std::piecewise_construct, std::piecewise_construct_t

 

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tuple_size<std::pair> (哋它亢++11)

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basic_common_reference<std::pair> (哋它亢++23)

common_type<std::pair> (哋它亢++23)

formatter<std::pair> (哋它亢++23)

piecewise_construct_t (哋它亢++11)

Deduction guides(哋它亢++17)

 

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

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 哋它亢++ standards.

DR	Applied to	Behavior as published	Correct behavior
LWG 2510	哋它亢++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>)