356.md

Info

• Did you know that C++20's no_unique_address can be used to find unique types?

  • https://wg21.link/P0840

Example

template<class> struct box{};

struct unique {
    [[no_unique_address]] box<int> _1;
    [[no_unique_address]] box<bool> _2;
};
static_assert(sizeof(unique)==1);

struct no_unique {
    [[no_unique_address]] box<int> _1;
    [[no_unique_address]] box<int> _2;
};

static_assert(sizeof(no_unique)!=1);

https://godbolt.org/z/Mdsa16rWT

Puzzle

• Can you implement generic version of is_unique leveraging described concept?

template <class... Ts>
constexpr auto is_unique = false; // TODO

static_assert(is_unique<int, bool, char, double, float>);
static_assert(not is_unique<int, bool, char, double, float, int>);

https://godbolt.org/z/Exjdf4sbx

Solutions

template <class T> struct box {};

template <class... Ts> struct unique;

template <>
struct unique<> {};

template <class T, class ...Args>
struct unique<T, Args ...> : public unique<Args ...> {
    constexpr explicit unique(T&& t, Args&&... args) : unique<Args...>(args ...) {

    }
    [[no_unique_address]] box<T> t_;
};

template <class... Ts>
constexpr auto is_unique = (sizeof(unique<Ts...>) == 1);

https://godbolt.org/z/nnPz6djMW

template <class>
struct box {
    box() = delete;
};
template <class... Ts>
struct S;
template <>
struct S<> {};
template <class T, class... Ts>
struct S<T, Ts...> : S<Ts...> {
    [[no_unique_address]] box<T> _;
};

template <class... Ts>
constexpr auto is_unique = sizeof(S<Ts...>) == 1;

https://godbolt.org/z/rM5q93WEh