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deduction guides for std::function

Defined in header <functional> 
template< class R, class... ArgTypes >
function( R(*)(ArgTypes...) ) -> function<R(ArgTypes...)>;
 (1) (since C++17) 
template< class F >
function( F ) -> function</*see below*/>;
 (2) (since C++17) 
template< class F >
function( F ) -> function</*see below*/>;
 (3) (since C++23) 
template< class F >
function( F ) -> function</*see below*/>;
 (4) (since C++23)
1) This deduction guide is provided for std::function to allow deduction from functions.
2) This overload participates in overload resolution only if &F::operator() is well-formed when treated as an unevaluated operand and decltype(&F::operator()) is of the form R(G::*)(A...) (optionally cv-qualified, optionally noexcept, optionally lvalue reference qualified). The deduced type is std::function<R(A...)>.
3) This overload participates in overload resolution only if &F::operator() is well-formed when treated as an unevaluated operand and F::operator() is an explicit object parameter function whose type is of form R(G, A...) or R(G, A...) noexcept. The deduced type is std::function<R(A...)>.
4) This overload participates in overload resolution only if &F::operator() is well-formed when treated as an unevaluated operand and F::operator() is a static member function whose type is of form R(A...) or R(A...) noexcept. The deduced type is std::function<R(A...)>.

Notes

These deduction guides do not allow deduction from a function with ellipsis parameter, and the ... in the types is always treated as a pack expansion.

The type deduced by these deduction guides may change in a later standard revision (in particular, this might happen if noexcept support is added to std::function in a later standard).

Example

#include <functional>
int func(double) { return 0; }
int main() {
  std::function f{func}; // guide #1 deduces function<int(double)>
  int i = 5;
  std::function g = [&](double) { return i; }; // guide #2 deduces function<int(double)>
}

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 3238 C++17 behavior of (2) was unclear when
F::operator() is &&-qualified 		clarified to be excluded from overload resolution

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