Question

I’ve recently (only on SO actually) run into uses of the C/C++ comma operator. From what I can tell, it creates a sequence point on the line between the left and right hand side operators so that you have a predictable (defined) order of evaluation.

I’m a little confused about why this would be provided in the language as it seems like a patch that can be applied to code that shouldn’t work in the first place. I find it hard to imagine a place it could be used that wasn’t overly complex (and in need of refactoring).

Can someone explain the purpose of this language feature and where it may be used in real code (within reason), if ever?

Answer 1

It can be useful in the condition of while() loops:

while (update_thing(&foo), foo != 0) {
    /* ... */
}

This avoids having to duplicate the update_thing() line while still maintaining the exit condition within the while() controlling expression, where you expect to find it. It also plays nicely with continue;.

It’s also useful in writing complex macros that evaluate to a value.