std::piecewise_construct, std::piecewise_construct_t
From cppreference.com
| Defined in header <utility>
|
||
struct piecewise_construct_t { explicit piecewise_construct_t() = default; };
|
(1) | (since C++11) |
constexpr std::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:
| implements binary tuple, i.e. a pair of values (class template) | |
| prepares the argument list matching the flavor of uses-allocator construction required by the given type (function template) | |
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
| constructs new pair (public member function of std::pair<T1,T2>)
|
