Is it possible to make the -init method private in Objective-C?

Question

I need to hide (make private) the -init method of my class in Objective-C.

How can I do that?

Answer 1

NS_UNAVAILABLE

- (instancetype)init NS_UNAVAILABLE;

This is a the short version of the unavailable attribute. It first appeared in macOS 10.7 and iOS 5. It is defined in NSObjCRuntime.h as #define NS_UNAVAILABLE UNAVAILABLE_ATTRIBUTE.

There is a version that disables the method only for Swift clients, not for ObjC code:

- (instancetype)init NS_SWIFT_UNAVAILABLE;

unavailable

Add the unavailable attribute to the header to generate a compiler error on any call to init.

-(instancetype) init __attribute__((unavailable("init not available")));  

If you don't have a reason, just type __attribute__((unavailable)), or even __unavailable:

-(instancetype) __unavailable init;  

doesNotRecognizeSelector:

Use doesNotRecognizeSelector: to raise a NSInvalidArgumentException. “The runtime system invokes this method whenever an object receives an aSelector message it can’t respond to or forward.”

- (instancetype) init {
    [self release];
    [super doesNotRecognizeSelector:_cmd];
    return nil;
}

NSAssert

Use NSAssert to throw NSInternalInconsistencyException and show a message:

- (instancetype) init {
    [self release];
    NSAssert(false,@"unavailable, use initWithBlah: instead");
    return nil;
}

raise:format:

Use raise:format: to throw your own exception:

- (instancetype) init {
    [self release];
    [NSException raise:NSGenericException 
                format:@"Disabled. Use +[[%@ alloc] %@] instead",
                       NSStringFromClass([self class]),
                       NSStringFromSelector(@selector(initWithStateDictionary:))];
    return nil;
}

[self release] is needed because the object was already allocated. When using ARC the compiler will call it for you. In any case, not something to worry when you are about to intentionally stop execution.

objc_designated_initializer

In case you intend to disable init to force the use of a designated initializer, there is an attribute for that:

-(instancetype)myOwnInit NS_DESIGNATED_INITIALIZER;

This generates a warning unless any other initializer method calls myOwnInit internally. Details will be published in Adopting Modern Objective-C after next Xcode release (I guess).

Answer 2

Apple has started using the following in their header files to disable the init constructor:

- (instancetype)init NS_UNAVAILABLE;

This correctly displays as a compiler error in Xcode. Specifically, this is set in several of their HealthKit header files (HKUnit is one of them).

Answer 3

Objective-C, like Smalltalk, has no concept of "private" versus "public" methods. Any message can be sent to any object at any time.

What you can do is throw an NSInternalInconsistencyException if your -init method is invoked:

- (id)init {
    [self release];
    @throw [NSException exceptionWithName:NSInternalInconsistencyException
                                   reason:@"-init is not a valid initializer for the class Foo"
                                 userInfo:nil];
    return nil;
}

The other alternative — which is probably far better in practice — is to make -init do something sensible for your class if at all possible.

If you're trying to do this because you're trying to "ensure" a singleton object is used, don't bother. Specifically, don't bother with the "override +allocWithZone:, -init, -retain, -release" method of creating singletons. It's virtually always unnecessary and is just adding complication for no real significant advantage.

Instead, just write your code such that your +sharedWhatever method is how you access a singleton, and document that as the way to get the singleton instance in your header. That should be all you need in the vast majority of cases.

Answer 4

You can declare any method to be not available using NS_UNAVAILABLE.

So you can put these lines below your @interface

- (instancetype)init NS_UNAVAILABLE;
+ (instancetype)new NS_UNAVAILABLE;

---

Even better define a macro in your prefix header

#define NO_INIT \
- (instancetype)init NS_UNAVAILABLE; \
+ (instancetype)new NS_UNAVAILABLE;

and

@interface YourClass : NSObject
NO_INIT

// Your properties and messages

@end

Answer 5

That depends on what you mean by "make private". In Objective-C, calling a method on an object might better be described as sending a message to that object. There's nothing in the language that prohibits a client from calling any given method on an object; the best you can do is not declare the method in the header file. If a client nevertheless calls the "private" method with the right signature, it will still execute at runtime.

That said, the most common way to create a private method in Objective-C is to create a Category in the implementation file, and declare all of the "hidden" methods in there. Remember that this won't truly prevent calls to init from running, but the compiler will spit out warnings if anyone tries to do this.

MyClass.m

@interface MyClass (PrivateMethods)
- (NSString*) init;
@end

@implementation MyClass

- (NSString*) init
{
    // code...
}

@end

There's a decent thread on MacRumors.com about this topic.

Answer 6

If you are talking about the default -init method then you can't. It's inherited from NSObject and every class will respond to it with no warnings.

You could create a new method, say -initMyClass, and put it in a private category like Matt suggests. Then define the default -init method to either raise an exception if it's called or (better) call your private -initMyClass with some default values.

One of the main reasons people seem to want to hide init is for singleton objects. If that's the case then you don't need to hide -init, just return the singleton object instead (or create it if it doesn't exist yet).

Answer 7

Put this in header file

- (id)init UNAVAILABLE_ATTRIBUTE;

Answer 8

well the problem why you can't make it "private/invisible" is cause the init method gets send to id (as alloc returns an id) not to YourClass

Note that from the point of the compiler (checker) an id could potencialy respond to anything ever typed (it can't check what really goes into the id at runtime), so you could hide init only when nothing nowhere would (publicly = in header) use a method init, than the compile would know, that there is no way for id to respond to init, since there is no init anywhere (in your source, all libs etc...)

so you cannot forbid the user to pass init and get smashed by the compiler... but what you can do, is to prevent the user from getting a real instance by calling a init

simply by implementing init, which returns nil and have an (private / invisible) initializer which name somebody else won't get (like initOnce, initWithSpecial ...)

static SomeClass * SInstance = nil;

- (id)init
{
    // possibly throw smth. here
    return nil;
}

- (id)initOnce
{
    self = [super init];
    if (self) {
        return self;
    }
    return nil;
}

+ (SomeClass *) shared 
{
    if (nil == SInstance) {
        SInstance = [[SomeClass alloc] initOnce];
    }
    return SInstance;
}

Note : that somebody could do this

SomeClass * c = [[SomeClass alloc] initOnce];

and it would in fact return a new instance, but if the initOnce would nowhere in our project be publicly (in header) declared, it would generate a warning (id might not respond ...) and anyway the person using this, would need to know exactly that the real initializer is the initOnce

we could prevent this even further, but there is no need

Answer 9

I have to mention that placing assertions and raising exceptions to hide methods in the subclass has a nasty trap for the well-intended.

I would recommend using __unavailable as Jano explained for his first example.

Methods can be overridden in subclasses. This means that if a method in the superclass uses a method that just raises an exception in the subclass, it probably won't work as intended. In other words, you've just broken what used to work. This is true with initialization methods as well. Here is an example of such rather common implementation:

- (SuperClass *)initWithParameters:(Type1 *)arg1 optional:(Type2 *)arg2
{
    ...bla bla...
    return self;
}

- (SuperClass *)initWithLessParameters:(Type1 *)arg1
{
    self = [self initWithParameters:arg1 optional:DEFAULT_ARG2];
    return self;
}

Imagine what happens to -initWithLessParameters, if I do this in the subclass:

- (SubClass *)initWithParameters:(Type1 *)arg1 optional:(Type2 *)arg2
{
    [self release];
    [super doesNotRecognizeSelector:_cmd];
    return nil;
}

This implies that you should tend to use private (hidden) methods, especially in initialization methods, unless you plan to have the methods overridden. But, this is another topic, since you don't always have full control in the implementation of the superclass. (This makes me question the use of __attribute((objc_designated_initializer)) as bad practice, although I haven't used it in depth.)

It also implies that you can use assertions and exceptions in methods that must be overridden in subclasses. (The "abstract" methods as in Creating an abstract class in Objective-C )

And, don't forget about the +new class method.