1.16 Objective-C++

gcc is at once a compiler for C, Objective-C, and C++. You can intermix C++ and Objective-C code to some degree. To instruct the compiler that a file contains C++ code as well as Objective-C, use the file extension .mm or .M instead of .m.

Following are some ways in which C++ and Objective-C code can be used together:

• Objective-C objects can have fields that point to C++ objects, and vice versa.

• Objective-C code can call methods on C++ objects, and vice versa.

• Objective-C objects can have C++ objects (as opposed to pointers) as fields, but only if the C++ class has no virtual methods.

However, Objective-C and C++ are not completely compatible. Here are some things you can't do:

• Objective-C classes can't inherit from C++ classes, and vice versa.

• You can't declare Objective-C classes in C++ namespaces or templates, or vice versa.

• You can't use C++ keywords for Objective-C variable names.

• You can't call Objective-C methods with C++ syntax, or vice versa.

Finally, there are some restrictions that are imposed to avoid ambiguity:

• You can't use the name id as a C++ template name. If you could, the declaration id<TypeName> var could be either a C++ template declaration or an Objective-C declaration using a protocol.

• If you are passing a globally-scoped C++ variable to an Objective-C method, you need a space between the first and second colons. For example:

  [obj methodName: ::aGlobal].