(Not So) Silly Objective-C inheritance problem when using property - GCC Bug?

Question

Update - Many people are insisting I need to declare an iVar for the property. Some are saying not so, as I am using Modern Runtime (64 bit). I can confirm that I have bee

Answer 1

I think this is the bug of GCC 4.2.1. I made the file foo.m with the content

#import 
@interface TestA : NSObject {
    NSString *testString;
}
@end

@implementation TestA  
@end

@interface TestB : TestA {
}
@property (retain) NSString *testProp;
@end

@implementation TestB
@synthesize testProp;
- (void)testing{
NSLog(@"test ivar is %@", testString);
}
@end

Note that it's OK in the 64 bit mode to omit the instance variable. My GCC 4.2.1 on OS X 10.6.3 gave me an error:

$ gcc -arch x86_64 -c foo.m
aho.m: In function ‘-[TestB testing]’:
aho.m:19: error: ‘testString’ undeclared (first use in this function)
aho.m:19: error: (Each undeclared identifier is reported only once
aho.m:19: error: for each function it appears in.)

This compiled without problem by changing

NSLog(@"test ivar is %@", testString);

to

NSLog(@"test ivar is %@", self->testString);

Clang compiled it without any problem.

( In the 32 bit mode, I got

$ gcc -arch i386 -c foo.m
aho.m:17: error: synthesized property ‘testProp’ must either be named 
the same as a compatible ivar or must explicitly name an ivar
aho.m: In function ‘-[TestB testing]’:
aho.m:19: error: ‘testString’ undeclared (first use in this function)
aho.m:19: error: (Each undeclared identifier is reported only once
aho.m:19: error: for each function it appears in.)

which is a perfectly expected behavior, as Manjunath wrote.)

However I think it's generally a rather bad idea to access an instance variable of the superclass: when you implement the methods the superclass, you cannot assume anything about the instance variable because it might be tweaked in a worst manner possible by the subclass. You at least need to write down what kind of operation on the instance variable is permitted or not... Remember you might need to maintain your code for years! I would prefer keeping programming contracts between various parts of the code at the level of methods and properties.

Finally you should change

@property NSString *testProp;

to

@property (copy) NSString *testProp;

or at least to

@property (retain) NSString *testProp;

if you're not using GC on OS X. Otherwise EXP_BAD_ACCESS will await you!