std::piecewise_construct, std::piecewise_construct_t - cppreference.com (original) (raw)

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.

[edit] Standard library

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

[edit] Example

#include #include #include   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

[edit] Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

[edit] See also

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