std::piecewise_construct_t (3) Linux Manual Page

std::piecewise_construct_t – std::piecewise_construct_t

Synopsis

Defined in header<utility>

    struct piecewise_construct_t {
};
(since C++ 11)(until C++ 17)

    struct piecewise_construct_t {
    explicit piecewise_construct_t() = default;
};
(since C++ 17)

std::piecewise_construct_t is an empty struct tag type used to disambiguate between different functions that take two tuple arguments.
The overloads that do not use std::piecewise_construct_t assume that each tuple argument becomes the element of a pair. The overloads that use std::piecewise_construct_t assume that each tuple argument is used to construct, piecewise, a new object of specified type, which will become the element of the pair.

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 an object of type piecewise_construct_t used to disambiguate functions for piecewise construction
                    (constant)
(C++11)
                    constructs new pair
constructor (public member function of std::pair<T1,T2>)