# deduction guides for `std::function`

From cppreference.com
< cpp‎ | utility‎ | functional‎ | 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

Run this code

    #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

Retrieved from "https://en.cppreference.com/mwiki/index.php?title=cpp/utility/functional/function/deduction_guides&oldid=141959"