Objective-C - Private vs Protected vs Public

Question

I am hoping for some clarification on how Private vs Protected vs Public works with respect to class members when programming in Objective-C - I thought I knew the differenc

Answer 1

You are setting the visibilities of the ivars, not the properties. Your properties generate public getter and setter methods.

To make private properties, you can put the properties in a private category in the .m file.

Answer 2

The usual trick is to create a class extension inside the .m file and put your private/protected property there instead of in the header.

//Person.m

@interface Person()

@property float height

@end

this hides the 'height' property

Another trick is if you want to create a readonly property is to declare it in the header as

@property(readonly) int myproperty

but in the class extension as readwrite which allows your .m to modify the value using the getter/setter

@property(readwrite) int myproperty

Answer 3

visibility does not affect methods. methods are as good as public when visible to clients (and potential pitfalls/bugs when invisible to clients). instead, visibility affects instance variables. try this:

#import <Foundation/Foundation.h>
#import "Person.h"
#import "Man.h"


    int main (int argc, const char * argv[]) 
        {
            NSAutoreleasePool * pool = [[NSAutoreleasePool alloc] init];

            //Create a Person object
            Person * aPerson = [[Person alloc]init];

            //Create a Man object
            Man * aMan = [[Man alloc]init];


            //Let's attempt to modify our Person class members
            aPerson->height = 5.11; //Protected
            aPerson->age = 21; //Protected
            aPerson->yob = 2010; //Private
            aPerson->alive = YES; //Public


            //Let's now attempt to modify the same members via our
            //derived class Man - in theory, the private members should
            //not be accessible by the derived class man
            aMan->height = 6; //Protected
            aMan->age = 26; //Protected
            aMan->yob = 2011; //Private
            aMan->alive = YES; //Public
            aMan->mWeight = 190; //Protected member of Man Class



            [pool drain];
            return 0;
        }

this prevents the subclasses from accessing ivars directly -- forcing them and clients to use the accessors (if provided).

this is all a bit weak because categories allow clients to overcome this.

also, older 32 bit objc programs did't really check that the visibility was declared correctly. fortunately, that's been deprecated in 32 and an error in 64.

if you really want something to be private to subclasses and categories, use PIMPL with an unpublished/opaque type.

method visibility (as found in Java, C++, etc.) is a feature i'd use in objc.

Answer 4

You aren't accessing the members - you are accessing the property on Person which does not have the access level specified.