NAME

std::piecewise_construct - std::piecewise_construct

Synopsis

Defined in header <utility>
struct piecewise_construct_t { explicit piecewise_construct_t() = (since C++11)
default; };
constexpr std::piecewise_construct_t piecewise_construct{}; (since C++11)
(until C++17)
inline constexpr std::piecewise_construct_t piecewise_construct{}; (since C++17)

std::piecewise_construct_t is an empty class tag type used to disambiguate between
different functions that take two tuple arguments. The constant
std::piecewise_construct is an instance of it.

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

Defect reports

The following behavior-changing defect reports were applied retroactively to
previously published C++ standards.

DR Applied to Behavior as published Correct behavior
LWG 2510 C++11 the default constructor was non-explicit, which made explicit
could lead to ambiguity

See also

constructor constructs new pair
(public member function of std::pair<T1,T2>)