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std::piecewise_construct - cppreference.com

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ヘッダ <utility> で定義

constexpr std::piecewise_construct_t piecewise_construct{};

 (C++11以上)
(C++17未満)

inline constexpr std::piecewise_construct_t piecewise_construct{};

 (C++17以上)

定数 std::piecewise_construct は空の構造体のタグ型 std::piecewise_construct_t のインスタンスです。

例

#include <iostream>
#include <utility>
#include <tuple>

struct Foo {
    Foo(std::tuple<int, float>) 
    {
        std::cout << "Constructed a Foo from a tuple\n";
    }
    Foo(int, float) 
    {
        std::cout << "Constructed a Foo from an int and a float\n";
    }
};

int main()
{
    std::tuple<int, float> t(1, 3.14);
    std::pair<Foo, Foo> p1(t, t);
    std::pair<Foo, Foo> p2(std::piecewise_construct, t, t);
}

出力: 

Constructed a Foo from a tuple
Constructed a Foo from a tuple
Constructed a Foo from an int and a float
Constructed a Foo from an int and a float

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