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std::piecewise_construct(3) C++ Standard Libary std::piecewise_construct(3) NAME std::piecewise_construct - std::piecewise_construct Synopsis Defined in header <utility> struct piecewise_construct_t { explicit piecewise_construct_t() = (since C++11) default; }; constexpr std::piecewise_construct_t piecewise_construct{}; (since C++11) (un- til 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 disam- biguate 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 el- ement of the pair. Example // 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 Defect reports The following behavior-changing defect reports were applied retroac- tively to previously published C++ standards. DR Applied to Behavior as published Correct behavior LWG 2510 C++11 the default constructor was non-explicit, which made explicit could lead to ambiguity See also constructor constructs new pair (public member function of std::pair<T1,T2>) http://cppreference.com 2022.07.31 std::piecewise_construct(3)
NAME | Synopsis | Example | Output: | See also
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