Questions tagged [objective-c++]

Objective-C++ is the union of the Objective-C and C++ syntax sets. Objective-C is, by definition, a strict superset of the C language; C++ is similarly regarded as such in practice (albeit without qualifying as a strict superset, q.v. this illustrated answer) and the conceptual union of these two supersets is the basis for the Objective-C++ language.

The practical use of Objective-C++ is contingent on the compiler in question. Briefly, compiling in Objective-C++ mode puts all of the basic C-family compiler settings in play, plus all C++ extensions, plus all Objective-C extensions, plus its own niche concerns (e.g. its ABI corner-case behavior). Despite this preponderance of abusable options, it is pretty straightforward to compile Objective-C++, as part of a larger project (as with parts of OpenFrameworks) or as the basis for a project in its own right (i.e. Pivotal Labs’ Cedar).

For the most part, two languages can be fairly freely intermixed – one important caveat being a class definition in C++ cannot inherit from an Objective-C class; likewise, Objective-C classes can’t inherit from C++ classes.

One can define Objective-C classes with:

• pointer ivars for pointers to dynamically allocated instances of C++ types

• value ivars for inline storage of C++ data, for default-constructable types

• class and instance methods that receive and/or return C++ types

  • pass by value, by pointer, by reference (lvalue or rvalue‡)

• std::atomic values accessible concurrently from both NSThread and GCD alongside std::thread primitives‡

One can define C++ classes:

• containing pointer-to-Objective-C-class members (e.g. NSURL*, NSString*, NSImage* etc)
  • With caveats on object lifecycle management qua ARC and GC
• as templates accepting NSObject subclasses as typename parameters
• with method members that accept and/or return NSObject subclass instances
  • By pointer as per [OCType alloc] message return value
• with method template members that accept NSObject subclasses as typename parameters
• that use block pointers as expected (e.g. in typedef and using‡ declarations)

Notably, Objective-C++ does not allow for:

• Templated Objective-C class structures of any kind – template <…> declaration syntax is incompatible with the @implementation/@interface definition-marker grammar rules
• Templated Objective-C class or instance methods – e.g. the following code is tempting to try, but totally invalid:

@implementation NSSomeThing : NSObject

    template <typename T>
    + (T) getValue {
        return static_cast<T>(value);
    }

@end

… C++ templates and Objective-C classes use separate, incompatible preprocessing paths in their respective compilation processes.

‡ may be used with C++11 compiler support