Probably Dance

C++20 added minimal support for coroutines. I think they’re done in a way that really doesn’t fit into C++, mostly because they don’t follow the zero-overhead principle. Calling a coroutine can be very expensive (requiring calls to new() and delete()) in a way that’s not entirely under your control, and they’re designed to make it extra hard for you to control how expensive they are. I think they were inspired by C# coroutines, and the design does fit into C# much better. But in C++ I don’t know who they are for, or who asked for this…

Before we get there I’ll have to explain what they are and what they’re useful for. Briefly, they’re very useful for code with concurrency. The classic example is if your code has multiple state machines that change state at different times: E.g. the state machine for reading data from the network is waiting for more bytes, and the code that provides bytes is also a state machine (maybe it’s decompressing) which in turn gets its bytes from another state machine (maybe it’s handling the TCP/IP layer). This is easier to do when all of these can pretend to operate on their own, as if in separate threads, maybe communicating through pipes. But the code looks nicer if the streamer can just call into the decompressor using a normal synchronous function call that returns bytes. Coroutines allow you to do that without blocking your entire system when more bytes aren’t immediately available, because code can pause and resume in the middle of the function.

One of the best things the C++ standard did is to define the word “coroutine” as different from related concepts like “fibers” or “green threads”. (this very much went against existing usage, so for example Lua coroutines are not the same thing as C++ coroutines. I think that’s fine, since the thing that was added to C++ could be useful, and there is a good case to be made that the existing usage was wrong) In the standard, a coroutine is simply a function that behaves differently when called multiple times: Instead of restarting from the start on each call, it will continue running after the return statement that last returned. To do this, it needs some state to store the information of where it last returned, and what the state of the local variables was at that point. In existing usage that meant that you need a program stack to store this state, but in C++ this is just stored in a struct.

To illustrate all of this, lets build a coroutine in plain C++, without using the language coroutines: