[MichaEiler]

There are a lot more possibilities. The reason why they are so complicated is simply to give the library developers all possible options. This features is afaik something targeted at them, so that boost, asio and others can implement powerful solutions without language restrictions.

The only way to enqueue a coroutine is to call schedule() within a co_await statement/expression. In this process the coroutine is suspended. Therefore there should not be any coroutine within the queue, which we cannot immediately resume.

I'm afraid I don't have any numbers available to compare coroutines with other approaches. But nevertheless in my opinion coroutines are benefitial because they keep their state (the stack frame, local variables) alive. If you use callbacks you would have to handle all these things yourself. Think about a generator for a sequence of numbers. You would have to store at least the counter variable manually. With a coroutine this happens automatically.