std::piecewise_construct

 

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tuple_size (C++11)

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common_type (C++23)

piecewise_construct_t (C++11)

 

Defined in header <utility>
struct piecewise_construct_t { explicit piecewise_construct_t() = default; };		(since C++11)
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 <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::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 could lead to ambiguity	made explicit

See also

(constructor)

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