private_ivars.m

@import Foundation;

@interface A : NSObject
@end
@interface B : A
@end

// Private objc ivars are really private:
@implementation A
{
    int _ivar;
}
- (void) assign_inA { _ivar = 42; }
- (int) access_fromA { return _ivar; }
@end

@implementation B
{
    char* _ivar;
}
- (void) assign_inB { _ivar = "forty-two"; }
- (char*) access_fromB { return _ivar; }
@end

int main() {
    id obj = [B new];
    [obj assign_inA];
    [obj assign_inB];
    printf("%i, %s\n", [obj access_fromA], [obj access_fromB]);
    // prints "42, forty-two"
}

public_ivars.m

@import Foundation;

// On the other hand, overriding public ivars is forbidden:
@interface A : NSObject
{
    int _ivar
}
@end
@interface B : A
{
    char* _ivar; // error: duplicate member '_ivar'
}
@end