# std::piecewise_construct, std::piecewise_construct_t

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< cpp‎ | utility

Defined in header `<utility>` |  |
struct piecewise_construct_t { explicit piecewise_construct_t() = default; }; | (1) | (since C++11)
constexpr std::piecewise_construct_t piecewise_construct{}; | (2) | (since C++11) (inline since C++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.

### Standard library

The following standard library types and functions use it as a disambiguation tag:

pair | implements binary tuple, i.e. a pair of values (class template)
uses_allocator_construction_args (C++20) | prepares the argument list matching the flavor of uses-allocator construction required by the given type (function template)
ranges::repeat_viewviews::repeat (C++23) | a `view` consisting of a generated sequence by repeatedly producing the same value (class template) (customization point object)

### 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 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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