Macro foreach loop in C

Question

I have this code

#define foreach(item, array) \
    for(int keep = 1, \
            count = 0,\
            size = sizeof (array) / sizeof *(array); \
        keep && count != size; \
        keep = !keep, count++) \
      for(item = (array) + count; keep; keep = !keep)

int main(void)
{
    int values[] = { 1, 2, 3 };

    foreach(int *v, values)
        printf("value: %d\n", *v);

    return 0;   
}

Can someone explain how it works this macro? especially this allocation keep = !keep what is this for

Answer 1

Expanding it out by hand:

foreach(int *v, values)
    printf("value: %d\n", *v);

becomes

for (int keep = 1, count = 0,  size = sizeof values / sizeof *(values); keep && count != size; keep = !keep, count++)
  for (int *v = (values) + count; keep; keep = !keep)
    printf("value: %d\n", *v);

The outer loop iterates over the array. The inner loop creates an item to which the current array element is assigned (int *v), and then the statements in the body of the foreach loop use that item (i.e., the body of the foreach loop becomes the body of the inner for loop). The keep flag makes sure that the inner loop only executes 1 time. Trace through the logic by hand for a few iterations and you should see the pattern.

Effectively, it's treating v as an iterator over the array values. As such, it's fairly clever; you can use this macro to iterate over an array of any type, such as

double dval[N];
...
foreach( double *d, dval )
  do_something_with( *d );

But...

As written, this code does have some pitfalls. It won't work with dynamically-allocated arrays:

int *vals = malloc( sizeof *vals * N );
...
foreach( int *v, vals ) // bzzzzt!
  printf( "%d\n", *v );

since sizeof vals only gives you the size of the pointer variable, not the size of the allocated buffer. Similarly, it won't work on any array passed as a function parameter.

Be very suspicious of any macro that attempts to extend language syntax; it's always a hack, and almost always has a fatal weakness or two.

Answer 2

The keep and keep = !keep is a trick, it can make break work in your foreach.

int *v;
foreach(v, values){
        printf("value: %d\n", *v);
        if(/*some condition*/){
            break;
        }
}
// After break, v can keep the status 
// just the same behaviour with for...break... in C language.

To get more details, please refer to Does C have a "foreach" loop construct?