std::packaged_task<R(Args...)>:: packaged_task

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std::packaged_task
packaged_task ( ) noexcept ;
(1) (since C++11)
template < class F >
explicit packaged_task ( F && f ) ;
(2) (since C++11)
template < class F, class Allocator >
explicit packaged_task ( std:: allocator_arg_t , const Allocator & a, F && f ) ;
(3) (since C++11)
(until C++17)
packaged_task ( const packaged_task & ) = delete ;
(4) (since C++11)
packaged_task ( packaged_task && rhs ) noexcept ;
(5) (since C++11)

Constructs a new std::packaged_task object.

1) Constructs a std::packaged_task object with no task and no shared state.
2,3) Constructs a std::packaged_task object with a shared state and a copy of the task, initialized with std:: forward < F > ( f ) . The allocator a is used to allocate memory necessary to store the task. (until C++17)
These overloads participate in overload resolution only if std:: decay < F > :: type is not the same type as std:: packaged_task < R ( Args... ) > .

Let t1 , t2 , ..., tN be values of the corresponding types in Args , if INVOKE<R> ( f, t1, t2, ..., tN ) is not a valid expression, the program is ill-formed.

(until C++17)

If std:: is_invocable_r_v < R, F & , Args... > is false , the program is ill-formed.

(since C++17)
If invoking f itself and invoking a copy of f behave differently, the behavior is undefined.
4) The copy constructor is deleted, std::packaged_task is move-only.
5) Constructs a std::packaged_task with the shared state and task formerly owned by rhs , leaving rhs with no shared state and a moved-from task.

Parameters

f - the callable target (function, member function, lambda expression, function object) to execute
a - the allocator to use when storing the task
rhs - the std::packaged_task to move from

Exceptions

2) Any exceptions thrown by copy/move constructor of f and possibly std::bad_alloc if the allocation fails.
3) Any exceptions thrown by copy/move constructor of f and by the allocator's allocate function if memory allocation fails.

Example

Run this code 
#include <future>
#include <iostream>
#include <thread>
 
int fib(int n)
{
    if (n < 3)
        return 1;
    else
        return fib(n - 1) + fib(n - 2);
}
 
int main()
{
    std::packaged_task<int(int)> fib_task(&fib); 
 
    std::cout << "Starting task\n";
    auto result = fib_task.get_future();
    std::thread t(std::move(fib_task), 42);
 
    std::cout << "Waiting for task to finish..." << std::endl;
    std::cout << result.get() << '\n';
 
    std::cout << "Task complete\n";
    t.join();
}

Output: 

Starting task
Waiting for task to finish...
267914296
Task complete

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 2067 C++11 the parameter type of the copy constructor was packaged_task& added const
LWG 2097 C++11 for overloads (2,3) , F could be std::packaged_task<R(Args...)> F is constrained
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