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

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The Workflow

The Workflow

Discover a dynamic aspect of coroutines called workflow.

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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.

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{
    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 promise workflow. Here are the main steps of this workflow:

  • Coroutine begins execution

    • allocates the coroutine frame if necessary
    • copies all function parameters to the coroutine frame
    • Creates the prom object prom (line 2)
    • Calls prom.get_return_object() to create the coroutine handle, and keeps it in a local variable. The result of the call will be returned to the caller when the coroutine first suspends.
    • Calls prom.initial_suspend()and
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