deductiongu...td::function(3) C++ Standard Libary deductiongu...td::function(3) 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::opera- tor() is well-formed when treated as an unevaluated operand and decltype(&F::op- erator()) 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::opera- tor() 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::opera- tor() 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 el- lipsis parameter, and the ... in the types is always treated as a pack expan- sion. The type deduced by these deduction guides may change in a later stan- dard 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 func- tion<int(double)> } Defect reports The following behavior-changing defect reports were applied retroac- tively to previously published C++ standards. DR Applied to Behavior as published Correct be- havior behavior of (2) was unclear clarified to be ex- cluded from LWG 3238 C++17 when overload resolution F::operator() is &&-qualified http://cppreference.com 2024.06.10 deductiongu...td::function(3)
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