deduction guides for std::function
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std::function
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Deduction guides(C++17)
Defined in header
<functional>
template< class R, class... ArgTypes >
function( R(*)(ArgTypes...) ) -> function<R(ArgTypes...)>;
(1)
(since C++17)
function( R(*)(ArgTypes...) ) -> function<R(ArgTypes...)>;
template< class F >
function( F ) -> function</*see below*/>;
(2)
(since C++17)
function( F ) -> function</*see below*/>;
template< class F >
function( F ) -> function</*see below*/>;
(3)
(since C++23)
function( F ) -> function</*see below*/>;
template< class F >
function( F ) -> function</*see below*/>;
(4)
(since C++23)
function( F ) -> function</*see below*/>;
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...)>.
[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
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)> }
[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 |