deduction guides for std::packaged_task
From cppreference.com
< cpp | thread | packaged task
| Defined in header <future>
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||
| template< class R, class... Args > packaged_task( R(*)(Args...) ) -> packaged_task<R(Args...)>; |
(1) | (since C++17) |
| template< class F > packaged_task( F ) -> packaged_task</*see below*/>; |
(2) | (since C++17) |
| template< class F > packaged_task( F ) -> packaged_task</*see below*/>; |
(3) | (since C++23) |
| template< class F > packaged_task( F ) -> packaged_task</*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::packaged_task<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::packaged_task<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::packaged_task<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.
Example
Run this code
#include <future> int func(double) { return 0; } int main() { std::packaged_task f{func}; // deduces packaged_task<int(double)> int i = 5; std::packaged_task g = [&](double) { return i; }; // => packaged_task<int(double)> }