std::packaged_task::packaged_task

1	`packaged_task();`

(1)

(since C++11)

template <class F>
explicit packaged_task( F&& f );

(2)

(since C++11)

(3)

template <class F, class Allocator>
explicit packaged_task( std::allocator_arg_t, const Allocator& a, F&& f );

(since C++11)
(until C++17)

template <class F, class Allocator>
packaged_task( std::allocator_arg_t, const Allocator& a, F&& f );

(since C++17)

(4)

1	`packaged_task( packaged_task& ) =` `delete;`

(since C++11)
(until C++14)

1	`packaged_task(` `const` `packaged_task& ) =` `delete;`

(since C++14)

1	`packaged_task( packaged_task&& rhs );`

(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) Constructs a std::packaged_task object with a shared state and a copy of the task, initialized with std::forward<F>(f). This constructor does not participate in overload resolution if std::decay<F>::type is the same type as std::packaged_task<R(ArgTypes...)>.

3) Constructs a std::packaged_task object with a shared state and a copy of the task, initialized with std::forward<F>(f). Uses the provided allocator to allocate memory necessary to store the task. This constructor does not participate in overload resolution if std::decay<F>::type is the same type as std::packaged_task<R(ArgTypes...)>.

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, functor) to execute
a	-	the allocator to use when storing the task
rhs	-	the `std::packaged_task` to move from

Exceptions

noexcept specification:

noexcept

2-3) Any exceptions thrown by copy/move constructor of f and possiblly std::bad_alloc if the allocation fails.

4) (none)

noexcept specification:

noexcept

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), 40);
  
    std::cout << "waiting for task to finish...\n";
    std::cout << result.get() << '\n';
  
    std::cout << "task complete\n";                                                                                                                                                                           
    t.join();
}

Output:

starting task
waiting for task to finish...
102334155
task complete

Links:

http://en.cppreference.com/w/cpp/thread/packaged_task/packaged_task

doc_CPP

2025-01-10 15:47:30

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