Uses of C comma operator [duplicate]

Answer 1

For me the one really useful case with commas in C is using them to perform something conditionally.

  if (something) dothis(), dothat(), x++;

this is equivalent to

  if (something) { dothis(); dothat(); x++; }

This is not about "typing less", it's just looks very clear sometimes.

Also loops are just like that:

while(true) x++, y += 5;

Of course both can only be useful when the conditional part or executable part of the loop is quite small, two-three operations.

Answer 2

You can overload it (as long as this question has a "C++" tag). I have seen some code, where overloaded comma was used for generating matrices. Or vectors, I don't remember exactly. Isn't it pretty (although a little confusing):

MyVector foo = 2, 3, 4, 5, 6;

Answer 3

It's very useful in adding some commentary into ASSERT macros:

ASSERT(("This value must be true.", x));

Since most assert style macros will output the entire text of their argument, this adds an extra bit of useful information into the assertion.

Answer 4

qemu has some code that uses the comma operator within the conditional portion of a for loop (see QTAILQ_FOREACH_SAFE in qemu-queue.h). What they did boils down to the following:

#include <stdio.h>

int main( int argc, char* argv[] ){
  int x = 0, y = 0;

  for( x = 0; x < 3 && (y = x+1,1); x = y ){
    printf( "%d, %d\n", x, y );
  }

  printf( "\n%d, %d\n\n", x, y );

  for( x = 0, y = x+1; x < 3; x = y, y = x+1 ){
    printf( "%d, %d\n", x, y );
  }

  printf( "\n%d, %d\n", x, y );
  return 0;
}

... with the following output:

0, 1
1, 2
2, 3

3, 3

0, 1
1, 2
2, 3

3, 4

The first version of this loop has the following effects:

• It avoids doing two assignments, so the chances of the code getting out of sync is reduced
• Since it uses &&, the assignment is not evaluated after the last iteration
• Since the assignment isn't evaluated, it won't try to de-reference the next element in the queue when it's at the end (in qemu's code, not the code above).
• Inside the loop, you have access to the current and next element

Answer 5

The only time I have ever seen the , operator used outside a for loop was to perform an assingment in a ternary statement. It was a long time ago so I cannot remeber the exact statement but it was something like:

int ans = isRunning() ? total += 10, newAnswer(total) : 0;

Obviously no sane person would write code like this, but the author was an evil genius who construct c statements based on the assembler code they generated, not readability. For instance he sometimes used loops instead of if statements because he preferred the assembler it generated.

His code was very fast but unmaintainable, I am glad I don't have to work with it any more.

Answer 6

It is sometimes used in macros, such as debug macros like this:

#define malloc(size) (printf("malloc(%d)\n", (int)(size)), malloc((size)))

(But look at this horrible failure, by yours truly, for what can happen when you overdo it.)

But unless you really need it, or you are sure that it makes the code more readable and maintainable, I would recommend against using the comma operator.