std::piecewise_construct

 

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constexpr piecewise_construct_t piecewise_construct     = std::piecewise_construct_t();		(since C++11) (until C++17)
inline constexpr piecewise_construct_t piecewise_construct     = std::piecewise_construct_t();		(since C++17)

The constant std::piecewise_construct is an instance of an empty struct tag type std::piecewise_construct_t.

Example

Run this code

#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);
}

Output:

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

See also

piecewise_construct_t (C++11)	tag type used to select correct function overload for piecewise construction (class)
(constructor)	constructs new pair (public member function of std::pair)