Re: Using C++ classes from Objective C

FROM : John Stiles
DATE : Thu Mar 20 18:07:52 2008

Wow. That is a really awesome trick there. Props to you!!
I wish Boost was easier to split apart so I didn't need to drag around
40MB of headers for just one feature…


Chris Meyer wrote:
> Don't listen to these C++ naysayers... we use it all the time very
> successfully.
>
> An easy way of reference counting using Obj-C++, I've used the boost
> libraries with boost::intrusive_ptr and these definitions:
>
> // file.h
>
>
> namespace boost
>
> {
>
>    void intrusive_ptr_add_ref( id object );
>
>    void intrusive_ptr_release( id object );
>
> }
>
>
> // file.m
>
> namespace boost
>
> {
>
>    void intrusive_ptr_add_ref( id object )
>
>    {
>
>        [object retain];
>
>    }
>
>
>    void intrusive_ptr_release( id object )
>
>    {
>
>        [object release];
>
>    }
>
> }
>
>
> Then you can do things like:
>
>    MyObjCObject *my_object = [[[MyObjCObject alloc] init] autorelease];
>
>    boost::intrusive_ptr< MyObjCObject> my_object_ptr = my_object;
>
> Now my_object_ptr can be safely stored in C++ classes, used in other
> classes such as
>
> stl::vector< boost::intrusive_ptr< MyObjCObject> > my_list;
>
> etc.
>
> Hope this helps.
>
>
> On Thu, Mar 20, 2008 at 9:40 AM, Jeremy <<email_removed>
> <mailto:<email_removed>>> wrote:
>
>    Thanks for the input guys, I really appreciate the help.
>
>    I do have a lot of C++ code that I would like to use and put behind a
>    Cocoa front end, so I think that using Objective C++ classes as
>    wrapper classes around my C++ classes is the route I would like to
>    take.
>
>    Anyone have an example of such a wrapper class so I can get an idea of
>    what is the preferred methodology?
>    I'm guessing that you create an Objective C++ class with a member
>    variable that is a C++ class and then mirror the methods from the C++
>    class?
>
>    I'm worried that I'll still end up having to import the header file
>    for my C++ class in my Objective C source file though, which caused a
>    lot of compile errors previously.
>
>
>    On Mar 20, 2008, at 12:32 PM, John Stiles wrote:
>
>    > 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>
>    <mailto:<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
>    >>
>
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