Here is how I did it. I am sure it can be done easier.
I assume you want to create a cross-plattform class that can be consumed by both c++ and objective-c where parts have a plattform dependend implementation and parts have a plattform independend implementation.
The public C++ interface clients will consume is declared in Resource.h:
static File ResourcePathFromName(const String& name);
In a file named Resources_ios.mm implement the ResourcePathFromName method for the iOS plattform using any Objective-C classes from Apples SDKs you need. Include this file in your Xcode project.
In a file named Resources_win32.mm implement ResourcePathFromName for the windows plattform using for example the Windows-API. Include this file in your Visual Studio project.
Common implementation if you have one can be put in a file named Resources.cpp. Include both files, Resources.h and Resources.cpp in your Visual Studio and Xcode project.
Consuming the Resources class in C++ from Visual Studio is easy. We exposed a C++ interface so we can use it by simply including it when needed. Consuming from Objective-C++ with Xcode works the same way. Simply include it where you need the functionality. Objective-C++ are the files ending with .mm . Consuming from Objective-C is not possible. Objective-C are the files ending with .m . .m files can only consume Objective-C or pure C and we created a C++ interface.
Hope that helped a bit.