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std::packag...ackaged_task(3) C++ Standard Libarystd::packag...ackaged_task(3) NAME std::packaged_task::packaged_task - std::packaged_task::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 > (since C++11) explicit packaged_task( std::allocator_arg_t, const Allocator& a, (3) (until C++17) F&& f ); 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::de- cay<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 (until C++17) is ill-formed. If std::is_invocable_r_v<R, F, Args...> is false, the program is (since C++17) ill-formed. 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. Parameters f - the callable target (function, member function, lambda expres- sion, function object) 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 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. 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), 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 Defect reports The following behavior-changing defect reports were applied retroac- tively 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 added const packaged_task& LWG 2097 C++11 for overloads (2,3), F could be F is constrained std::packaged_task<R(Args...)> http://cppreference.com 2024.06.10 std::packag...ackaged_task(3)
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