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Showing content from http://en.cppreference.com/w/cpp/language/../utility/functional/function/deduction_guides.html below:

deduction guides for std::function - cppreference.com

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) 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...)

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...)

R(A...) noexcept

. The deduced type is

std::function<R(A...)>

[edit] 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).

[edit] 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)>
}

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

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