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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?
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The comma operator in c comes with the lowest precedence in the C language. The comma operator is basically a binary operator that initially operates the first available operand, discards the obtained result from it, evaluates the operands present after this, and then returns the result/value accordingly.
The comma operator ( , ) evaluates each of its operands (from left to right) and returns the value of the last operand. This lets you create a compound expression in which multiple expressions are evaluated, with the compound expression's final value being the value of the rightmost of its member expressions.
Use a comma after the if-clause when the if-clause precedes the main clause. If I'd had time, I would have cleaned the house. If the main clause precedes the if-clause, no punctuation is necessary. I would have cleaned the house if I'd had time.
On the left-hand side of an assignment, the comma indicates that sequence unpacking should be performed according to the rules you quoted: a will be assigned the first element of the tuple, b the second.
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.
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These are comma separated expressions and are evaluated from left to right, with the result of the entire expression being the last expression evaluated.
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