std::piecewise_construct_t (3) - Linux Manuals

std::piecewise_construct_t: std::piecewise_construct_t

NAME

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