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The All-in-One Guide to C++20

The Workflow

Explore the detailed workflow of C++20 coroutines, including the promise and awaiter mechanisms. Understand how the compiler transforms coroutine code, manages suspension points, handles exceptions, and coordinates execution flow to enable asynchronous and event-driven programming.

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The compiler transforms your coroutine and runs two workflows: the outer promise workflow and the inner awaiter workflow.

The promise workflow

When you use co_yield, co_await, or co_return in a function, the function becomes a coroutine, and the compiler transforms its body to something equivalent to the following lines.

C++
{
    Promise prom;
    co_await prom.initial_suspend();
    try {
        <function body having co_return, co_yield, or co_wait>
    }
    catch (...) {
        prom.unhandled_exception();
    }
    FinalSuspend:
        co_await prom.final_suspend();
}

The compiler automatically runs the transformed code using the functions of the promise object. In short, I call this workflow the ...