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)
(until 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 disambiguate
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 element 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 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 made explicit
could lead to ambiguity
See also¶
constructor constructs new pair
(public member function of std::pair<T1,T2>)
2022.07.31 | http://cppreference.com |