deduction guides for std::function

Since C++17

Header: <functional>

This deduction guide is provided for std::function to allow deduction from functions.

# Declarations

template< class R, class... ArgTypes >
function( R(*)(ArgTypes...) ) -> function<R(ArgTypes...)>;

(since C++17)

template< class F >
function( F ) -> function</*see below*/>;

(since C++17)

template< class F >
function( F ) -> function</*see below*/>;

(since C++23)

template< class F >
function( F ) -> function</*see below*/>;

(since C++23)

# 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

DR	Applied to	Behavior as published	Correct behavior
LWG 3238	C++17	behavior of (2) was unclear whenF::operator() is &&-qualified	clarified to be excluded from overload resolution