Programming iOS 4

Header File and Implementation File

It’s perfectly possible for the interface and implementation of a class to appear in the same file, or for multiple classes to be defined in a single file, but this is not the usual convention. The usual convention is one class, two files: one file containing the interface section, the other file containing the implementation section. For example, let’s suppose you are defining a class MyClass. Then you have two files, MyClass.h and MyClass.m. (The file naming is not magical or necessary; it’s just part of the convention. The file extensions are pretty much necessary, though, because the build process and Xcode itself rely on them.) The interface section goes into MyClass.h, which is called the header file. The implementation section goes into MyClass.m, which is called the implementation file. This separation into two files is not inconvenient, because Xcode, expecting you to follow this convention, makes it easy to jump from editing a .h file to the corresponding .m file and vice versa (Navigate → Jump to Next Counterpart; in Xcode 3.2.x, View → Switch to Header/Source File). Finally, the implementation file imports the header file (see Chapter 1 on the #import directive); this effectively unites the full class definition, making the definition legal even though it is split between two files, and allowing the implementation section to “see” any method declarations in the interface section.

With this arrangement in place, further imports become ...