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# std::piecewise_construct_t

From cppreference.com
< cpp‎ | utility

Defined in header `<utility>` |  |
struct piecewise_construct_t { }; |  | (since C++11) (until C++17)
struct piecewise_construct_t { explicit piecewise_construct_t() = default; }; |  | (since C++17)

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

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.

### [edit] Example

[edit]
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

### [edit] See also

piecewise_construct (C++11) | an object of type `piecewise_construct_t` used to disambiguate functions for piecewise construction (constant) [edit]
(constructor) | constructs new pair (public member function of `std::pair`)

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