C++20 Coroutines: Stackless Asynchronous Execution & Generator Pipelines

A standard function in C++ runs from start to finish and cannot be paused. A Coroutine is a generalized function that can suspend execution to yield control back to the caller, and later resume from the exact point of suspension with all local variables preserved.

The 3 Coroutine Keywords

co_yield expr;: Produces a value to the caller and pauses execution. Perfect for infinite number generators and streams.
co_await expr;: Suspends execution until an asynchronous task (like a disk read or HTTP network request) finishes.
co_return expr;: Returns a final value and terminates the coroutine frame.

Frequently Asked Questions (FAQ) - Student & Interview Review

Q1. What makes a function a Coroutine in C++20?

A function automatically becomes a coroutine if its body contains any of the three coroutine keywords: 1. co_await (suspends execution until an event completes), 2. co_yield (produces a value and suspends), or 3. co_return (finishes execution and returns a final value).

Q2. Are C++20 Coroutines stackful or stackless?

C++20 Coroutines are STACKLESS! When suspended, their local variables and execution state are preserved in a tiny heap-allocated 'Coroutine Frame' rather than holding onto an OS thread stack frame, allowing millions of concurrent coroutines on a single machine.