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Objective-C++ Tips

May 22nd, 2008

C++ Objects as Instance Data

Say you create a custom view with arbitrary many tracking rectangles (i.e. dynamically added).

Each time you add a rectangle you get back an identifier for this rectangle which can’t be stored in an NSArray as-is since it is of the primitive type NSTrackingRectTag (an integer).

If you use Objective-C++ then you can use a std::vector<NSTrackingRectTag> to avoid having to box/unbox your identifiers but if you have tried to put non-POD in the interface declaration of your Objective-C class you have probably seen that gcc does not like that.

Well, starting with 10.4 (so actually, some time ago) Apple added a switch to gcc which allows C++ objects as part of the instance data, and it will call both constructor and destructor for your C++ objects when allocating/deallocating the Objective-C object.

The flag you need to set is -fobjc-call-cxx-cdtors.

C++ Objects as Method Arguments

Occasionally it is convenient to pass a C++ object to an Objective-C method. For example I have an NSString initializer that takes a std::string as argument.

This works as long as you pass the object as a reference (i.e. pass a pointer), but you can use the “reference of” operator in the method signature rather than at the call-site. By using a const reference it will work for temporary/implicit objects.

So with the following method:

+ (NSString*)stringWithCxxString:(std::string const&)cxxString
{
   return [[[NSString alloc] initWithBytes:cxxString.data()
                                    length:cxxString.size()
                                  encoding:NSUTF8StringEncoding] autorelease];
}

We can have code like this:

std::string dir  = get_some_dir();
std::string file = get_some_file();

NSString* str    = [NSString stringWithCxxString:dir + file];

[by Allan Odgaard]


2 Responses to “Objective-C++ Tips”

  1. Pedro Says:
    September 27th, 2008 at 20:36

    Where do you put the -fobjc-call-cxx-cdtors option in the Project settings? Other C++ flags field?

  2. Allan Odgaard Says:
    September 27th, 2008 at 20:56

    That would be a good place, yes.


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