std::piecewise_construct, std::piecewise_construct_t
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piecewise_construct_t
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Defined in header
<utility>
struct piecewise_construct_t { explicit piecewise_construct_t() = default; };
(1)
(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.
[edit] Standard library
The following standard library types and functions use it as a disambiguation tag:
(C++20)
(function template) [edit]
(C++23)
view consisting of a generated sequence by repeatedly producing the same value(class template) (customization point object)[edit]
[edit] 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
[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 |