c++ std::promise

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c++ std::promise

c++ std::future

std::promise

c++ std::promise promises setting a value or an excecption in one thread, and returning that value or exception in another thread.

std::promise is commonly conjunction with std::future.

class :

std::promise<R>;

Methods :

.swap
.get_future
.set_value
.set_value_at_thread_exit
.set_exception
.set_exception_at_thread_exit

c++ Example

Promise in one thread must set a value or an exception once and only once.

The future bound to that promise can get the value or exception.

#include <future>	// std::promise, std::future
#include <iostream>
#include <thread>

int main()
{
	std::promise<int> promise;
	std::future<int> future = promise.get_future();

	std::thread thread{
		[promise = std::move(promise)]
			mutable
		{
			try
			{
				std::string get;
				std::cout << "Input: " << std::flush;
				std::getline(std::cin, get);
				int value = std::stoi(get);
				if (get != std::to_string(value))
					throw std::runtime_error{
						"std::stoi error, Invalid input."
					};
				promise.set_value(value);
			}
			catch (...)
			{
				promise.set_exception(std::current_exception());
			}
		}
	};

	try
	{
		int value = future.get();
		std::cout << "Get: " << value << std::endl;
	}
	catch (const std::exception & e)
	{
		std::cerr << "Error: " << e.what() << std::endl;
	}

	thread.join();
}

c++ Example 2

#include <future>
#include <iostream>
#include <thread>
#include <random>

int main()
{
	std::promise<void> promise;
	std::future<void> future = promise.get_future();

	static std::mt19937 rnd{std::mt19937_64{std::random_device{}()}()};

	std::thread thread{
		[promise = std::move(promise)]
			mutable
		{
			unsigned long value = rnd()%2u;
			if (value %2 == 1)
				promise.set_value();
			else
				promise.set_exception(
					std::make_exception_ptr(
						std::runtime_error{"Even for test"}
					)
				);
		}
	};

	try
	{
		future.get();
		std::cout << "OK" << std::endl;
	}
	catch (const std::exception & e)
	{
		std::cerr << "Error: " << e.what() << std::endl;
	}

	thread.join();
}

///////////////////////////////////////////////////////////////////////////////

Mon Oct 12 01:21:01 PM UTC 2026

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