Instance variables in implementation files — private vs. protected

Question

In Objective-C, what is the actual difference between putting an instance variable in .h files vs. putting it in .m files? Is the end-result any different? What particular mechanism allows for this? How come this is possible in Objective-C and not in, say, C++?

If I adopt the style of putting instance variables in .m files, this is only suitable for private instance variables, right? I should always put my protected instance variables in .h files so they can be properly inherited by subclasses, right? Or is there a way to let protected ivars in .m files be inherited somehow?

Sergey Kalinichenko · Accepted Answer · 2012-03-02 16:46:53Z

2

Putting additional instance variables in class extensions was not possible prior to Clang/LLVM 2.0. Objective C compiler "assembles" the ivars of the class from the definitions in the header and in the class extension, arriving at the final layout.

Class extensions is a relatively new language feature of Objective-C, so its absence in other languages, such is C++, should come as no surprise. Yet other languages, such as C#, may implement a comparable feature with partial classes.

You are correct about the second part: only private instance variables and properties are suitable for use in class extensions; protected and of course public instance variables should be in the .h file.

answered Mar 2, 2012 at 16:46

Sergey Kalinichenko

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