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

From cppreference.com
 
 
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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.

Contents

[edit] 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

[edit] 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.

[edit] Example

#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

[edit] 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