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Showing content from https://en.cppreference.com/w/cpp/thread/packaged_task/deduction_guides.html below:

deduction guides for std::packaged_task - cppreference.com

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...)

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...)

R(A...) noexcept

. The deduced type is

std::packaged_task<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.

[edit] Example

#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)>
}

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