objective-c - extension - objective c test private methods




Best way to define private methods for a class in Objective-C (8)

I just started programming Objective-C and, having a background in Java, wonder how people writing Objective-C programs deal with private methods.

I understand there may be several conventions and habits and think about this question as an aggregator of the best techniques people use dealing with private methods in Objective-C.

Please include an argument for your approach when posting it. Why is it good? Which drawbacks does it have (that you know of) and how you deal with them?


As for my findings so far.

It is possible to use categories [e.g. MyClass (Private)] defined in MyClass.m file to group private methods.

This approach has 2 issues:

  1. Xcode (and compiler?) does not check if you define all methods in private category in corresponding @implementation block
  2. You have to put @interface declaring your private category in the begin of MyClass.m file, otherwise Xcode complains with a message like "self may not respond to message "privateFoo".

The first issue can be worked around with empty category [e.g. MyClass ()].
The second one bothers me a lot. I'd like to see private methods implemented (and defined) near the end of the file; I do not know if that's possible.


As other people said defining private methods in the @implementation block is OK for most purposes.

On the topic of code organization - I like to keep them together under pragma mark private for easier navigation in Xcode

@implementation MyClass 
// .. public methods

# pragma mark private 
// ...

@end

Defining your private methods in the @implementation block is ideal for most purposes. Clang will see these within the @implementation, regardless of declaration order. There is no need to declare them in a class continuation (aka class extension) or named category.

In some cases, you will need to declare the method in the class continuation (e.g. if using the selector between the class continuation and the @implementation).

static functions are very good for particularly sensitive or speed critical private methods.

A convention for naming prefixes can help you avoid accidentally overriding private methods (I find the class name as a prefix safe).

Named categories (e.g. @interface MONObject (PrivateStuff)) are not a particularly good idea because of potential naming collisions when loading. They're really only useful for friend or protected methods (which are very rarely a good choice). To ensure you are warned of incomplete category implementations, you should actually implement it:

@implementation MONObject (PrivateStuff)
...HERE...
@end

Here's a little annotated cheat sheet:

MONObject.h

@interface MONObject : NSObject

// public declaration required for clients' visibility/use.
@property (nonatomic, assign, readwrite) bool publicBool;

// public declaration required for clients' visibility/use.
- (void)publicMethod;

@end

MONObject.m

@interface MONObject ()
@property (nonatomic, assign, readwrite) bool privateBool;

// you can use a convention where the class name prefix is reserved
// for private methods this can reduce accidental overriding:
- (void)MONObject_privateMethod;

@end

// The potentially good thing about functions is that they are truly
// inaccessible; They may not be overridden, accidentally used,
// looked up via the objc runtime, and will often be eliminated from
// backtraces. Unlike methods, they can also be inlined. If unused
// (e.g. diagnostic omitted in release) or every use is inlined,
// they may be removed from the binary:
static void PrivateMethod(MONObject * pObject) {
    pObject.privateBool = true;
}

@implementation MONObject
{
    bool anIvar;
}

static void AnotherPrivateMethod(MONObject * pObject) {
    if (0 == pObject) {
        assert(0 && "invalid parameter");
        return;
    }

    // if declared in the @implementation scope, you *could* access the
    // private ivars directly (although you should rarely do this):
    pObject->anIvar = true;
}

- (void)publicMethod
{
    // declared below -- but clang can see its declaration in this
    // translation:
    [self privateMethod];
}

// no declaration required.
- (void)privateMethod
{
}

- (void)MONObject_privateMethod
{
}

@end

Another approach which may not be obvious: a C++ type can be both very fast and provide a much higher degree of control, while minimizing the number of exported and loaded objc methods.


One more thing that I haven't seen mentioned here - Xcode supports .h files with "_private" in the name. Let's say you have a class MyClass - you have MyClass.m and MyClass.h and now you can also have MyClass_private.h. Xcode will recognize this and include it in the list of "Counterparts" in the Assistant Editor.

//MyClass.m
#import "MyClass.h"
#import "MyClass_private.h"

There is a benefit of private methods absence. You can move the logic that you intended to hide to the separate class and use it as delegate. In this case you can mark delegate object as private and it will not be visible from outside. Moving logic to the separate class (maybe several) makes better design of your project. Cause your classes become simpler and your methods are grouped in classes with proper names.


There isn't, as others have already said, such a thing as a private method in Objective-C. However, starting in Objective-C 2.0 (meaning Mac OS X Leopard, iPhone OS 2.0, and later) you can create a category with an empty name (i.e. @interface MyClass ()) called Class Extension. What's unique about a class extension is that the method implementations must go in the same @implementation MyClass as the public methods. So I structure my classes like this:

In the .h file:

@interface MyClass {
    // My Instance Variables
}

- (void)myPublicMethod;

@end

And in the .m file:

@interface MyClass()

- (void)myPrivateMethod;

@end

@implementation MyClass

- (void)myPublicMethod {
    // Implementation goes here
}

- (void)myPrivateMethod {
    // Implementation goes here
}

@end

I think the greatest advantage of this approach is that it allows you to group your method implementations by functionality, not by the (sometimes arbitrary) public/private distinction.


There's no way of getting around issue #2. That's just the way the C compiler (and hence the Objective-C compiler) work. If you use the XCode editor, the function popup should make it easy to navigate the @interface and @implementation blocks in the file.


You could try defining a static function below or above your implementation that takes a pointer to your instance. It will be able to access any of your instances variables.

//.h file
@interface MyClass : Object
{
    int test;
}
- (void) someMethod: anArg;

@end


//.m file    
@implementation MyClass

static void somePrivateMethod (MyClass *myClass, id anArg)
{
    fprintf (stderr, "MyClass (%d) was passed %p", myClass->test, anArg);
}


- (void) someMethod: (id) anArg
{
    somePrivateMethod (self, anArg);
}

@end

You could use blocks?

@implementation MyClass

id (^createTheObject)() = ^(){ return [[NSObject alloc] init];};

NSInteger (^addEm)(NSInteger, NSInteger) =
^(NSInteger a, NSInteger b)
{
    return a + b;
};

//public methods, etc.

- (NSObject) thePublicOne
{
    return createTheObject();
}

@end

I'm aware this is an old question, but it's one of the first I found when I was looking for an answer to this very question. I haven't seen this solution discussed anywhere else, so let me know if there's something foolish about doing this.







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