Thiết kế website giá rẻ

Question

I was trying to specialize std::formatter<T> for a basic custom type Vector3D, but I’m receiving a compiler error. Here is a minimal reproducible example:

vector.h defines the Vector3D class and defines the std::formatter<Vector3D> specialization, but only declares its format function:

#ifndef VECTOR_H
#define VECTOR_H

#include <format>
using T = float;

/* Define `Vector3D` */
struct Vector3D {
    T x, y, z;

    Vector3D() {}
    Vector3D(T x_, T y_, T z_) : x{x_}, y{y_}, z{z_} {}
};

/* Declare specialization of `std::formatter` for `Vector3D<T>` */
template <>
struct std::formatter<Vector3D> : public std::formatter<std::string> {
    auto format(const Vector3D &item, std::format_context &format_context) const;
};

#endif

vector.cpp includes vector.h and defines std::formatter<Vector3D>::format()

#include "base/vector.h"

/* Define `format` function in the specialization of `std::formatter` for `Vector3D<T>` */
auto std::formatter<Vector3D>::format(
    const Vector3D &item, 
    std::format_context &format_context
) const {
    return std::format_to(format_context.out(), "({}, {}, {})", item.x, item.y, item.z);
}

Finally, main.cpp attempts to std::format a value of type Vector3D:

#include "base/vectors.h"

int main()
{
    std::format("{}n", Vector3D(1, 2, 3));
}

However, compiling this gives a huge compiler error:

/main.cpp: In function ‘int main()’:
/main.cpp:5:43: warning: ignoring return value of ‘std::string std::format(format_string<_Args ...>, _Args&& ...) [with _Args = {Vector3D}; string = __cxx11::basic_string<char>; format_string<_Args ...> = basic_format_string<char, Vector3D>]’, declared with attribute ‘nodiscard’ [-Wunused-result]
    5 |     std::format("{}n", Vector3D(1, 2, 3));
      |                                           ^
In file included from vectors.h:4,
                 from /main.cpp:1:
/usr/include/c++/13/format:3742:5: note: declared here
 3742 |     format(format_string<_Args...> __fmt, _Args&&... __args)
      |     ^~~~~~
/usr/include/c++/13/format: In instantiation of ‘static std::__format::_Arg_store<_Context, _Args>::_Element_t std::__format::_Arg_store<_Context, _Args>::_S_make_elt(_Tp&) [with _Tp = Vector3D; _Context = std::basic_format_context<std::__format::_Sink_iter<char>, char>; _Args = {std::basic_format_arg<std::basic_format_context<std::__format::_Sink_iter<char>, char> >::handle}; _Element_t = std::__format::_Arg_store<std::basic_format_context<std::__format::_Sink_iter<char>, char>, std::basic_format_arg<std::basic_format_context<std::__format::_Sink_iter<char>, char> >::handle>::_Element_t]’:
/usr/include/c++/13/format:3281:23:   required from ‘std::__format::_Arg_store<_Context, _Args>::_Arg_store(_Tp& ...) [with _Tp = {Vector3D}; _Context = std::basic_format_context<std::__format::_Sink_iter<char>, char>; _Args = {std::basic_format_arg<std::basic_format_context<std::__format::_Sink_iter<char>, char> >::handle}]’
/usr/include/c++/13/format:3330:14:   required from ‘auto std::make_format_args(_Args&& ...) [with _Context = basic_format_context<__format::_Sink_iter<char>, char>; _Args = {Vector3D&}]’
/usr/include/c++/13/format:3743:61:   required from ‘std::string std::format(format_string<_Args ...>, _Args&& ...) [with _Args = {Vector3D}; string = __cxx11::basic_string<char>; format_string<_Args ...> = basic_format_string<char, Vector3D>]’
/main.cpp:5:16:   required from here
/usr/include/c++/13/format:3270:38: error: no matching function for call to ‘std::basic_format_arg<std::basic_format_context<std::__format::_Sink_iter<char>, char> >::basic_format_arg(Vector3D&)’
 3270 |           basic_format_arg<_Context> __arg(__v);
      |                                      ^~~~~
/usr/include/c++/13/format:3025:9: note: candidate: ‘template<class _Tp>  requires  __formattable_with<_Tp, _Context, typename _Context::formatter_type<typename std::remove_const<_Tp>::type>, std::basic_format_parse_context<typename _Context::char_type> > std::basic_format_arg<_Context>::basic_format_arg(_Tp&) [with _Context = std::basic_format_context<std::__format::_Sink_iter<char>, char>]’
 3025 |         basic_format_arg(_Tp& __v) noexcept
      |         ^~~~~~~~~~~~~~~~
/usr/include/c++/13/format:3025:9: note:   template argument deduction/substitution failed:
/usr/include/c++/13/format:3025:9: note: constraints not satisfied
/usr/include/c++/13/format: In substitution of ‘template<class _Tp>  requires  __formattable_with<_Tp, _Context, typename _Context::formatter_type<typename std::remove_const<_Tp>::type>, std::basic_format_parse_context<typename _Context::char_type> > std::basic_format_arg<std::basic_format_context<std::__format::_Sink_iter<char>, char> >::basic_format_arg(_Tp&) [with _Tp = std::basic_format_context<std::__format::_Sink_iter<char>, char>]’:
/usr/include/c++/13/format:3270:31:   required from ‘static std::__format::_Arg_store<_Context, _Args>::_Element_t std::__format::_Arg_store<_Context, _Args>::_S_make_elt(_Tp&) [with _Tp = Vector3D; _Context = std::basic_format_context<std::__format::_Sink_iter<char>, char>; _Args = {std::basic_format_arg<std::basic_format_context<std::__format::_Sink_iter<char>, char> >::handle}; _Element_t = std::__format::_Arg_store<std::basic_format_context<std::__format::_Sink_iter<char>, char>, std::basic_format_arg<std::basic_format_context<std::__format::_Sink_iter<char>, char> >::handle>::_Element_t]’
/usr/include/c++/13/format:3281:23:   required from ‘std::__format::_Arg_store<_Context, _Args>::_Arg_store(_Tp& ...) [with _Tp = {Vector3D}; _Context = std::basic_format_context<std::__format::_Sink_iter<char>, char>; _Args = {std::basic_format_arg<std::basic_format_context<std::__format::_Sink_iter<char>, char> >::handle}]’
/usr/include/c++/13/format:3330:14:   required from ‘auto std::make_format_args(_Args&& ...) [with _Context = basic_format_context<__format::_Sink_iter<char>, char>; _Args = {Vector3D&}]’
/usr/include/c++/13/format:3743:61:   required from ‘std::string std::format(format_string<_Args ...>, _Args&& ...) [with _Args = {Vector3D}; string = __cxx11::basic_string<char>; format_string<_Args ...> = basic_format_string<char, Vector3D>]’
/main.cpp:5:16:   required from here
/usr/include/c++/13/format:2226:13:   required for the satisfaction of ‘__formattable_with<_Tp, _Context, typename _Context::formatter_type<typename std::remove_const<_Tp>::type>, std::basic_format_parse_context<typename _Context::char_type> >’ [with _Tp = Vector3D; _Context = std::basic_format_context<std::__format::_Sink_iter<char>, char>]
/usr/include/c++/13/format:2228:7:   in requirements with ‘const _Formatter __cf’, ‘_Tp&& __t’, ‘_Context __fc’ [with _Formatter = std::formatter<Vector3D, char>; _Tp = Vector3D; _Context = std::basic_format_context<std::__format::_Sink_iter<char>, char>]
/usr/include/c++/13/format:2230:20: note: the required expression ‘__cf.format(__t, __fc)’ is invalid
 2230 |       { __cf.format(__t, __fc) } -> same_as<typename _Context::iterator>;
      |         ~~~~~~~~~~~^~~~~~~~~~~
/usr/include/c++/13/format: In instantiation of ‘static std::__format::_Arg_store<_Context, _Args>::_Element_t std::__format::_Arg_store<_Context, _Args>::_S_make_elt(_Tp&) [with _Tp = Vector3D; _Context = std::basic_format_context<std::__format::_Sink_iter<char>, char>; _Args = {std::basic_format_arg<std::basic_format_context<std::__format::_Sink_iter<char>, char> >::handle}; _Element_t = std::__format::_Arg_store<std::basic_format_context<std::__format::_Sink_iter<char>, char>, std::basic_format_arg<std::basic_format_context<std::__format::_Sink_iter<char>, char> >::handle>::_Element_t]’:
/usr/include/c++/13/format:3281:23:   required from ‘std::__format::_Arg_store<_Context, _Args>::_Arg_store(_Tp& ...) [with _Tp = {Vector3D}; _Context = std::basic_format_context<std::__format::_Sink_iter<char>, char>; _Args = {std::basic_format_arg<std::basic_format_context<std::__format::_Sink_iter<char>, char> >::handle}]’
/usr/include/c++/13/format:3330:14:   required from ‘auto std::make_format_args(_Args&& ...) [with _Context = basic_format_context<__format::_Sink_iter<char>, char>; _Args = {Vector3D&}]’
/usr/include/c++/13/format:3743:61:   required from ‘std::string std::format(format_string<_Args ...>, _Args&& ...) [with _Args = {Vector3D}; string = __cxx11::basic_string<char>; format_string<_Args ...> = basic_format_string<char, Vector3D>]’
/main.cpp:5:16:   required from here
cc1plus: note: set ‘-fconcepts-diagnostics-depth=’ to at least 2 for more detail
/usr/include/c++/13/format:2837:7: note: candidate: ‘std::basic_format_arg<_Context>::basic_format_arg() [with _Context = std::basic_format_context<std::__format::_Sink_iter<char>, char>]’
 2837 |       basic_format_arg() noexcept : _M_type(__format::_Arg_none) { }
      |       ^~~~~~~~~~~~~~~~
/usr/include/c++/13/format:2837:7: note:   candidate expects 0 arguments, 1 provided
/usr/include/c++/13/format:2775:11: note: candidate: ‘constexpr std::basic_format_arg<std::basic_format_context<std::__format::_Sink_iter<char>, char> >::basic_format_arg(const std::basic_format_arg<std::basic_format_context<std::__format::_Sink_iter<char>, char> >&)’
 2775 |     class basic_format_arg
      |           ^~~~~~~~~~~~~~~~
/usr/include/c++/13/format:2775:11: note:   no known conversion for argument 1 from ‘Vector3D’ to ‘const std::basic_format_arg<std::basic_format_context<std::__format::_Sink_iter<char>, char> >&’
/usr/include/c++/13/format:2775:11: note: candidate: ‘constexpr std::basic_format_arg<std::basic_format_context<std::__format::_Sink_iter<char>, char> >::basic_format_arg(std::basic_format_arg<std::basic_format_context<std::__format::_Sink_iter<char>, char> >&&)’
/usr/include/c++/13/format:2775:11: note:   no known conversion for argument 1 from ‘Vector3D’ to ‘std::basic_format_arg<std::basic_format_context<std::__format::_Sink_iter<char>, char> >&&’

When I just define the format function directly in the header file vector.h, though, everything works. Can someone explain what is going on here?

Thiết kế website giá rẻ

Danh mục

Why does separating the interface and implementation of a `std::formatter` specialization cause constraints to fail?