std::piecewise_construct,std::piecewise_construct_t

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Defined in header`<utility>`
struct piecewise_construct_t{explicit piecewise_construct_t()=default;};	(1)	(since C++11)
constexprstd::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 constantstd::piecewise_construct is an instance of(1).

The overloads that do not usestd::piecewise_construct_t assume that each tuple argument becomes the element of a pair. The overloads that usestd::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:

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

[edit]Example

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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 tupleConstructed a Foo from a tupleCreating p2...Constructed a Foo from an int and a floatConstructed a Foo from an int and a float