std::constructible_from(C++20)

namespace std {
  template <class T, class... Args>
  concept constructible_from = destructible<T> && is_constructible_v<T, Args...>;
}

概要

constructible_fromは、任意の型Tが引数型Args...から構築可能であることを表すコンセプトである。

例

#include <iostream>
#include <concepts>
#include <vector>
#include <optional>
#include <memory>
#include <span>

template<typename T, typename... Args>
requires std::constructible_from<T, Args...>
void f(const char* name, const char* args) {
  std::cout << name << " is constructible from " << args << std::endl;
}

template<typename T, typename... Args>
void f(const char* name, const char* args) {
  std::cout << name << " is not constructible from " << args << std::endl;
}


int main() {
  f<std::vector<int>, std::initializer_list<int>>("std::vector<int>", "std::initializer_list<int>");
  f<std::optional<std::vector<int>>, std::in_place_t, std::initializer_list<int>, std::allocator<int>>("std::optional<std::vector<int>>", "std::in_place_t, std::initializer_list<int>, std::allocator<int>");
  f<std::unique_ptr<const int>, std::unique_ptr<int>>("std::unique_ptr<const int>", "std::unique_ptr<int>");
  f<std::span<int, 4>, std::array<int, 4>&>("std::span<int, 4>", "std::array<int, 4>&");

  std::cout << "\n";

  f<std::vector<int>, std::vector<double>>("std::vector<int>", "std::vector<double>");
  f<std::optional<std::vector<int>>, std::optional<std::vector<double>>>("std::optional<std::vector<int>>", "std::optional<std::vector<double>>");
  f<std::unique_ptr<int>, std::unique_ptr<const int>>("std::unique_ptr<int>", "std::unique_ptr<const int>");
  f<std::span<int, 4>, std::array<int, 4>>("std::span<int, 4>", "std::array<int, 4>");
}

出力

std::vector<int> is constructible from std::initializer_list<int>
std::optional<std::vector<int>> is constructible from std::in_place_t, std::initializer_list<int>, std::allocator<int>
std::unique_ptr<const int> is constructible from std::unique_ptr<int>
std::span<int, 4> is constructible from std::array<int, 4>&

std::vector<int> is not constructible from std::vector<double>
std::optional<std::vector<int>> is not constructible from std::optional<std::vector<double>>
std::unique_ptr<int> is not constructible from std::unique_ptr<const int>
std::span<int, 4> is not constructible from std::array<int, 4>

std::constructible_from(C++20)

概要

例

出力

バージョン

言語

処理系

関連項目

参照