c++ - What is the proper use of the comma operator?

Question

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);
}

Answer 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 the actual "evaluation" of the while loop is governed solely on the last expression.