NAME¶

deductionguidesforstd::function - deductionguidesforstd::function

Synopsis¶

Defined in header <functional>
template< class R, class... ArgTypes > (1) (since C++17)
function( R(*)(ArgTypes...) ) -> function<R(ArgTypes...)>;
template< class F > (2) (since C++17)
function( F ) -> function</*see below*/>;
template< class F > (3) (since C++23)
function( F ) -> function</*see below*/>;
template< class F > (4) (since C++23)
function( F ) -> function</*see below*/>;

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