NAME¶

std::piecewise_construct,std::piecewise_construct_t - std::piecewise_construct,std::piecewise_construct_t

Synopsis¶

Defined in header <utility>
struct piecewise_construct_t { explicit (1) (since C++11)
piecewise_construct_t() = default; };
constexpr std::piecewise_construct_t (since C++11)
piecewise_construct{}; (2) (until C++17)
inline constexpr std::piecewise_construct_t (since C++17)
piecewise_construct{};

1) std::piecewise_construct_t is an empty class tag type used to disambiguate
between different functions that take two tuple arguments.
2) The constant std::piecewise_construct is an instance of (1).

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 <tuple>
#include <utility>

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::cout << "Creating p1...\n";
std::pair<Foo, Foo> p1(t, t);

std::cout << "Creating p2...\n";
std::pair<Foo, Foo> p2(std::piecewise_construct, t, t);
}

Output:¶

Creating p1...
Constructed a Foo from a tuple
Constructed a Foo from a tuple
Creating p2...
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>)