整型</li>
<li>默认底层为int</li>
<li>调用格式为name::x</li>
</ul>
</li>
<li>:type 放在枚举名后以指定底层, 否则将随实现而异.</li>
</ul>
<hr>
<h2 id="char-字符型"><a href="#char-字符型" class="headerlink" title="char 字符型"></a>char 字符型</h2><ul>
<li>char类型被输入输出流区别对待。cout << (int*) st << endl;</li>
<li>char类型数组的初始化被C++区别对待：<figure class="highlight cpp"><table><tr><td class="code"><pre><span class="line"><span class="type">char</span> s1[<span class="number">5</span>] = <span class="string">"abcd"</span>; 合法,<span class="string">"abcd"</span>被转换为{<span class="string">'a'</span>,<span class="string">'b'</span>,<span class="string">'c'</span>,<span class="string">'d'</span>}, 可供修改</span><br><span class="line"><span class="type">char</span> s2[<span class="number">5</span>] = s1; 错误，即使s1是<span class="type">const</span>。</span><br><span class="line"><span class="type">char</span>* s3 = <span class="string">"abcd"</span>; 警告, <span class="string">"abcd"</span>是常量,不能修改. 参数<span class="type">char</span> s3[]=<span class="string">"abcd"</span>亦等价警告.</span><br></pre></td></tr></table></figure></li>
<li><code>const char []</code>所定义的字符串被保护，有内存位置隔离，不允许跟踪（但允许查看）地址。</li>
<li>char是否带符号取决于系统。</li>
<li>wchar_t可以表示系统使用的最大扩展字符集，输入输出用wcin,wcout</li>
<li>raw r"(a/b\c回车defg*)", 标识符 "( )" 可变为 "*@( )*@" 等等，保持平移对称</li>
</ul>
<h3 id="lt-cstring-gt-常用函数"><a href="#lt-cstring-gt-常用函数" class="headerlink" title="<cstring>常用函数"></a><cstring>常用函数</h3><ul>
<li><code>char* strcpy(char* dest, const char* src)</code><ul>
<li>以src覆盖dest, 直到遇到src的终止符.返回dest.</li>
</ul>
</li>
<li><code>char* strncpy(char* dest, const char* src, size_t count)</code><ul>
<li>以src的前n位覆盖dest, 若遇到src的终止符则用0填充剩余位.返回dest.</li>
</ul>
</li>
<li><code>char* strcat(char *dest, char *src)</code><ul>
<li>将src复制到dest的末尾.返回dest.</li>
</ul>
</li>
<li><code>int strcmp(char *str1, char *str2)</code><ul>
<li>按字典序比较, 返回-1,0,1.</li>
</ul>
</li>
<li><code>int stricmp(char *str1, char *str2)</code><ul>
<li>按字典序但对大小写不敏感比较.</li>
</ul>
</li>
<li><code>int strncmp(char *str1, char *str2, size_t count)</code><ul>
<li>按字典序比较前n位</li>
</ul>
</li>
<li><code>int strnicmp(char *str1, char *str2, size_t count)</code><ul>
<li>按字典序但对大小写不敏感比较前n位</li>
</ul>
</li>
</ul>
<hr>
<h2 id="vector与array-模板类"><a href="#vector与array-模板类" class="headerlink" title="vector与array 模板类"></a>vector与array 模板类</h2><ul>
<li><code>vector<typename> arr(n_elem);</code> 也可以不指定长度</li>
<li><code>array<typename, n_elem> arr;</code> 定长数组。 等长数组可以直接复制。</li>
<li>下标可越界，欲防止越界用 <code>arr.at(x)</code></li>
</ul>
<hr>
<h2 id="读入行"><a href="#读入行" class="headerlink" title="读入行"></a>读入行</h2><ul>
<li><code>cin.getline(arr, arsize)</code> 空行不设置failbit</li>
<li><code>cin.get(arr, arsize)</code> 行读取结束后不丢弃换行符（无法跨越）</li>
<li><code>cin.get()</code> 读取一个任意字符，返回char值</li>
<li><code>cin.get(char)</code>返回cin</li>
<li><code>string</code>类读行 <code>getline(cin, str);</code></li>
</ul>
<hr>
<h2 id="fstream-文件流（以fin-fout为例）"><a href="#fstream-文件流（以fin-fout为例）" class="headerlink" title="fstream 文件流（以fin, fout为例）"></a>fstream 文件流（以fin, fout为例）</h2><ul>
<li>打开文件： <code>.open("filename");</code></li>
<li>关闭文件： <code>.close();</code></li>
<li>检测最后一次读入遇到EOF： <code>fin.eof();</code></li>
<li>检测最后一次读入遇到类型不匹配（包括EOF）： <code>fin.fail();</code></li>
<li>检测最后一次读入文件损坏、故障： <code>fin.bad();</code></li>
<li>检测最后一次读入完全正常： <code>fin.good();</code> 等价于 <code>(bool) fin >> value;</code></li>
<li>清空错误标记，准许读入： <code>.clear();</code></li>
</ul>
<hr>
<h2 id="switch-结构控制"><a href="#switch-结构控制" class="headerlink" title="switch 结构控制"></a>switch 结构控制</h2><figure class="highlight cpp"><table><tr><td class="code"><pre><span class="line"><span class="keyword">switch</span>(num)</span><br><span class="line">{</span><br><span class="line"> <span class="keyword">case</span> constValue1 : statement1</span><br><span class="line"> <span class="keyword">break</span>;(optional)</span><br><span class="line"></span><br><span class="line"> <span class="keyword">case</span> constValue2 : statement2</span><br><span class="line"> <span class="keyword">break</span>;(optional)</span><br><span class="line"></span><br><span class="line"> <span class="keyword">default</span> : statement3</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<hr>
<h2 id="杂项"><a href="#杂项" class="headerlink" title="杂项"></a>杂项</h2><ul>
<li><code>exit(EXIT_FAILURE);</code> ： cstdlib, 同操作系统通信的参数值</li>
<li><code>nullptr</code> : C++11 关键字, 空指针值.</li>
</ul>
<hr>
<h2 id="指针"><a href="#指针" class="headerlink" title="指针"></a>指针</h2><ul>
<li>应当将空指针写为<code>nullptr</code>. <code>delete</code>和<code>delete []</code> 都可以作用于空指针.</li>
<li>const 指针 将受保护，非const 指针 不能复制其值（除非利用强制类型转换），不允许通过const 指针 修改所指向值。<br>为了防止欺骗const检查，不允许令const 二级指针指向非const 一级指针</li>
</ul>
<hr>
<h2 id="函数指针"><a href="#函数指针" class="headerlink" title="函数指针"></a>函数指针</h2><ul>
<li>声明：<code>typename (*pointer name)(parameter list...);</code></li>
<li>获得某函数的地址：<code>pointer = function name;</code></li>
<li>使用函数指针：<code>(*pointer)(parameter list...);</code> 或 <code>pointer(parameter list...);</code></li>
<li>举例： <figure class="highlight cpp"><table><tr><td class="code"><pre><span class="line">原函数：</span><br><span class="line"></span><br><span class="line"><span class="function"><span class="type">const</span> <span class="type">double</span>* <span class="title">f1</span><span class="params">(<span class="type">const</span> <span class="type">double</span> [], <span class="type">int</span>)</span></span>;</span><br><span class="line"></span><br><span class="line">指向f1的指针：</span><br><span class="line"></span><br><span class="line"><span class="type">const</span> <span class="type">double</span>* (*p1)(<span class="type">const</span> <span class="type">double</span> [], <span class="type">int</span>) = f1; 或 <span class="keyword">auto</span> p1 = f1;</span><br><span class="line"></span><br><span class="line">由f1,f2,f3组成数组：<span class="comment">//[]优先级高于*</span></span><br><span class="line"></span><br><span class="line"><span class="type">const</span> <span class="type">double</span>* (*ar[<span class="number">3</span>])(<span class="type">const</span> <span class="type">double</span> [], <span class="type">int</span>) = {f1, f2, f3}; 不允许<span class="keyword">auto</span></span><br><span class="line"></span><br><span class="line">指向ar[<span class="number">3</span>]的指针：</span><br><span class="line"></span><br><span class="line"><span class="type">const</span> <span class="type">double</span>* (*(*arp)[<span class="number">3</span>])(<span class="type">const</span> <span class="type">double</span>[], <span class="type">int</span>) = &ar; 或 <span class="keyword">auto</span> arp = &ar；</span><br><span class="line"></span><br><span class="line">还可以用<span class="keyword">typedef</span>简化声明：</span><br><span class="line"></span><br><span class="line"><span class="keyword">typedef</span> <span class="type">const</span> <span class="type">double</span>* (*p_fun)<span class="built_in">const</span>(<span class="type">const</span> <span class="type">double</span> [], <span class="type">int</span>);</span><br><span class="line"></span><br><span class="line">p_fun ar[<span class="number">3</span>] = {f1, f2, f3};</span><br></pre></td></tr></table></figure></li>
<li>分析方法: 从变量名开始, 往右往左结合, 逐步解释结合体.<ul>
<li>当遇到一个指针, 总是关心它指向什么类型</li>
<li>当遇到一个数组, 总是关心它的元素的类型</li>
<li>当遇到一个函数, 总是关心它的返回值类型</li>
</ul>
</li>
</ul>
<hr>
<h2 id="参数传递"><a href="#参数传递" class="headerlink" title="参数传递"></a>参数传递</h2><ul>
<li>参量（argument）是实参，参数（parameter）是形参。</li>
</ul>
<hr>
<h2 id="函数重载（函数多态）"><a href="#函数重载（函数多态）" class="headerlink" title="函数重载（函数多态）"></a>函数重载（函数多态）</h2><ul>
<li>同名函数共存要求：函数特征标（函数参数列表）不同。</li>
<li>当需要自动类型转换但选项不唯一时，编译不通过。</li>
<li>不区分const和非const变量（包括指针）。</li>
<li>区分指向const的指针和指向非const的指针。</li>
<li>区分指向const的引用和指向非const的引用。（编译器调用最匹配的版本）</li>
<li>如果没有右值引用参数版本，右值参量将赋给const引用参量。</li>
</ul>
<hr>
<h2 id="引用"><a href="#引用" class="headerlink" title="引用"></a>引用</h2><ul>
<li>左值引用<ul>
<li>非const引用可以引自可修改的左值（防止修改const值和修改临时变量）</li>
<li>const引用可以引自任何左值、右值（毕竟数值不会变）</li>
<li>基类引用可以指向派生类对象</li>
</ul>
</li>
<li>指针也能被引用, 例如 int*& x = ptr;</li>
</ul>
<hr>
<h2 id="短路运算符"><a href="#短路运算符" class="headerlink" title="短路运算符"></a>短路运算符</h2><ul>
<li>||, &&具有短路作用,结果必然时短路,不计算右边的表达式</li>
</ul>
<hr>
<h2 id="inline"><a href="#inline" class="headerlink" title="inline"></a>inline</h2><ul>
<li>在声明时用或在第一次调用前的定义用都行</li>
<li>不能处理递归, 编译器有权拒绝采用.</li>
<li>函数默认值必须放在原型声明中, 不能放在和声明分离的定义中.只能从右到左地提供默认值</li>
</ul>
<hr>
<h2 id="decltype-expression"><a href="#decltype-expression" class="headerlink" title="decltype(expression)"></a>decltype(expression)</h2><ul>
<li><p><strong>decltype(expression)核对表</strong></p>
<ol>
<li>如果expression是一个不带括号的标识符, 则返回相同类型, 包括const,*等限定符</li>
<li>否则,如果expression是一个函数调用(需要提供参数),则返回相同类型</li>
<li>否则,如果expression是一个带括号的左值,则返回其引用</li>
<li>否则,返回expression的类型(例:右值)</li>
</ol>
</li>
<li><p>可以利用<strong>typedef</strong> decltype(expression) somename;简化声明</p>
</li>
<li><p><strong>decltype 用于函数返回类型声明</strong></p>
<ul>
<li>声明返回类型时未知参数列表, 所以需要后置声明 <figure class="highlight cpp"><table><tr><td class="code"><pre><span class="line"><span class="keyword">template</span> <<span class="keyword">typename</span> T1, <span class="keyword">class</span> <span class="title class_">T2</span>></span><br><span class="line"><span class="function"><span class="keyword">auto</span> <span class="title">gt</span><span class="params">(T1 x, T2 y)</span> -> <span class="title">decltype</span><span class="params">(x + y)</span> <span class="comment">// "-> 已知类型" 也行</span></span></span><br></pre></td></tr></table></figure></li>
</ul>
</li>
</ul>
<hr>
<h2 id="模板"><a href="#模板" class="headerlink" title="模板"></a>模板</h2><ul>
<li><p><strong>创建模板</strong></p>
<figure class="highlight cpp"><table><tr><td class="code"><pre><span class="line"><span class="keyword">template</span> <<span class="keyword">typename</span> T> <span class="comment">//也可以用class代替typename</span></span><br><span class="line"><span class="function"><span class="type">void</span> <span class="title">swap</span><span class="params">(T&, T&)</span></span>;</span><br></pre></td></tr></table></figure>
</li>
<li><p><strong>显式具体化</strong></p>
<figure class="highlight cpp"><table><tr><td class="code"><pre><span class="line"><span class="keyword">template</span> <> <span class="function"><span class="type">void</span> <span class="title">swap</span><span class="params">(<span class="type">int</span>&, <span class="type">int</span>&)</span></span>;</span><br><span class="line"><span class="keyword">template</span> <> <span class="type">void</span> <span class="built_in">swap</span><<span class="type">int</span>>(<span class="type">int</span>&, <span class="type">int</span>&);</span><br></pre></td></tr></table></figure>
<p> <em>为什么需要显示具体化?常规函数不能替代吗?</em></p>
</li>
<li><p><strong>显式实例化</strong></p>
<ul>
<li>不使用函数就能依照模板生成实例, 常用在库中</li>
<li><code>template void swap<int>(int&, int&);</code></li>
<li>使用函数时<code>add<double>((int)a, (double)b);</code> 会强制使用<double>模板.注意引用类型不能隐式类型转换.后续的强制使用仍需加<double></li>
</ul>
</li>
</ul>
<hr>
<h2 id="变量名"><a href="#变量名" class="headerlink" title="::变量名"></a>::变量名</h2><ul>
<li>表明采用全局变量而非局部变量</li>
</ul>
<hr>
<h2 id="储存空间"><a href="#储存空间" class="headerlink" title="储存空间"></a>储存空间</h2><ol>
<li>静态储存: 保证初始化为0, 生命周期和程序相同.</li>
<li>自动储存(栈内存): 不保证初始化;</li>
<li>动态储存(堆内存): 不保证初始化;</li>
</ol>
<hr>
<h2 id="储存说明符"><a href="#储存说明符" class="headerlink" title="储存说明符"></a>储存说明符</h2><ul>
<li>static用于变量 <em>编译阶段尽量初始化, 运行时直接分配空间,程序结束时销毁, 首次遇到时再保证初始化</em><ul>
<li>静态外部链接性变量: 直接在非被包括区域定义, 在其他单元中可用extern引用声明(不允许再次初始化)(ODR单定义规则)</li>
<li>静态内部链接性变量: 在非被包括区域加static, 限定在本单元内可以访问</li>
<li>静态无链接性变量:在被包括区域加static, 限定在本包括区域内可以访问</li>
</ul>
</li>
<li>static用于函数 覆盖函数的默认外部链接性为内部链接性,必须同时用于原型和定义中.</li>
<li>thread_local 变量持续性与所属线程的持续性相同, 可与static, extern结合使用(其他声明中不能使用多个储存说明符)</li>
<li>mutable 使得const对象中的属性能被修改,而不受const限制</li>
<li>register C++11之前是寄存器变量,不能取地址; 之后是显式指明自动变量(无实际作用,避免旧代码非法)</li>
</ul>
<hr>
<h2 id="显式指出语言链接性以帮助链接程序寻找匹配函数"><a href="#显式指出语言链接性以帮助链接程序寻找匹配函数" class="headerlink" title="显式指出语言链接性以帮助链接程序寻找匹配函数:"></a>显式指出语言链接性以帮助链接程序寻找匹配函数:</h2><pre><code>extern "C" void spiff(int);
</code></pre>
<hr>
<h2 id="cv-限定符"><a href="#cv-限定符" class="headerlink" title="cv-限定符"></a>cv-限定符</h2><pre><code>- const全局变量带有隐式static使得变量链接性为内部, 导致多文件同时include一个头文件时不会发生定义冲突 *可以使用extern覆盖隐式static使变量链接性为外部. 在其他单元中仍需用extern来引用它.*
- volatile 提示编译器该变量会在程序之外被修改, 不要进行寄存器缓存优化. 多见于驱动程序.
</code></pre>
<hr>
<h2 id="new运算符"><a href="#new运算符" class="headerlink" title="new运算符"></a>new运算符</h2><ul>
<li>原函数: <figure class="highlight cpp"><table><tr><td class="code"><pre><span class="line"><span class="function"><span class="type">void</span> * <span class="keyword">operator</span> <span class="title">new</span><span class="params">(std::<span class="type">size_t</span>)</span></span>;</span><br><span class="line"><span class="type">void</span> * <span class="keyword">operator</span> <span class="keyword">new</span>[] (std::size_T); <span class="comment">// new int[40] --> new(40*sizeof(int))</span></span><br><span class="line"><span class="function"><span class="type">void</span> <span class="keyword">operator</span> <span class="title">delete</span><span class="params">(<span class="type">void</span> *)</span></span>;</span><br><span class="line"><span class="type">void</span> <span class="keyword">operator</span> <span class="keyword">delete</span>[] (<span class="type">void</span> *);</span><br></pre></td></tr></table></figure></li>
<li>new, delete函数可以替换.</li>
<li>new在堆中查找满足要求的内存块</li>
<li>定位new运算符<ul>
<li><code>typename* p = new (MemAddress) typename</code></li>
<li>返回(void *)指定的内存地址</li>
<li>允许重载</li>
<li><strong>注意非堆内存不可delete</strong></li>
<li><strong>若内存中存放了对象，则需要手动调用析构函数，再由MemAddress释放内存，且需注意new []与delete []对应</strong></li>
</ul>
</li>
<li>元素的创建与销毁应遵循FILO顺序</li>
</ul>
<hr>
<h2 id="名称空间"><a href="#名称空间" class="headerlink" title="名称空间"></a>名称空间</h2><ul>
<li>声明区域: 可以声明变量的区域, 例如所在的文件, 代码块.</li>
<li>潜在作用域: 从声明点开始到声明区域的结尾. 可能被局部变量隐藏,故称潜在.</li>
<li>自定义名称空间:<code>namespace Somewhat {...}</code><ul>
<li>只能在函数外部定义, 允许在另一个名称空间中嵌套</li>
<li>因此, 默认情况下声明的名称的链接性为外部(除非它引用了常量)</li>
<li>可以在其他合法位置继续添加名称, 提供定义.</li>
</ul>
</li>
<li><code>using namespace Somewhat;</code>编译指令<ul>
<li>每个声明区域中都有一条隐式的<code>using namespace 全局名称空间;</code></li>
<li>若某处使用过<code>using namespace</code>编译指令,则其子声明区域也隐式添加这条语句.</li>
<li>局部变量拥有最高优先权,能隐藏各种using namespace同名变量(因为名称空间都在函数外部定义)</li>
</ul>
</li>
<li><code>using</code>声明<ul>
<li>类似于声明了一个局部变量, 在代码块中不允许同级同名.</li>
<li>因此使用using声明比使用using编译指令更安全.</li>
</ul>
</li>
<li>名称空间可以拥有别名, 用于简化代码: namespace MEF = myth::elements::fire;</li>
<li>名称空间可以匿名, 声明之后自动隐式using, 用于避免其他using并替代static全局变量.</li>
<li><.h>文件是不支持名称空间的版本.新版一般将函数搬到std中.</li>
<li>使用建议: 在大型程序/多单元程序使用<ul>
<li>少用<code>using namespace</code></li>
<li>避免在头文件中使用using编译指令,若必须使用,则在所有#include之后使用</li>
<li>避免直接声明外部全局变量和静态全局变量,改用已命名的名称空间</li>
<li>using声明首选用在局部而不是全局</li>
</ul>
</li>
</ul>
<hr>
<h2 id="类"><a href="#类" class="headerlink" title="类"></a>类</h2><ul>
<li>构造函数<ul>
<li>调用示例: <figure class="highlight cpp"><table><tr><td class="code"><pre><span class="line">Classname object; <span class="comment">// 调用空参数构造函数</span></span><br><span class="line"><span class="function">Classname <span class="title">object</span><span class="params">()</span></span>; <span class="comment">// 警告, 正在声明函数</span></span><br><span class="line"><span class="function">Classname <span class="title">object</span><span class="params">(one, two, ...)</span></span>; <span class="comment">// 调用对应的构造函数</span></span><br><span class="line">Classname object = <span class="built_in">Classname</span>(one, ...); <span class="comment">// 有可能先构造临时对象</span></span><br><span class="line">Classname* p = <span class="keyword">new</span> <span class="built_in">Classname</span>(...);</span><br><span class="line">Classname object{...}; Classname object = Classname{...}; <span class="comment">// C++11</span></span><br><span class="line">Classname* p = <span class="keyword">new</span> Classname{}; <span class="comment">// C++11</span></span><br><span class="line">Classname object = value; <span class="comment">// 调用一个参数的构造函数, 可用explicit(修饰构造函数)关闭这种特性</span></span><br></pre></td></tr></table></figure></li>
<li>重载构造函数中使用的new或new []必须对应，因为析构函数只有一个。</li>
<li>初始化列表<ul>
<li>参数列表后由冒号引出的初始化信息。在此初始化对象将使用复制构造函数，而不是空构造函数加赋值运算符，因此效率更高。</li>
<li>初始化的顺序按照成员变量的声明顺序，而与初始化列表顺序无关。</li>
<li>一旦进入花括号，成员变量将完成默认的初始化，对象初步创建完成。因此，成员变量中，非静态常量与引用必须由此列表进行初始化。</li>
<li>类内初始化可等价于默认的列表初始化。列表初始化会覆盖类内初始化。</li>
<li>初始化列表负责调用基类构造函数（基类已在派生类类域中，无需加作用域解析符）</li>
<li>初始化列表负责调用成员对象的非默认构造函数</li>
</ul>
</li>
<li>默认不能继承构造函数（C++11）</li>
</ul>
</li>
<li>析构函数<ul>
<li>只能有一个析构函数, 且参数必须为空</li>
<li>注意用delete对应构造函数或其他过程的new</li>
<li>若对象由定位new运算符创建，则需要手动调用析构函数，且遵循FILO顺序。</li>
<li>必须以delete或delete []对应构造函数中的new或new []。</li>
<li>最后会自动调用基类的析构函数</li>
<li>应当将析构函数声明为虚函数</li>
<li>不能继承析构函数</li>
</ul>
</li>
<li>封装是一个编译期的概念<ul>
<li>编译期不存在实例，编译器仅针对类型做检查</li>
<li>可以在类方法中访问同类对象的私有成员</li>
</ul>
</li>
<li><em>同struct</em> : 避免环形构造<ul>
<li>编译器禁用简单环形定义, 如 <code> struct A { A a; }; // 使用了未完成的定义</code></li>
<li>编译器不能辨别复杂环形定义, 如<code>struct A { A(){} A* a = new A(); }</code></li>
</ul>
</li>
<li>非静态变量在运行时才会创建, 所以如int arr[MAXN]的MAXN必须是静态量,可以是全局const, 类中static const, enum{MAXN=x}.</li>
<li>友元函数<ul>
<li>在共有部分声明友元函数原型, 也可以紧接定义以设为内联函数.后置的定义无需friend修饰</li>
<li>友元函数视为类外函数, 但可以访问类私有成员变量.</li>
<li>类的显式二元重载运算符应当用友元, 尽管没有直接修改类私有成员变量</li>
<li>重载<< : <code>std::ostream& operator << (const std::ostream& os, const Classname& obj)</code>以方便输出.</li>
<li>友元函数可方便隐式类型转换, 不必苛求由对象发起函数调用.例如cmp("asd", obj),可以对应原型cmp(string, string);</li>
<li>可以在派生类友元函数中，强制向上转型并使用基类友元函<br> 数据类型, 否则易有二义性<br> 函数, 避免隐式转换出错<br> ass_name&)`<br> 始化、按值传递、按值返回、上转型并使用基类友元函数。</li>
</ul>
</li>
<li>转换函数<code>operator typeName()</code><ul>
<li>用途:将类转换成基础数据类型</li>
<li>必须是成员方法</li>
<li>不能指定返回类型</li>
<li>不能有参数</li>
<li>用于cout时应显式标明类型</li>
<li>应当用explicit修饰</li>
</ul>
</li>
<li>复制构造函数<code>Class_name(const Class_name&)</code><ul>
<li>在初始化对象时使用(显示的初始化、按值传递、按值返回、编译器生成临时对象)</li>
<li><strong>新版本C++可用移动构造函数</strong></li>
<li>例: <figure class="highlight cpp"><table><tr><td class="code"><pre><span class="line"><span class="function">Class_name <span class="title">a</span><span class="params">(b)</span></span>;</span><br><span class="line">Class_name* pa = <span class="keyword">new</span> <span class="built_in">Class_name</span>(b);</span><br><span class="line">Class_name a = b; <span class="comment">// 可能生成临时对象后调用赋值运算符函数,根据实现而异.</span></span><br><span class="line">Class_name a = <span class="built_in">Class_name</span>(b); <span class="comment">// 同上</span></span><br><span class="line">按值传递函数调用<span class="built_in">func</span>(b); <span class="comment">// 按值传递也初始化了参数</span></span><br></pre></td></tr></table></figure></li>
<li>默认的复制构造函数，是在初始化列表中调用所有成员的复制构造函数。默认复制派生类对象的基类部分</li>
<li>应注意<strong>深度复制</strong>, 即妥善处理指针所指向内容的复制</li>
</ul>
</li>
<li>赋值运算符函数<ul>
<li>考虑妥善处理原来已有的,即将被抛弃的数据.</li>
<li>考虑自己赋值给自己的情况, 小心赋值前删除了自身数据.</li>
<li>记住返回自身引用, 即 <code>return *this</code></li>
<li>不能继承赋值运算符函数</li>
<li>不建议设置为虚方法，为了避免同基类的不同派生类互相赋值</li>
</ul>
</li>
<li>默认方法的潜在危险<ul>
<li>某个类在开发初期不需要复制构造函数/赋值运算符重载，但在以后可能需要</li>
<li>可能写出符合常理的代码，但由于未覆盖默认方法而运行时异常</li>
<li>可能在未察觉的情况下调用了默认方法（用重载加法时按值传递、按值返回将调用复制构造函数）</li>
<li>解决办法：<ul>
<li>不管默认方法是否需要，总是提供正确的代码</li>
<li>将空方法设为private，并留下错误信息</li>
<li>使用delete（C++11）</li>
</ul>
</li>
</ul>
</li>
<li><code>[ ]</code>(取位)运算符: 两个操作数分立于左中括号两侧。只能是成员函数。</li>
<li>const对象<ul>
<li>只能使用const函数, 若返回引用, 则返回类型为const type&</li>
</ul>
</li>
<li>静态类成员函数<ul>
<li>原型含static, 定义不含</li>
<li>和Java不同, 不能通过对象来调用静态成员函数</li>
<li>调用格式 <code>Class_name::s_method();</code></li>
</ul>
</li>
<li>静态类成员变量<ul>
<li>在类中声明, 不可定义</li>
<li>在类外定义并分配内存, 可以不初始化</li>
<li>静态常量在声明同时定义.</li>
</ul>
</li>
<li>派生/继承<ul>
<li><code>class newClass : public baseClase</code></li>
<li>默认继承为private继承。私有继承的向上转型必须显式写出</li>
<li>多重继承的向上转型也应显示写出，以防不同基类的方法冲突</li>
<li>派生类中一旦定义某方法与基类方法同名，则基类所有该名方法被隐藏。与参数列表（特征标）无关。<ul>
<li>可用基类名::方法名的途径调用隐藏的方法</li>
<li>重新定义继承的方法，应确保与原来的原型完全相同</li>
<li>若返回类型为基类引用或指针，则可以修改为派生类的引用或指针（返回类型协变）</li>
<li>若基类虚方法被重载了，则应在派生类中重新定义所有的基类版本</li>
</ul>
</li>
<li>若派生类构造函数使用了new，则应提供复制构造函数/赋值运算符（含<code>base::operator=(o);</code>）/虚析构函数的定义</li>
<li>公有继承表达了is-a关系，私有继承/包含表达了has-a关系。通常使用组合包含层次化来建立has-a关系。如果需要访问原有类的保护成员或重新定义虚方法，才使用私有继承。</li>
<li>欲调用基类对象，对(*this)强制向上转型(const Base &)即可</li>
<li>用using改变继承成员权限：在派生类的public处<code>using Base::methodName;</code>(省略using的技术是即将被抛弃的旧技术)</li>
</ul>
</li>
<li>虚方法<ul>
<li>若希望通过基类引用或指针调用派生类方法，则需要将基类方法声明为virtual虚方法（一般也将派生类方法声明为virtual）</li>
<li>应当将析构函数定义为虚方法，以确保正确地销毁对象</li>
<li>在类中欲调用基类方法(而不是本类方法)，需使用作用域解析符<code>Base::baseMethod();</code></li>
<li>编译器对虚方法的一般实现：把类的所有虚函数的地址制作成表，在对象中添加隐藏的虚函数表指针，在运行时通过指针检索虚函数。</li>
<li>与Java不同，派生类虚方法的访问权限允许变严格，但由基类引用或指针的多态效果仍然生效。</li>
<li>派生类中一旦定义虚方法，就将隐藏基类所有同名方法，故应在派生类中重新定义所有的基类重载方法</li>
<li>可以令虚方法声明结尾处为<code>=0</code>，使方法成为纯虚方法。纯虚方法可以不提供定义。含有至少一个纯虚方法的类为抽象类，不能声明对象。</li>
</ul>
</li>
<li>访问权限<ul>
<li>private：仅本类和友元函数有权访问。对数据成员的理想访问控制。</li>
<li>protected：本类和派生类有权访问。在派生链中，此权限类似public；在类外部，此权限类似peivate。此权限通常只给成员函数。</li>
<li>public：在同一域中就能访问。</li>
</ul>
</li>
<li>引用限定成员函数<ul>
<li>函数签名的最后可以标注<code>&</code>或<code>&&</code>来做「引用限定 ref-qualified」。<ol>
<li>非限定成员函数可由左值调用，也可由右值调用。此时不能再定义引用限定成员函数。</li>
<li>左值限定成员函数仅可由左值调用。</li>
<li>右值限定成员函数仅可由右值调用。</li>
</ol>
</li>
</ul>
</li>
</ul>
<hr>
<h2 id="Lambda"><a href="#Lambda" class="headerlink" title="Lambda"></a>Lambda</h2><p>Lambda是一种匿名函数，它在普通函数的基础上增加了一些功能。Lambda是通过函数对象实现的，具有在编译器内联的能力，因此性能可以比函数指针更好。</p>
<h3 id="使用举例"><a href="#使用举例" class="headerlink" title="使用举例"></a>使用举例</h3><figure class="highlight cpp"><table><tr><td class="code"><pre><span class="line">[](<span class="type">int</span> q, <span class="type">int</span> p) { <span class="keyword">return</span> q > p; } <span class="comment">// 常见的比较函数</span></span><br><span class="line">[mask](<span class="type">int</span> x) { <span class="keyword">return</span> mask & x; } <span class="comment">// 判断有无交集</span></span><br><span class="line">[&flag](<span class="type">int</span> x) { flag[x] = <span class="literal">true</span>; } <span class="comment">// 修改自动变量flag数组</span></span><br><span class="line">[&](<span class="type">int</span> x)-><span class="type">int</span> { <span class="comment">// 按引用捕获当前作用域的全部自动变量。不是简单地return故要声明类型</span></span><br><span class="line"> <span class="keyword">if</span> (x > mx) mx = x;</span><br><span class="line"> <span class="keyword">return</span> tot += x;</span><br><span class="line">}</span><br><span class="line">[k, &](<span class="type">int</span> del) { <span class="comment">// 按值捕获k、按引用捕获其他变量</span></span><br><span class="line"> sth += del;</span><br><span class="line"> cout << k << endl; <span class="comment">// k是只读的？？！！</span></span><br><span class="line">}</span><br><span class="line"></span><br></pre></td></tr></table></figure>
<hr>
<h2 id="右值"><a href="#右值" class="headerlink" title="右值"></a>右值</h2><p>rvalue,（非严格定义: ）匿名的临时值，常出现在等号的右方。最大特征是不可取地址，例如</p>
<figure class="highlight cpp"><table><tr><td class="code"><pre><span class="line"><span class="type">int</span> a;</span><br><span class="line">a = <span class="number">2</span> + <span class="number">3</span>; <span class="comment">// (2+3) 就是一个右值， &(2+3)没有意义</span></span><br></pre></td></tr></table></figure>
<h3 id="右值引用"><a href="#右值引用" class="headerlink" title="右值引用"></a>右值引用</h3><p>有时候运算过程中会生成一些体积大的临时对象，在完成表达式后这些临时对象会析构，例如</p>
<figure class="highlight cpp"><table><tr><td class="code"><pre><span class="line"><span class="keyword">struct</span> <span class="title class_">MyString</span> {</span><br><span class="line"> <span class="type">char</span>* data = <span class="literal">nullptr</span>;</span><br><span class="line"> <span class="comment">//... 使用new和delete的模仿string类，具有拷贝构造函数</span></span><br><span class="line">};</span><br><span class="line"></span><br><span class="line">vector<MyString> arr; arr.<span class="built_in">reverse</span>(<span class="number">10</span>); <span class="comment">// 保留10个string的空间</span></span><br><span class="line"><span class="keyword">for</span> (<span class="type">int</span> i=<span class="number">0</span>; i<<span class="number">10</span>; ++i) {</span><br><span class="line"> arr.<span class="built_in">push_back</span>(<span class="built_in">MyString</span>(<span class="string">"Hello_rvalue"</span>));</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<p>第8行发生如下事件：</p>
<ol>
<li>MyString("Hello_rvalue") 产生一个右值。</li>
<li>push_back接收一个右值参数，并调用MyString的<strong>拷贝构造函数</strong>新建一个对象。</li>
<li>MyString("Hello_rvalue")生命周期结束，析构。</li>
</ol>
<p>发现拷贝构造函数存在资源浪费：既然右值的资源已经没有后续价值，大可以将其资源“偷”过来使用。</p>
<p>于是，MyString类新增<strong>移动构造函数</strong>（还有<strong>移动赋值函数</strong>）</p>
<figure class="highlight cpp"><table><tr><td class="code"><pre><span class="line"><span class="keyword">struct</span> <span class="title class_">MyString</span> {</span><br><span class="line"> <span class="comment">// ... 同上</span></span><br><span class="line"> <span class="built_in">MyString</span>(MyString&& x) {</span><br><span class="line"> <span class="keyword">this</span>->data = x->data;</span><br><span class="line"> x->data = <span class="literal">nullptr</span>; <span class="comment">// 资源被偷走</span></span><br><span class="line"> <span class="comment">// ...</span></span><br><span class="line"> }</span><br><span class="line"> </span><br><span class="line"> MyString& <span class="keyword">operator</span>=(MyString&& x) {</span><br><span class="line"> <span class="comment">// 类似移动构造函数，不赘述</span></span><br><span class="line"> }</span><br><span class="line">};</span><br></pre></td></tr></table></figure>
<p>如此一来，push_back将自动调用移动构造函数来创建对象，避免大量new内存。</p>
<p>右值引用的存在，就是为了<strong>尽量榨干临时对象的价值</strong>。</p>
<p>要利用右值引用，最重要的是<strong>合理地定义移动构造和移动赋值</strong>。</p>
<h3 id="移动语义"><a href="#移动语义" class="headerlink" title="移动语义"></a>移动语义</h3><p>有时候一些左值就像右值一样即将消亡，弃之可惜，例如</p>
<figure class="highlight cpp"><table><tr><td class="code"><pre><span class="line"><span class="keyword">for</span> (<span class="type">int</span> i=<span class="number">0</span>; i<<span class="number">10</span>; ++i) {</span><br><span class="line"> <span class="function">string <span class="title">tmp</span><span class="params">(<span class="string">"hello"</span>)</span></span>;</span><br><span class="line"> tmp += <span class="string">" rvalue?"</span>; <span class="comment">// tmp经过一系列复杂的处理</span></span><br><span class="line"> arr.<span class="built_in">push_back</span>(tmp);</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<p>如果能像右值引用一般把tmp的资源“偷”走就好了，于是</p>
<figure class="highlight cpp"><table><tr><td class="code"><pre><span class="line"><span class="keyword">for</span> (<span class="type">int</span> i=<span class="number">0</span>; i<<span class="number">10</span>; ++i) {</span><br><span class="line"> <span class="function">string <span class="title">tmp</span><span class="params">(<span class="string">"hello"</span>)</span></span>;</span><br><span class="line"> tmp += <span class="string">" rvalue?"</span>; <span class="comment">// tmp经过一系列复杂的处理</span></span><br><span class="line"> arr.<span class="built_in">push_back</span>(std::<span class="built_in">move</span>(tmp)); <span class="comment">// 强制转换为右值</span></span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<p>注意，move(tmp)后由于资源已经被偷走，tmp可能像野指针一样危险。被move后对象的行为由程序员负责。</p>
<h3 id="完美转发"><a href="#完美转发" class="headerlink" title="完美转发"></a>完美转发</h3><p>完美转发是针对c++模板函数的概念。</p>
<p>我们不得不先介绍c++引用折叠概念：</p>
<table>
<thead>
<tr>
<th>typename T</th>
<th>T&</th>
<th>T&&</th>
</tr>
</thead>
<tbody><tr>
<td>G</td>
<td>G&</td>
<td>G&&</td>
</tr>
<tr>
<td>G&</td>
<td>G&</td>
<td><strong>G&</strong></td>
</tr>
<tr>
<td>G&&</td>