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)

(until C++17)

If std::is_invocable_r_v <R, F&, Args...> is false

(since C++17)
If invoking f itself and invoking a copy of f
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

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
3) Any exceptions thrown by copy/move constructor of f and by the allocator's allocate

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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