why objective-c does not support same method name in difference class? - objective-c

Question

Here is a code snippet from Learning objective-c 2.0
Full code:
ClassWithFloat.h
#import <Foundation/Foundation.h>
#interface ClassWithFloat : NSObject
{
float value;
}
-(void)setValue:(float)aValue;
#end
ClassWithFloat.m
#import "ClassWithFloat.h"
#implementation ClassWithFloat
-(void)setValue:(float)aValue
{
value = aValue;
}
#end
ClassWithInt.h
#import <Foundation/Foundation.h>
#interface ClassWithInt : NSObject
{
int value;
}
-(void)setValue:(int)aValue;
#end
ClassWithInt.m
#import "ClassWithInt.h"
#implementation ClassWithInt
-(void)setValue:(int)aValue
{
value = aValue;
}
#end
main.m:
#import <Foundation/Foundation.h>
#import "ClassWithFloat.h"
#import "ClassWithInt.h"
int main(int argc, const char * argv[])
{
#autoreleasepool {
id number = [[ClassWithInt alloc] init];
[number setValue:3];
}
return 0;
}
failed to compile, after changing to ClassWithInt* number it works.
Error message:
/Users/jcyangzh/src/oc/SameName/SameName/main.m:17:9: Multiple methods named 'setValue:' found with mismatched result, parameter type or attributes
But since objective-c is somehow a dynamic programming language, the message call will be translated to native C method call.
obj_msgSend(number, #selector(setValue:), 3)
the obj_msgSend method find the class structure for the number object by isa variable. Which should make no difference between id or ClassWithInt type.
Why objective-c compiler could not recognize the right method?
Note: I am asking this question, because having same method name, but different argument type for different class is reasonable to me. But it seems that it is not possible either because the compiler limitation or the language design (do not supporting method overloading etc).

Answer 1

The problem really is that your object is only typed as id within the lexical scope.
The compiler doesn't know which method of the same name/selector to use.
You have multiple classes that have that selector but with different signatures because their arguments are different types.
You should avoid id in this case
Or typecast your object in the message send brackets to tell the compiler what class's method to use
Or
Bracket the same message call repeatedly in a sequence of if ([obj isKindOf:
checks. (Crazy here)
Or
Best take a hint from NSNumber class on good method naming conventions and do something like setFloatValue: and setIntValue: which is more readable and clear and helps the compiler a bit.
But any time you have and id type only, you need to be checking if the object isKindOf: or you are asking for trouble.

Answer 2

It is very very bad to have methods with same name but different signatures. (It is documented somewhere but I can't find now)
The calling conversion between calling setValue:(float) is different to setValue:(int), compiler have to generate different binary code.
As you said, it end up with something like
obj_msgSend(number, #selector(setValue:), 3)
but they are different
obj_msgSend(number, #selector(setValue:), (int)3)
obj_msgSend(number, #selector(setValue:), (float)3.0f)
Compiler have to decide at compile-time to generate the which version. Because the calling conversion between pass parameter with int type and float type are different.
Given code
ClassWithInt *number = [[ClassWithInt alloc] init];
[number setValue:3];
Compile know it need to generate the version with int with the help of type information.
but without type information
id number = [[ClassWithInt alloc] init];
[number setValue:3]; // is this takes int or float? if it is float then 3 need to be convert to float value first
There are two possible way to call it. Compiler can't figure it out without help. Hence the error message.