Defined in header <future> | ||
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template< class > class packaged_task; //not defined | (1) | (since C++11) |
template< class R, class ...Args > class packaged_task<R(Args...)>; | (2) | (since C++11) |
The class template std::packaged_task wraps any Callable target (function, lambda expression, bind expression, or another function object) so that it can be invoked asynchronously. Its return value or exception thrown is stored in a shared state which can be accessed through std::future objects.
Just like std::function, std::packaged_task is a polymorphic, allocator-aware container: the stored callable target may be allocated on heap or with a provided allocator.
| constructs the task object (public member function) |
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| destructs the task object (public member function) |
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| moves the task object (public member function) |
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| checks if the task object has a valid function (public member function) |
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| swaps two task objects (public member function) |
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Getting the result |
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returns a std::future associated with the promised result (public member function) |
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Execution |
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| executes the function (public member function) |
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| executes the function ensuring that the result is ready only once the current thread exits (public member function) |
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| resets the state abandoning any stored results of previous executions (public member function) |
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(C++11) | specializes the std::swap algorithm (function template) |
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(C++11)(until C++17) | specializes the std::uses_allocator type trait (class template specialization) |
#include <iostream>
#include <cmath>
#include <thread>
#include <future>
#include <functional>
// unique function to avoid disambiguating the std::pow overload set
int f(int x, int y) { return std::pow(x,y); }
void task_lambda()
{
std::packaged_task<int(int,int)> task([](int a, int b) {
return std::pow(a, b);
});
std::future<int> result = task.get_future();
task(2, 9);
std::cout << "task_lambda:\t" << result.get() << '\n';
}
void task_bind()
{
std::packaged_task<int()> task(std::bind(f, 2, 11));
std::future<int> result = task.get_future();
task();
std::cout << "task_bind:\t" << result.get() << '\n';
}
void task_thread()
{
std::packaged_task<int(int,int)> task(f);
std::future<int> result = task.get_future();
std::thread task_td(std::move(task), 2, 10);
task_td.join();
std::cout << "task_thread:\t" << result.get() << '\n';
}
int main()
{
task_lambda();
task_bind();
task_thread();
}Output:
task_lambda: 512 task_bind: 2048 task_thread: 1024
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(C++11) | waits for a value that is set asynchronously (class template) |
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