std::piecewise_construct, std::piecewise_construct_t

 

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Utilities library

Language support Type support (basic types, RTTI) Library feature-test macros(C++20) Program utilities Variadic functions initializer_list (C++11) is_constant_evaluated (C++20) is_within_lifetime (C++26) source_location (C++20) Coroutine support(C++20) Contract support(C++26) Three-way comparison three_way_comparablethree_way_comparable_with (C++20)(C++20) strong_ordering (C++20) weak_ordering (C++20) partial_ordering (C++20) common_comparison_category (C++20) compare_three_way_result (C++20) compare_three_way (C++20) strong_order (C++20) weak_order (C++20) partial_order (C++20) compare_strong_order_fallback (C++20) compare_weak_order_fallback (C++20) compare_partial_order_fallback (C++20)   is_eqis_ltis_lteq (C++20)(C++20)(C++20)   is_neqis_gtis_gteq (C++20)(C++20)(C++20)

General utilities Function objects Bit manipulation(C++20) bitset hash (C++11) Relational operators(deprecated in C++20) rel_ops::operator!=rel_ops::operator>    rel_ops::operator<=rel_ops::operator>= Integer comparison functions cmp_equalcmp_lesscmp_less_than (C++20)(C++20)(C++20)   cmp_not_equalcmp_greatercmp_greater_than (C++20)(C++20)(C++20) in_range (C++20) Swap and type operations swap ranges::swap (C++20) exchange (C++14) declval (C++11) to_underlying (C++23) forward (C++11) forward_like (C++23) move (C++11) move_if_noexcept (C++11) as_const (C++17) Common vocabulary types pair tuple (C++11) optional (C++17) any (C++17) variant (C++17) tuple_size (C++11) tuple_element (C++11) apply (C++17) make_from_tuple (C++17) expected (C++23)

 

std::pair

Member functions

pair::pair

pair::operator=

pair::swap (C++11)

Non-member functions

make_pair

operator==operator!=operator<operator<=operator>operator>=operator<=> (until C++20)(until C++20)(until C++20)(until C++20)(until C++20)(C++20)

swap(std::pair) (C++11)

get(std::pair) (C++11)

Helper classes

tuple_size<std::pair> (C++11)

tuple_element<std::pair> (C++11)

basic_common_reference<std::pair> (C++23)

common_type<std::pair> (C++23)

formatter<std::pair> (C++23)

piecewise_construct_t (C++11)

Deduction guides(C++17)

 

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 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>)