std::piecewise_construct

C++

Language

Headers

Freestanding and hosted implementations

Named requirements

Language support library

Concepts library (C++20)

Diagnostics library

Utilities library

Strings library

Containers library

Algorithms library

Iterators library

Numerics library

Input/output library

Localizations library

Regular expressions library (C++11)

Atomic operations library (C++11)

Thread support library (C++11)

Filesystem library (C++17)

Technical Specifications

Utilities library

Type support (basic types, RTTI, type traits)
Dynamic memory management
Error handling
Program utilities
Variadic functions
Library feature-test macros
Date and time
Function objects
initializer_list (C++11)
bitset
hash (C++11)
integer_sequence (C++14)
Relational operators (deprecated)
rel_ops::operator!=rel_ops::operator>rel_ops::operator<=rel_ops::operator>=
Comparisons (C++20)
strong_order weak_order partial_order strong_equal weak_equal	strong_ordering weak_ordering partial_ordering strong_equality weak_equality	is_eqis_neqis_ltis_lteqis_gtis_gteq
common_comparison_category
Common vocabulary types
pair tuple (C++11) apply (C++17) make_from_tuple (C++17)	optional (C++17) any (C++17) variant (C++17)
Swap, forward and move
swap
exchange (C++14)
forward (C++11)
move (C++11)
move_if_noexcept (C++11)
Elementary string conversions
to_chars (C++17)
from_chars (C++17)
chars_format (C++17)
Type operations
declval (C++11)
as_const (C++17)

std::pair

Member functions

pair::pair

pair::operator=

pair::swap

Non-member functions

make_pair

operator==operator!=operator<operator<=operator>operator>=

std::swap

get (C++11)

Deduction guides(C++17)

Helper classes

tuple_size (C++11)

tuple_element (C++11)

constexpr piecewise_construct_t piecewise_construct = std::piecewise_construct_t();		(since C++11) (until C++17)
inline constexpr piecewise_construct_t piecewise_construct = std::piecewise_construct_t();		(since C++17)

The constant std::piecewise_construct is an instance of an empty struct tag type std::piecewise_construct_t.

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

See also

piecewise_construct_t (C++11)	tag type used to select correct function overload for piecewise construction (class)
(constructor)	constructs new pair (public member function of std::pair)