Uses of C comma operator

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.

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