Using the Comma Operator

Operators will be covered later in this chapter; however, it is useful to introduce the comma operator here. You can have a sequence of expressions separated by a comma as a single statement. For example, the following code is legal in C++:

    int a = 9;
    int b = 4;
    int c;
    c = a + 8, b + 1;

The writer intended to type c = a + 8 / b + 1; and : they pressed comma instead of a /. The intention was for c to be assigned to 9 + 2 + 1, or 12. This code will compile and run, and the variable c will be assigned with a value of 17 (a + 8). The reason is that the comma separates the right-hand side of the assignment into two expressions, a + 8 and b + 1, and it uses the value of the first expression to assign c. Later in this chapter, we will look at operator precedence. However, it is worth saying here that the comma has the lowest precedence and + has a higher precedence than =, so the statement is executed in the order of the addition: the assignment and then the comma operator (with the result of b + 1 thrown away).

You can change the precedence using parentheses to group expressions. For example, the mistyped code could have been as follows:

    c = (a + 8, b + 1);

The result of this statement is: variable c is assigned to 5 (or b + 1). The reason is that with the comma operator expressions are executed from left to right so the value of the group of expressions is the tight-most one. There are some cases, for example, in the initialization or loop expression of a for loop, where you will find the comma operator useful, but as you can see here, even used intentionally, the comma operator produces hard-to-read code.