Using an Objective-C++ class with C++ instance variables in Objective-C code

Question

I'm writing an Objective-C++ class interface that has to be usable from both Objective-C and Objective-C++. The problem is that, because it must be usable from Objective-C, I cannot simply use a C++ type. I want to do it using pointers and I came up with this:

@interface SXDiff : NSObject {
@private
#ifdef __cplusplus
  dtl::Diff<std::string, std::vector<std::string> > *_diff;
#else
  void *_diff;
#endif
}

...

@end

Can any problems occur when doing this? Is there a better way of doing this?

Note that the use of pointers is just to get the size of the ivar to be the same in both Objective-C and Objective-C++. The ivar can only be used from within the class implementation of SXDiff (it's @private). The implementation is written in Objective-C++.

I have an application written in Objective-C and I want to use C++ code in just one of its classes. — user142019
– user142019, Commented Oct 18, 2011 at 14:03
Which compiler are you using? With LLVM 2.0+ you can declare your dtl::Diff in the implementation file using a class extension. — Joe
– Joe, Commented Oct 18, 2011 at 14:07
@Joe I am using Apple clang version 3.0, based on LLVM 3.0svn. — user142019
– user142019, Commented Oct 18, 2011 at 14:08
Why not avoid the #ifdef altogether and just use a void*. Another option would be to wrap the c++ class into a struct and use that for hiding (pimpl idiom). Since C knows structs and pointers to structs that should work, too. — cli_hlt
– cli_hlt, Commented Oct 18, 2011 at 14:08

Joe · Accepted Answer · 2011-10-18 14:13:55Z

5

With Apple LLVM version 2.0+ try moving all of your C++ code from the header into a class extension.

//SXDiff.h
@interface SXDiff : NSObject {
}

...

@end

//SXDiff.mm
#include "dtl/dtl.hpp"

@interface SXDiff()
{
    dtl::Diff<std::string, std::vector<std::string> > *_diff;
}
@end

@implementation SXDiff

...

@end

answered Oct 18, 2011 at 14:13

Joe

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2 Comments

user142019 Over a year ago

I didn't know this were possible, because GCC always gave me an error when trying that. clang doesn't. Thanks! This also eliminates the need for a pointer.

Rob Napier Over a year ago

BTW, an interesting new feature in the latest SDK: You can now add ivars using a {} block after @implementation rather than using a class extension.

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