In Objective-C, what does a comma do when used as a statement separator?

Question

I am looking though some source code from a third party and am repeatedly seeing a syntax that is new to me. Basically they are separating statements with commas instead of semicolons. It compiles and works, but I don't understand what it is doing. It looks like so

if(url)[url release], url = nil;

and they also use it without the if sometimes

[url release], url = nil;

What's going on here?

Answer 1

As in C and C++, the comma operator computes the thing on its left side and then computes the thing on the right; the overall value of the expression is the value of the right side. Basically, this lets a single expression do two things (the one on the left side being presumably for side effects such as a method call or assignment). Yes, the syntax is somewhat ambiguous with that used in function calls and variable declarations.

I prefer to use an honest block containing multiple statements where possible. Longer, but ultimately cleaner. Easier to debug too.

Answer 2

This is just a combined assignment statement.

Does this look familiar to you?

int a, b = 1;

Answer 3

These are comma separated expressions and are evaluated from left to right, with the result of the entire expression being the last expression evaluated.