std::packaged_task<R(Args...)>::packaged_task (3) - Linux Manuals

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

NAME

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

Synopsis

packaged_task() noexcept; (1) (since C++11)
template <class F> (2) (since C++11)
explicit packaged_task( F&& f );
template <class F, class Allocator> (3) (since C++11)
explicit packaged_task( std::allocator_arg_t, const Allocator& a, F&& f ); (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) 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

2) Any exceptions thrown by copy/move constructor of f and possiblly 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.
4) (none)

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 deleted copy constructor took reference to non-const made const

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), 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