What is a coroutine?

Coroutines focus on:

How a function can pause and later resume execution from the point at which it paused.

It is a control flow tool; it is not equivalent to a thread, nor does it inherently imply parallelism.

1.An intuitive understanding

A normal function call is usually:

进入函数
一直执行
返回结果
函数结束

Coroutines are more like:

进入函数
执行一段
暂停
之后恢复
继续执行
再暂停或结束

This capability is very suitable for expressing asynchronous flows.

2.A coroutine is not a thread

Threads are scheduled by the operating system, and multiple threads may run simultaneously on multiple CPU cores.

Coroutines are usually cooperative:

协程自己在合适的位置暂停
调度器再恢复其他协程

Therefore:

多个协程可以在一个线程里交错执行
协程本身不保证并行

Only if the coroutine is backed by a thread pool or an asynchronous runtime can it be combined with parallel execution.

3.C++ Coroutine keywords

C++20 introduced language-level coroutine support, with common keywords including:

• co_await: wait for an awaitable object, and possibly suspend the current coroutine;
• co_yield: yield a value, and suspend the current coroutine;
• co_return: return a result from the coroutine.

However, C++ the standard library only provides the low-level capabilities of the coroutine mechanism and does not directly provide a complete high-level asynchronous framework.

Therefore, when learning C++ coroutines, it is usually also necessary to understand them in conjunction with a specific library or runtime.

4.What scenarios are coroutines suitable for?

Coroutines are commonly found in:

• 异步 IO；
• 网络服务器；
• 定时任务；
• 生成器；
• 状态机；
• 需要把回调式代码写成顺序流程的场景。

5.Direction of the rest of this chapter

Currently, reserve a place for the coroutine concept first.

If further additions are made later, they can be expanded from these questions:

1. co_awaitWhat exactly are we waiting for?
2. What is the promise type?
3. What is a coroutine handle?
4. What is the difference between a generator and a task?
5. How do coroutines combine with asynchronous IO and thread pools?

It will be much easier if you first understand that "a coroutine is a control flow that can be suspended and resumed," and then look at the specific syntax.