Odd variable assignment

Question

Good day, everyone.

I've come across a peculiar piece of code today, which I don't quite understand. I don't even know how to search for this particular problem.

In this code, which works, a variable assignment is done like this:

if(condition) {
    Var1 = false, Var2 = false;
}

Now, I was under the impression, that ALL commands need to be terminated by a semicolon instead of a comma. I am familiar with the syntax

Var1 = Var2 = false;

but not with the one posted above. The compiler (g++) doesn't even throw me a warning or anything...am I missing something from the specification here? Or is the compiler generous with me and just replaces the , with a ; internally? If so, shouldn't he at least throw a warning?

Thank you for your time.

http://stackoverflow.com/questions/1613230/uses-of-c-comma-operator — Alexandru Barbarosie, Oct 01 '13 at 08:54
@AlexandruBarbarosie Ah, thank you very much. I was at an absolute loss as to what to search for. I'll check that link right away. — ATaylor, Oct 01 '13 at 08:56

score 1 · Answer 1 · answered Oct 01 '13 at 08:58

1

In the C and C++ programming languages, the comma operator (represented by the token ,) is a binary operator that evaluates its first operand and discards the result, and then evaluates the second operand and returns this value (and type). (read more)

answered Oct 01 '13 at 08:58

Lajos Arpad

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score 1 · Answer 2 · edited May 23 '17 at 10:32

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As Alexandru Barbarosie pointed out, there's quite a thorough explanation on what's happening at https://stackoverflow.com/questions/1613230/uses-of-c-comma-operator

To quickly summarize it for whomever stumbles across this post: When used outside of for loops and stuff, the , actually has the same effect as the ;.

For more information, please visit the link.

edited May 23 '17 at 10:32

Community

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answered Oct 01 '13 at 09:00

ATaylor

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score 1 · Accepted Answer · answered Oct 01 '13 at 09:06

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am I missing something from the specification here?

Yes, it's the "comma operator", specified by C++11 5.18. It evaluates the sub-expression to the left, then the one to the right, and the overall result is that of the right-hand one.

In this case, it's equivalent to two expression statements separated by ;

It's useful in places like if/while/for where you're only allowed one expression, but might want to do more than one thing:

while (++i, --j != 0)

and also if you like to jam multiple statements together to make life difficult for whoever has to read your code.

answered Oct 01 '13 at 09:06

Mike Seymour

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Hmm, this one's interesting. I may be able to use it in the future. You learn something new every day. Thank you. :) – ATaylor Oct 01 '13 at 09:07
@ATaylor You don't want to use it (except maybe for obfuscation). Mike made that quite clear. There are very few, if any valid uses of the comma operator. – James Kanze Oct 01 '13 at 09:12
@JamesKanze Yeah, I came to the same conclusion by myself. Still, thank you for the heads up. – ATaylor Oct 01 '13 at 09:13

Odd variable assignment

3 Answers3