How to check that a type is 'formattable' using type traits / concepts?

user_10576494 0
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I would like to check if a certain type can be used with std::format.

This is my naive attempt:

template<typename Object>
concept formattable = requires(const Object & obj)
{
    std::format("{}", obj);
};

But this does not work. It basically returns true for all types. Even those that can't be used with std::format.

static_assert(!formattable<std::vector<int>>); // should return false

Why doesn't it work?

c++-concepts typetraits c++20 c++
1 Answers

Since std::format is not a constrained function, the expression std::format("{}", obj) is always well-formed. You might want to do

#include <format>
    
template<typename T>
concept formattable = requires (T& v, std::format_context ctx) {
  std::formatter<std::remove_cvref_t<T>>().format(v, ctx);
};

which mainly based on the constraints of the basic_format_arg's constructor in [format.arg].

Demo

user_116387180
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