# std::piecewise_construct

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< cpp‎ | utility

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>`)

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