Comma operator

The comma operator separates expressions (which have value) in a way analogous to how the semicolon terminates statements, and sequences of expressions are enclosed in parentheses analogously to how sequences of statements are enclosed in braces:[1] (a, b, c) is a sequence of expressions, separated by commas, which evaluates to the last expression c, while {a; b; c;} is a sequence of statements, and does not evaluate to any value. A comma can only occur between two expressions – commas separate expressions – unlike the semicolon, which occurs at the end of a (non-block) statement – semicolons terminate statements.

The comma operator has the lowest precedence of any C operator, and acts as a sequence point. In a combination of commas and semicolons, semicolons have lower precedence than commas, as semicolons separate statements but commas occur within statements, which accords with their use as ordinary punctuation: a, b; c, d is grouped as (a, b); (c, d) because these are two separate statements.

The comma operator has been deprecated in subscripting expressions (as of C++20);[2] to reduce confusion, and open up the future possibility of repurposing the syntax for multidimensional array indexing. In C++23, the ability to overload operator[] with multiple arguments was added making unparenthesised comma expressions unusable in subscripts.[3] The comma operator is still usable and not deprecated in this context if the comma expression is surrounded by parentheses (as in a[(b,c)]).

In this example, the differing behavior between the second and third lines is due to the comma operator having lower precedence than assignment. The last example differs as well since the return expression must be fully evaluated before the function can return.

/**
 *  Commas act as separators in this line, not as an operator.
 *  Results: a=1, b=2, c=3, i=0
 */
int a=1, b=2, c=3, i=0;

/**
 *  Assigns value of b into i.
 *  Commas act as separators in the first line and as an operator in the second line.
 *  Results: a=1, b=2, c=3, i=2
 */
int a=1, b=2, c=3;              
int i = (a, b);           
                      
/**
 *  Assigns value of a into i.
 *  Equivalent to: int i = a; int b;
 *  Commas act as separators in both lines.
 *  The braces on the second line avoid variable redeclaration in the same block,
 *  which would cause a compilation error.
 *  The second b declared is given no initial value.
 *  Results: a=1, b=2, c=3, i=1
 */
int a=1, b=2, c=3;                                
{ int i = a, b; }

/**
 *  Increases value of a by 2, then assigns value of resulting operation a + b into i.
 *  Commas act as separators in the first line and as an operator in the second line.
 *  Results: a=3, b=2, c=3, i=5
 */
int a=1, b=2, c=3;
int i = (a += 2, a + b);
          
/**
 *  Increases value of a by 2, then stores value of a to i, and discards unused
 *  values of resulting operation a + b.
 *  Equivalent to: (i = (a += 2)), a + b;
 *  Commas act as separators in the first line and as an operator in the third line.
 *  Results: a=3, b=2, c=3, i=3
 */
int a=1, b=2, c=3;
int i;
i = a += 2, a + b;

/**
 *  Assigns value of a into i.
 *  Commas act as separators in both lines.
 *  The braces on the second line avoid variable redeclaration in the same block,
 *  which would cause a compilation error.
 *  The second b and c declared are given no initial value.
 *  Results: a=1, b=2, c=3, i=1
 */
int a=1, b=2, c=3;
{ int i = a, b, c; }

/**
 *  Commas act as separators in the first line and as an operator in the second line.
 *  Assigns value of c into i, discarding the unused a and b values.
 *  Results: a=1, b=2, c=3, i=3
 */
int a=1, b=2, c=3;
int i = (a, b, c);

/**
 *  Returns 6, not 4, since comma operator sequence points following the keyword 
 *  return are considered a single expression evaluating to rvalue of final 
 *  subexpression c=6.
 *  Commas act as operators in this line.
 */
return a=4, b=5, c=6;

/**
 *  Returns 3, not 1, for same reason as previous example.
 *  Commas act as operators in this line.
 */
return 1, 2, 3;

/**
 *  Returns 3, not 1, still for same reason as above. This example works as it does
 *  because return is a keyword, not a function call. Even though compilers will 
 *  allow for the construct return(value), the parentheses are only relative to "value"
 *  and have no special effect on the return keyword.
 *  Return simply gets an expression and here the expression is "(1), 2, 3".
 *  Commas act as operators in this line.
 */
return(1), 2, 3;

The comma operator has relatively limited use cases. Because it discards its first operand, it is generally only useful where the first operand has desirable side effects that must be sequenced before the second operand. Further, because it is rarely used outside of specific idioms, and easily mistaken with other commas or the semicolon, it is potentially confusing and error-prone. Nevertheless, there are certain circumstances where it is commonly used, notably in for loops and in SFINAE.[4] For embedded systems which may have limited debugging capabilities, the comma operator can be used in combination with a macro to seamlessly override a function call, to insert code just before the function call.

void rev(char *s, size_t len)
{
    char *first;
    for (first = s, s += len; s >= first; --s) {
        putchar(*s);
    }
}

++p, ++q;
++p; ++q;

#include <stdio.h>
#include <assert.h>

int main ( void )
{
    int i;
    for (i=0; i<=9; i++)
    {
        assert( ( "i is too big!", i <= 4 ) );
        printf("i = %i\n", i);
    }
    return 0;
}

i = 0
i = 1
i = 2
i = 3
i = 4
assert: assert.c:6: test_assert: Assertion `( "i is too big!", i <= 4 )' failed.
Aborted

if (y = f(x), y > x) {
    ... // statements involving x and y
}

if (failure)
    return (errno = EINVAL, -1);

if (failure) {
    errno = EINVAL;
    return -1;
}

if (x == 1) y = 2, z = 3;

if (x == 1)
    y = 2, z = 3;

if (x == 1) {y = 2; z = 3;}

if (x == 1) {
    y = 2; z = 3;
}

In the OCaml and Ruby programming languages, the semicolon (";") is used for this purpose. JavaScript[7] and Perl[8] utilize the comma operator in the same way C/C++ does. In Java, the comma is a separator used to separate elements in a list in various contexts.[9] It is not an operator and does not evaluate to the last element in the list.[10]

• Sequence point
• Operators in C and C++
• Operator (programming)

• "Effect of using a comma instead of a semi-colon in C and C++", Stack Overflow
• Boost.Assignment – facility from the Boost library that makes it easy to fill containers by overloading the comma operator