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

packaged_task() noexcept;

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

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

packaged_task( const packaged_task& ) = delete;

packaged_task( packaged_task&& rhs ) noexcept;

Constructs a new std::packaged_task object.

• These constructors do not (until C++17)This constructor does not (since C++17) participate in overload resolution if std::decay<F>::type is the same type as std::packaged_task<R(ArgTypes...)>.
• The program is ill-formed if the INVOKE<R>(std::forward<F>(f), std::declval<Args>()...) expression (described in Callable) is ill-formed when treated as an unevaluated operand (i.e. std::is_invocable_r_v<R, F, Args...> is not true) (since C++17).
• The behavior is undefined if the invocation on a copy of f behaves different from that on f.

Parameters

Exceptions

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();
}

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.