Question

I am learning object orientated programming from the online Stanford courses there is a part I am unsure of regarding declarations. I thought that you must always declare the prototype in the header and then write the code in the implementation file, but the professor wrote a method in the implementation without a declaration prototype in the header file, how come?

Also, may someone please clear the difference between private and public and if the method without a prototype is public or private? The method without a prototype is not from a super class.

Answer 1

That is a perfectly legal way to declare methods that are not to be used outside the class implementation itself.

The compiler will find methods in the implementation file as long as they precede the method in which they are used. However that will not always be the case, as the new LLVM compiler allows methods to be declared in any order and referenced from a given file.

There are a couple of different styles for declaring methods inside an implementation file:

//In the Header File, MyClass.h
@interface MyClass : NSObject
@end

//in the implementation file, MyClass.m
//Method Decls inside a Private Category
@interface MyClass (_Private)
    - (void)doSomething;
@end

//As a class extension (new to LLVM compiler)
@interface MyClass ()
    - (void)doSomething;
@end

@implementation MyClass
//You can also simply implement a method with no formal "forward" declaration
//in this case you must declare the method before you use it, unless you're using the
//latest LLVM Compiler (See the WWDC Session on Modern Objective C)
- (void)doSomething {
}

- (void)foo {
    [self doSomething];
}
@end