Why no switch on pointers?
- by meeselet
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.)
