comma-operator

In C.1.3 of the C++ IS (2003. It's in the C++11 IS, too), the standard points out a difference between ISO C and C++; namely, for char arr[100]; sizeof(0, arr) returns sizeof(char*) in C, but 100 in C++. I can find no documentation for sizeof taking two arguments. The obvious fallback is the comma operator, but I don't think so: sizeof(arr) in C is 100 ; sizeof(0, arr) is sizeof(char*) . Both sizeof(0, arr) and sizeof(arr) are 100 in C++. I may be missing the whole point of the IS in this context. Can anyone help? This is similar to a question discussed back in '09, but no one referred to the

I saw this code: if (cond) { perror("an error occurred"), exit(1); } Why would you do that? Why not just: if (cond) { perror("an error occurred"); exit(1); } In your example it serves no reason at all. It is on occasion useful when written as if(cond) perror("an error occured"), exit(1) ; -- then you don't need curly braces. But it's an invitation to disaster. The comma operator is to put two or more expressions in a position where the reference only allows one. In your case, there is no need to use it; in other cases, such as in a while loop, it may be useful: while (a = b, c < d) ... where

I would like to increment two variables in a for -loop condition instead of one. So something like: for (int i = 0; i != 5; ++i and ++j) do_something(i, j); What is the syntax for this? A common idiom is to use the comma operator which evaluates both operands, and returns the second operand. Thus: for(int i = 0; i != 5; ++i,++j) do_something(i,j); But is it really a comma operator? Now having wrote that, a commenter suggested it was actually some special syntactic sugar in the for statement, and not a comma operator at all. I checked that in GCC as follows: int i=0; int a=5; int x=0; for(i; i