struct piecewise_construct_t { explicit piecewise_construct_t() = default; };
(1) (since C++11) (2) (since C++11)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.
The following standard library types and functions use it as a disambiguation tag:
[edit] Example#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[edit] 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 [edit] See also constructs new pairstd::pair<T1,T2>)
RetroSearch is an open source project built by @garambo | Open a GitHub Issue
Search and Browse the WWW like it's 1997 | Search results from DuckDuckGo
HTML:
3.2
| Encoding:
UTF-8
| Version:
0.7.4