Re: Using C++ classes from Objective C

FROM : John Stiles
DATE : Thu Mar 20 17:32:56 2008

Without starting a religious war, I have to disagree with this.

ObjC++ is probably a bad idea if you are a novice programmer in general,
but I think it also has some really good things going for it, and having
written huge amounts of ObjC++ code, I think it's perfectly
straightforward to use. It is by far your best bet for writing
cross-platform code that uses a native Cocoa front end.


Rob Napier wrote:
> On Wed, Mar 19, 2008 at 4:12 PM, Jeremy <<email_removed>> wrote:
> 

>> Hi.
>>
>>  I am just starting to learn Cocoa and would like to use standard C++
>>  classes from my Objective C/C++ classes.
>>   

>
> You really don't. You think you do (probably because you know C++),
> but then you enter the crazy messed up world of Objective-C++ and
> discover that you really wish you hadn't. A bit of an overstatement,
> yes, but really I'd recommend against using ObjC++ unless you have
> existing C++ code that you have to bridge to, and then it's best used
> just to wrap the C++ classes so that ObjC can deal with them. Learn
> Cocoa in pure ObjC.
>
> ObjC and C++ have very different models for managing memory, types,
> pointers and well, just about everything. Memory management in
> particular is a real hassle. ObjC++ strips away the safety nets from
> both ObjC and C++ because now you have to track two very different
> ways of managing memory (retain counting versus ref variables for
> instance).
>
> 

>>  Is there any known documentation on how to do this, or does anyone
>>  have any pointers?
>>
>>  I tried creating a new object of my C++ class and calling a method on
>>  it in a .m file and received a bunch of errors (including new not
>>  being found).
>>
>>  I then changed my .m file to a .mm file and all of the errors went
>>  away except for one:
>>  "cannot find interface declaration for 'MyClass'"
>>   

>
> This probably means that ObjC expects there to be an ObjC class called
> MyClass (rather than a C++ class called MyClass; they're completely
> unrelated class structures).  I suspect that you accidentally put
> "@class MyClass" somewhere rather than "class MyClass", or possibly
> wrote "@interface MyClass" somewhere. Alternately, you failed to put
> "class MyClass" above the declaration of m_pMemberVariable and so the
> compiler assumed MyClass was an ObjC class (this is less likely since
> you should have gotten a compiler error elsewhere for that error).
>
> I strongly recommend naming your ObjC and ObjC++ classes differently.
> I typically name the ObjC++ classes ending in "Wrapper" but that's
> because I only ever use these classes to wrap existing C++ classes
> defined in separate C++-only projects.
>
> Also, make sure that you're working in the right namespace. You may
> need to say "new myNamespace::MyClass" here.
>
> ObjC++ is deep magic. It is not a good place to learn Cocoa. It is a
> useful tool once you know Cocoa and need to integrate it at key points
> with C++.
>
> -Rob
>