cocoa - Objective-C: instance variables out of scope in debugger

Question

I have a superclass and a subclass, both of which define instance variables.

Rough outline of superclass:

/* GenericClass.h */
@interface GenericClass : NSObject {
    /* some variables */
}
@end
/* GenericClass.m */
@implementation GenericClass
    /* ... */
@end

Outline of subclass:

/* SpecificClass.h */
#import "GenericClass.h"
@interface SpecificClass : GenericClass {
    NSMutableString *str;
}
/* SpecificClass.m */
#import "SpecificClass.h"
@implementation SpecificClass
- (void)aMethod {
    //Debugger reports str as out of scope
    str = [[NSMutableString alloc] initWithCapacity:100];
    //Works fine:
    self->str = [[NSMutableString alloc] initWithCapacity:100];
    //Doesn't compile as I haven't defined @property/@synthesize:
    self.str = [[NSMutableString alloc] initWithCapacity:100];
}

When I am using classes that inherit directly from NSObject, one doesn't need the self-> pointer. Note that there is no object with the name str defined in the parent GenericClass. So, my question is, why is str out of scope when not referenced as self->str? The code in itself works, but I can't read the variable with the debugger

Answer 1

GDB is not an Objective-C compiler. The compiler knows about things like lexical scope within Objective-C methods, but GDB does not. It does, however, understand local variables.

In Objective-C, every method has an implicit self parameter passed to it when it's called. So when you look at self->str, GDB is interpreting that like it would interpret any other local variable evaluation.

When you try to evaluate str on its own, GDB will look for a local variable called str and, not finding one, reports that it's not in scope. This is not an error; this is the expected behavior.