Obj-c method override/polymorphism problem - objective-c

Question

Ok, so I'm using Objective-C. Now, say I have:
TopClass : NSObject
- (int) getVal {return 1;}
MidClass : TopClass
- (int) getVal {return 2;}
BotClass : MidClass
- (int) getVal {return 3;}
I then put objects of each type into an NSMutableArray and take one out. What I want to do is run the getVal func on the appropriate object type, but when I put
id a = [allObjects objectAtIndex:0];
if ([a isKindOfClass:[TopClass class]])
{
int i;
i = [a getVal];
}
I get firstly a warning about multiple methods called getVal (presumably because the compiler can't determine the actual object type until runtime). But more seriously I also get an error "void value not ignored as it should be" and it won't compile.
If I don't try and use the return from [a getVal] then it compiles fine e.g.
[a getval]; //obviously no good if I want to use the return value
It will also work if I use isMemberOfClass statements to cast the object to a class before running the function e.g.
if ([a isMemberOfClass:[BotClass]) i = [(BotClass*) a getVal];
But surely I shouldn't have to do this to get the functionality I require? Otherwise I'll have to put in a statement for every single subclass, and worse have to add a new line if I add a new sub class, which rather defeats the point of method overriding doesn't it?
Surely there is a better way?

Answer 1

Since a BotClass is a MidClass and a MidClass is a TopClass, you could just set the type of a to TopClass*.
TopClass* a = [allObjects objectAtIndex:0];
if ([a isKindOfClass:[TopClass class]]) {
int i;
i = [a getVal];
}
The better way is to add -getVal to the #interface and #import it. Then the compiler will know such this method is likely to return an int and won't complain even if a is an id. Make sure the method name won't coincide with others, though.
(BTW, in ObjC, getters won't be named as -getFoo. The convention is just call it -foo.)

Answer 2

Your code compiles without any warning:
#import <Foundation/Foundation.h>
#interface TopClass : NSObject
{
}
- (int) getVal;
#end
#interface MidClass : TopClass
{
}
- (int) getVal;
#end
#interface BotClass : MidClass
{
}
- (int) getVal;
#end
#implementation TopClass
- (int) getVal {
return 1;
}
#end
#implementation MidClass
- (int) getVal
{
return 2;
}
#end
#implementation BotClass
- (int) getVal
{
return 3;
}
#end
int main (int argc, const char * argv[]) {
NSAutoreleasePool * pool = [[NSAutoreleasePool alloc] init];
NSMutableArray *array = [NSMutableArray array];
[array addObject:[[[TopClass alloc]init]autorelease]];
[array addObject:[[[MidClass alloc]init]autorelease]];
[array addObject:[[[BotClass alloc]init]autorelease]];
for (int objectNumber = 0; objectNumber < [array count]; objectNumber++) {
id a = [array objectAtIndex:objectNumber];
if ([a isKindOfClass:[TopClass class]])
{
int i = [a getVal];
NSLog(#"%d",i);
}
}
[pool drain];
return 0;
}
The test "isKindOfClass" is always true in this case, as MidClass and BotClass inherit from TopClass.
Are you sure the compiler knows about the method signatures that you call?