Why no switch on pointers?

Question

For instance:

#include <stdio.h>  void why_cant_we_switch_him(void *ptr) {     switch (ptr) {         case NULL:             printf("NULL!\n");             break;         default:             printf("%p!\n", ptr);             break;     } }  int main(void) {     void *foo = "toast";     why_cant_we_switch_him(foo);     return 0; }  gcc test.c -o test test.c: In function 'why_cant_we_switch_him': test.c:5: error: switch quantity not an integer test.c:6: error: pointers are not permitted as case values 

Just curious. Is this a technical limitation?

EDIT

People seem to think there is only one constant pointer expression. Is that is really true, though? For instance, here is a common paradigm in Objective-C (it is really only C aside from NSString, id and nil, which are merely a pointers, so it is still relevant — I just wanted to point out that there is, in fact, a common use for it, despite this being only a technical question):

#include <stdio.h> #include <Foundation/Foundation.h>  static NSString * const kMyConstantObject = @"Foo";  void why_cant_we_switch_him(id ptr) {     switch (ptr) {         case kMyConstantObject: // (Note that we are comparing pointers, not string values.)             printf("We found him!\n");             break;         case nil:             printf("He appears to be nil (or NULL, whichever you prefer).\n");             break;         default:             printf("%p!\n", ptr);             break;     } }  int main(void) {     NSString *foo = @"toast";     why_cant_we_switch_him(foo);     foo = kMyConstantObject;     why_cant_we_switch_him(foo);      return 0; }  gcc test.c -o test -framework Foundation test.c: In function 'why_cant_we_switch_him': test.c:5: error: switch quantity not an integer test.c:6: error: pointers are not permitted as case values 

It appears that the reason is that switch only allows integral values (as the compiler warning said). So I suppose a better question would be to ask why this is the case? (though it is probably too late now.)

Answer 1

Because there is only one constant pointer expression

Given that only a single constant pointer expression exists, the switch statement has little to offer pointer expressions. You have cited essentially the only possible construction.

Answer 2

A switch compares the variable with a set of compile-time constants. Other than null, I can't see any valid compile time constants that you might compare a pointer with. For example:

switch (ptr) {     case &var1: printf ("Pointing to var1"); break;    case &var2: printf ("Pointing to var2"); break; } 

var1 and var2 are likely different in each run of the program, and would not be compile time constants. One possibility might be that they are addresses of memory-mapped ports that are always fixed, but otherwise I don't see how you could easily expand this from your two cases (null / not-null).