deduction guides for std::function
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C++
Utilities library
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Function objects
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std::function
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(until C++17)
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(until C++20)
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(until C++17)
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| Deduction guides(C++17) |
Defined in header <functional>
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template
<
class R, class... ArgTypes
>
function( R( * ) (ArgTypes...) ) - > function<R(ArgTypes...) > ; |
(1) | (since C++17) |
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template
<
class F >
function( F ) - > function< /*see below*/ > ; |
(2) | (since C++17) |
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template
<
class F >
function( F ) - > function< /*see below*/ > ; |
(3) | (since C++23) |
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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...)
>
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
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() |
clarified to be excluded from overload resolution |