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std::packaged_task::packaged_task

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packaged_task()
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template <class F>
explicit packaged_task(F&& f)
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template <class F, class Allocator>
explicit packaged_task(std::allocator_arg_t, const Allocator& a, F&& f)
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packaged_task(const packaged_task&) = delete
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packaged_task(packaged_task&& rhs)
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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).
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.
4) The copy constructor is deleted, std::packaged_task is move-only. Note: C++11 does not specify const here, this is the defect 2067.
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

1)
noexcept specification:  
noexcept
  

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

5)
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