std::piecewise_construct, std::piecewise_construct_t

Since C++11

Header: <utility>

std::piecewise_construct_t is an empty class tag type used to disambiguate between different functions that take two tuple arguments.

# Declarations

struct piecewise_construct_t { explicit piecewise_construct_t() = default; };

(since C++11)

constexpr std::piecewise_construct_t piecewise_construct{};

(since C++11) (inline since C++17)

# Example

#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);
}

# Defect reports

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

# See also

(constructor)