[Learnbyexamples]

Comma-separated expressions were inherited from C. It’s likely that you use such expressions in for- and while-loops rather often. Yet, the language rules in this regard are far from being intuitive. First, let’s see what a comma separated expression is.
An expression may consist of one or more sub-expressions separated by commas. For example:

Code:

if(++x, --y, cin.good()) /*three expressions*/

The if condition contains three expressions separated by commas. C++ ensures that each of the expressions is evaluated and its side effects take place. However, the value of an entire comma-separated expression is only the result of the rightmost expression. Therefore, the if condition above evaluates as true only if cin.good() returns true. Here’s another example of a comma expression:

Code:

int j=10;  
int i=0;  
while( ++i, --j)  
{  
 /*..repeat as long as j is not 0*/  
}

[Ken Peterson]

The C++ standard guarantees that each of the expressions shall be evaluated and its side effects shall take place. However, the value of an entire comma-separated expression is only the result of the rightmost expression. Thus, the if condition evaluates as true only if cin.good() returns true. Comma separated expressions are used in loops.