So I'm looking over C++ operator rules as I do when my programs start behaving wonkily. And I come across the comma operator. Now, I have known it was there for a while but never used it, so I began reading, and I come across this little gem:
if (int y = f(x), y > x){ // statements that use y}I had never thought about using commas' first arguments' side-effects to get locally-scoped variables without the need for bulky block-delimited code or repeated function calls. Naturally, this all excited me greatly, and I immediately ran off to try it.
test_comma.cpp: In function 'int main()':test_comma.cpp:9:18: error: expected ')' before ',' token if (int y = f(x), y > x) {I tried this on both a C and C++ compiler, and neither of them liked it. I tried instead declaring y in the outer scope, and it compiled and ran just fine without the int in the if condition, but that defeats the purpose of the comma here. Is this just a GCC implementation quirk? The opinion of the Internet seems to be that this should be perfectly valid C (and ostensibly, to my eye, C++) code; there is no mention of this error on any GCC or C++ forum that I've seen.
EDIT: Some more information. I am using MinGW GCC 4.8.1-4 on Windows 7 64-bit (though obviously my binaries are 32-bit; I need to install mingw-w64 one of these days).
I also tried using this trick outside of a conditional statement, as below:
int y = (int z = 5, z);This threw up two different errors:
test_comma.cpp: In function 'int main()':test_comma.cpp:9:11: error: expected primary-expression before 'int' int y = (int z = 5, z); ^test_comma.cpp:9:11: error: expected ')' before 'int'With creative use of parentheses in my if statement above, I managed to get the same errors there, too.