What are the advantages of squashing assignment and error checking in one line?

Question

This question is inspired by this question, which features the following code snippet.

int s;
if((s = foo()) == ERROR)
    print_error();

I find this style hard to read and prone to error (as the original question demonstrates -- it was prompted by missing parentheses around the assignment). I would instead write the following, which is actually shorter in terms of characters.

int s = foo();
if(s == ERROR)
    print_error();

This is not the first time I've seen this idiom though, and I'm guessing there are reasons (perhaps historical) for it being so often used. What are those reasons?

Answer 1

I think it's for hysterical reasons, that early compilers were not so smart at optimizing. By putting it on one line as a single expression, it gives the compiler a hint that the same value fetched from foo() can be tested rather than specifically loading the value from s.

I prefer the clarity of your second example, with the assignment and test done later. A modern compiler will have no trouble optimizing this into registers, avoiding unnecessary loads from memory store.