Error on assigment to std::string lvalue reference

[VaderDev]

sol::state lua;
lua.script("var = \"hi\"");

std::string var0;
var0 = lua["var"]; // error: call of overloaded 'assign(sol::proxy<sol::basic_table_core<true, sol::reference>&, const char (&)[4]>&)' is ambiguous
std::string var1 = lua["var"]; // good

[VaderDev]

Ohh, Lovely... It is using gcc 7.1.0 with C++17 enabled.

The problem seams to be that the standard implementation has some issues with string_view and basic_string overloads. I wonder if a workaround is even possible in usercode (without to much pain).

[VaderDev]

Duplicate of #414

[VaderDev]

It turned out the usercode workaround is viable:
An additional sfiane for disabling std::string_view conversion in proxy_base if C++17 is present.
I don't have the time for a proper pull request nor proper testing, so be careful.

For reading from lua I dont think the general use case would involve std::string_view due to lifetime issues.

Big thanks for the folks at nlohmann/json/issues/464 for the macro

template<typename T,
	meta::enable<meta::neg<meta::is_string_constructible<T>>, meta::neg<is_proxy_primitive<meta::unqualified_t<T>>>> = meta::enabler
#if (defined(__cplusplus) && __cplusplus == 201703L) || (defined(_MSC_VER) && _MSC_VER >1900 && defined(_HAS_CXX17) && _HAS_CXX17 == 1)
	, typename = std::enable_if_t<!std::is_same_v<T, typename std::string_view>>
#endif
>
operator T& () const {
	const Super& super = *static_cast<const Super*>(static_cast<const void*>(this));
	return super.template get<T&>();
}

[ThePhD]

I forgot about this.

But yes, I need to add that is_string_view thing to the meta::is_string_constructible trait when I detect someone has C++17 or greater. I was hesitant to do so because VC++ still doesn't have header-inclusion-test-macros nor feature-test macros, so it's janky to do the test (as you can see from Modern JSON's big ol' macro work around).

I'll integrate this into the code-base and it should make things clean everywhere. Sorry about the problems with C++17-and-beyond stuff: it's still a WIP!

[VaderDev]

I don't think you have to apologize. It was a silent backward compatibility breaking change and I am still not convinced that the string_view works as intended in this corner case.

Thank you for your hard work.

[ThePhD]

So here's a juicy side note: nlohmann's code is correct, insofar as you actually include a std library header. If you don't, _HAS_CXX_17 doesn't exist, and code that attempts to inspect it without including an MSVC header will barf.

[ThePhD]

Check the latest, it should be a little bit more okay now.

[VaderDev]

With 48fc90b

#include <iostream>
#include <sol/state.hpp>

int main(int, char**) {
	sol::state lua;

	lua.script("var = \"world\"");

	std::string x;
	x = lua["var"];

	std::cout << x << std::endl;
}

Results in:

In file included from project/ext/sol/include/sol/usertype.hpp:26:0,
                 from project/ext/sol/include/sol/table_core.hpp:28,
                 from project/ext/sol/include/sol/table.hpp:25,
                 from project/ext/sol/include/sol/state_view.hpp:26,
                 from project/ext/sol/include/sol/state.hpp:25,
                 from project/sandbox/lua.cpp:4:
project/ext/sol/include/sol/usertype_metatable.hpp: In function 'std::__cxx11::string sol::usertype_detail::make_string(Arg&&)':
project/ext/sol/include/sol/usertype_metatable.hpp:216:25: error: 'sol::string_detail::string_shim {aka class std::basic_string_view<char>}' has no member named 'c_str'; did you mean '_M_str'?
    return std::string(s.c_str(), s.size());
                         ^~~~~
                         _M_str
In file included from project/ext/sol/include/sol/usertype.hpp:27:0,
                 from project/ext/sol/include/sol/table_core.hpp:28,
                 from project/ext/sol/include/sol/table.hpp:25,
                 from project/ext/sol/include/sol/state_view.hpp:26,
                 from project/ext/sol/include/sol/state.hpp:25,
                 from project/sandbox/lua.cpp:4:
project/ext/sol/include/sol/simple_usertype_metatable.hpp: In member function 'void sol::simple_usertype_metatable<T>::add(lua_State*, N&&, sol::constructor_wrapper<Fxs ...>)':
project/ext/sol/include/sol/simple_usertype_metatable.hpp:266:71: error: expected primary-expression before '>' token
    object o(L, in_place<detail::tagged<T, constructor_wrapper<Fxs...>>>, std::move(c));
                                                                       ^
project/ext/sol/include/sol/simple_usertype_metatable.hpp:266:72: error: expected primary-expression before ',' token
    object o(L, in_place<detail::tagged<T, constructor_wrapper<Fxs...>>>, std::move(c));
                                                                        ^
project/ext/sol/include/sol/simple_usertype_metatable.hpp: In member function 'void sol::simple_usertype_metatable<T>::add(lua_State*, N&&, sol::constructor_list<Lists ...>)':
project/ext/sol/include/sol/simple_usertype_metatable.hpp:276:70: error: expected primary-expression before '>' token
    object o(L, in_place<detail::tagged<T, constructor_list<Lists...>>>, std::move(c));
                                                                      ^
project/ext/sol/include/sol/simple_usertype_metatable.hpp:276:71: error: expected primary-expression before ',' token
    object o(L, in_place<detail::tagged<T, constructor_list<Lists...>>>, std::move(c));
                                                                       ^
project/ext/sol/include/sol/simple_usertype_metatable.hpp: In member function 'void sol::simple_usertype_metatable<T>::add(lua_State*, N&&, sol::destructor_wrapper<void>)':
project/ext/sol/include/sol/simple_usertype_metatable.hpp:286:68: error: expected primary-expression before '>' token
    object o(L, in_place<detail::tagged<T, destructor_wrapper<void>>>, std::move(c));
                                                                    ^
project/ext/sol/include/sol/simple_usertype_metatable.hpp:286:69: error: expected primary-expression before ',' token
    object o(L, in_place<detail::tagged<T, destructor_wrapper<void>>>, std::move(c));
                                                                     ^
project/ext/sol/include/sol/simple_usertype_metatable.hpp: In member function 'void sol::simple_usertype_metatable<T>::add(lua_State*, N&&, sol::destructor_wrapper<Fx>)':
project/ext/sol/include/sol/simple_usertype_metatable.hpp:296:66: error: expected primary-expression before '>' token
    object o(L, in_place<detail::tagged<T, destructor_wrapper<Fx>>>, std::move(c));
                                                                  ^
project/ext/sol/include/sol/simple_usertype_metatable.hpp:296:67: error: expected primary-expression before ',' token
    object o(L, in_place<detail::tagged<T, destructor_wrapper<Fx>>>, std::move(c));
                                                                   ^

[VaderDev]

The problem is unrelated of any code, the sole inclusion produces the same error.

[ThePhD]

Sorry about that: I'm still vetting C++17 support. It's fixed now.

[VaderDev]

Fix confirmed. Thank you.