asio::co_spawn std::promise Exception Chain [c++]

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asio::co_spawn boost

c++ std::promise

asio::thread_pool

Exception Chain: exception propagation inside them

Namespace: boost::asio

asio::co_spawn

asio::co_spawn spawns a coroutine-based execution thread.

deduced_type asio::co_spawn(
	executor_or_context,
	awaitable_or_lambda,
	completion_token,
	constraint_ = 0
);

executor_or_context can be an executor or an object of asio::io_context.

awaitable_or_lambda can be an awaitable coroutine or a lambda object.

A lambda is a c++ object.

completion_token is a token, which will handle the notification when the execution has completed.

Handler Signature:

void(std::exception_ptr, T);

Token Signature:

void(std::exception_ptr, T);

std::promise

std::promise for cross-threading exception handling.

In my practice, the exception thrown in awaitable_or_lambda is propagated to completion_token, which is managed by asio::co_spawn itself. If the execution spawned by asio::co_spawn is a sub-thread, how to retrieve the exception from it? The way is std::promise .

c++ Example

#include <boost/asio.hpp>
#include <future>
#include <iostream>
#include <random>

namespace asio = boost::asio;

namespace ut
{
	inline std::mt19937 rnd{std::mt19937_64{std::random_device{}()}()};
}

int main()
{
	asio::thread_pool pool{32u};
	std::promise<void> promise;
	std::future<void> future = promise.get_future();

	asio::co_spawn(
		pool.get_executor(),
		[]
			->
				asio::awaitable<void>
		{
			if (ut::rnd() %2 == 0)
				throw std::runtime_error{
					"Not an error: just a test."
				};
			std::cout << "OK\n";
			co_return;
		},
		[promise = std::move(promise)]
		(std::exception_ptr eptr)	// The completion token
			mutable
		{
			if (eptr)
				promise.set_exception(eptr);
			else
				promise.set_value();
		}
	);

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

	pool.join();
}

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

Tue Oct 13 12:36:38 AM UTC 2026

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