Comma operator

In the C and C++ programming languages, the comma operator (represented by the token ,) is a binary operator that evaluates its first operand and discards the result, and then evaluates the second operand and returns this value (and type). The comma operator has the lowest precedence of any C operator, and acts as a sequence point.

The use of the comma token as an operator is distinct from its use in function calls and definitions, variable declarations, enum declarations, and similar constructs, where it acts as a separator.

In this example, the differing behavior between the second and third lines is due to the comma operator having lower precedence than assignment.

int a=1, b=2, c=3, i;   // comma acts as separator in this line, not as an operator
i = (a, b);             // stores b into i                                ... a=1, b=2, c=3, i=2
i = a, b;               // stores a into i. Equivalent to (i = a), b;     ... a=1, b=2, c=3, i=1
i = (a += 2, a + b);    // increases a by 2, then stores a+b = 3+2 into i ... a=3, b=2, c=3, i=5
i = a += 2, a + b;      // increases a by 2, then stores a = 5 into i   ... a=5, b=2, c=3, i=5
i = a, b, c;            // stores a into i                                ... a=5, b=2, c=3, i=5
i = (a, b, c);          // stores c into i                                ... a=5, b=2, c=3, i=3

A handy reminder is that

(a, b)

has the same effect as using C's ternary operator like

(a ? b : b)

Because the comma operator discards its first operand, it is generally only useful where the first operand has desirable side effects, such as in the initialiser or the counting expression of a for loop. For example, the following terse linked list cycle detection algorithm (a version of Floyd's "tortoise and hare" algorithm):

bool loops(List *list)
{
    List *tortoise, *hare; /* advance hare 2 times faster than tortoise */
    for (tortoise = hare = list;
                 hare && (hare = hare->next); /* tests for valid pointers + one step of hare */
                 tortoise = tortoise->next, hare = hare->next) /* comma separates hare and tortoise step */
        if (tortoise == hare) /* loop found */
            return true;
    return false;
}

In the OCaml and Ruby programming languages, the semicolon (";") is used for this purpose. JavaScript utilizes the comma operator in the same way C/C++ does.

See also

• Sequence point
• Operators in C and C++
• Operator (programming)
• Facility from the Boost library which makes it easy to fill containers using comma-operator