1.1.2 How about porting between Cocoa and GNUstep?

It’s easier from GNUstep to Cocoa than Cocoa to GNUstep. Cocoa is constantly changing, much faster than GNUstep could hope to keep up. They have added extensions and new classes that aren’t available in GNUstep yet. Plus there are some other issues. If you start with Cocoa:

• Use #ifndef GNUSTEP for Apple only code.
• Do not use CoreFoundation
• Do not use Objective-C++ (except with gcc 4.1 or later)
• Do not use Quicktime or other proprietary extension
• GNUstep should be able to read Cocoa nib files automatically, so there is no need to port these, although you might want to have GNUstep specific versions of them anyway.

See also http://mediawiki.gnustep.org/index.php/Writing_portable_code for more information.