Strange C/C++ syntax

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What's this C++ syntax that puts a brace-surrounded block where an expression is expected?

I've just come across this strange C/C++ syntax:

#include <stdio.h>
int main() {
    printf("%s",
        ({
        static char b__[129];
        b__[0] = 55;
        b__[1] = 55;
        b__[2] = 0;
        b__;
        })
    );
}

This compiles and runs fine using both gcc and g++ (4.5.2). This is the first time I see something like this, and I wonder what exactly this syntax means. I've tried to Google it, but I have no idea what this construct is called.

Well, I've come across this piece of code at: http://gcc.gnu.org/bugzilla/show_bug.cgi?id=50179. — enobayram, Feb 24 '12 at 14:02

score 27 · Accepted Answer · answered Feb 24 '12 at 13:44

27

They're called statement expressions, it's a GNU extension. In your example the result of the expression is b__.

answered Feb 24 '12 at 13:44

cnicutar

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I would add that they are very convenient when used in macros, since they allow macros to act (almost) like functions returning a result. – Blagovest Buyukliev Feb 24 '12 at 13:52
@BlagovestBuyukliev Yup, I especially like the `maxint` macro example in the docs. – cnicutar Feb 24 '12 at 13:53
So is that how shorthand lambdas work? – Lee Louviere Feb 24 '12 at 14:02

Strange C/C++ syntax

1 Answers1