`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) |

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...)>`

.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).

#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)> }

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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