Re: Private Methods

FROM : Jonathan Hess
DATE : Sat Feb 23 23:15:28 2008

On Feb 22, 2008, at 6:08 AM, Adam P Jenkins wrote:

> On Feb 22, 2008, at 6:48 AM, Rob Petrovec wrote:
>

>> Lack of private methods is a serious flaw in Obj-C IMO.

>
> This is silly.  Private is really just about as voluntary in C++ as 
> it is in ObjC.  If a user of your class wants to ignore your 
> decisions about what should be private methods, they have only to 
> write:
>
> #define private public
> #include "yourclass.h"
> #undef private
>
> void foo() {
>  YourClass obj;
>  obj.aPrivateMethod();  // no compiler errors or warnings
> }
>
> So declaring things private in C++ is just an advisory to users of 
> your class that certain methods and fields aren't intended to be 
> used by clients of the class, it's not meant as any kind of security 
> mechanism.  In ObjC you can achieve the same thing by not declaring 
> methods in the interface file, so if a user of your class still 
> finds out about the method somehow and calls it, they'll get a 
> compiler warning and will know they're doing something not intended 
> by the class's author.
>
> Note that this is in contrast to Java or C#, which run in managed 
> environments, and where private really can be used to fairly 
> securely block certain methods and fields from being accessed, when 
> using the correct security settings in the VM.
>
> That said I do agree that it's nicer to have a formal language 
> construct for specifying what's private, like in C++, than the ad-
> hoc mechanisms available in ObjC.  I just don't agree that it's 
> really a serious flaw, more of a minor 
> flaw._______________________________________________


If a method is private, why would you ever want to place it in a 
public header? I could see a justification for @proected applying to 
methods, but it seems much better to just not have the private methods 
in the interface at all. Putting a private method in the interface 
says "This exists, and you can't use it." If I can't use it, why are 
you telling me it is there?

I believe that in c++ the MyClass.h file is the *only* interface to 
your class. In objective-c, the MyClass.h file is simply the public 
interface to your class. You're free to have more than one @interface 
and you are encouraged to use them to declare things that shouldn't be 
part of your outward facing public interface.

Things would be best if we didn't even have to put our private 
instance variables in the header -
Jon Hess

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