Creating an abstract class in Objective-C

问题:

I\'m originally a Java programmer who now works with Objective-C. I\'d like to create an abstract class, but that doesn\'t appear to be possible in Objective-C. Is this possible?

If not, how close to an abstract class can I get in Objective-C?

回答1:

Typically, Objective-C class are abstract by convention only—if the author documents a class as abstract, just don\'t use it without subclassing it. There is no compile-time enforcement that prevents instantiation of an abstract class, however. In fact, there is nothing to stop a user from providing implementations of abstract methods via a category (i.e. at runtime). You can force a user to at least override certain methods by raising an exception in those methods implementation in your abstract class:

[NSException raise:NSInternalInconsistencyException 
            format:@\"You must override %@ in a subclass\", NSStringFromSelector(_cmd)];

If your method returns a value, it\'s a bit easier to use

@throw [NSException exceptionWithName:NSInternalInconsistencyException
                               reason:[NSString stringWithFormat:@\"You must override %@ in a subclass\", NSStringFromSelector(_cmd)]
                             userInfo:nil];

as then you don\'t need to add a return statement from the method.

If the abstract class is really an interface (i.e. has no concrete method implementations), using an Objective-C protocol is the more appropriate option.

回答2:

No, there is no way to create an abstract class in Objective-C.

You can mock an abstract class - by making the methods/ selectors call doesNotRecognizeSelector: and therefore raise an exception making the class unusable.

For example:

- (id)someMethod:(SomeObject*)blah
{
     [self doesNotRecognizeSelector:_cmd];
     return nil;
}

You can also do this for init.

回答3:

Just riffing on @Barry Wark\'s answer above (and updating for iOS 4.3) and leaving this for my own reference:

#define mustOverride() @throw [NSException exceptionWithName:NSInvalidArgumentException reason:[NSString stringWithFormat:@\"%s must be overridden in a subclass/category\", __PRETTY_FUNCTION__] userInfo:nil]
#define methodNotImplemented() mustOverride()

then in your methods you can use this

- (void) someMethod {
     mustOverride(); // or methodNotImplemented(), same thing
}

Notes: Not sure if making a macro look like a C function is a good idea or not, but I\'ll keep it until schooled to the contrary. I think it\'s more correct to use NSInvalidArgumentException (rather than NSInternalInconsistencyException) since that\'s what the runtime system throws in response to doesNotRecognizeSelector being called (see NSObject docs).

回答4:

The solution I came up with is:

1. Create a protocol for everything you want in your \"abstract\" class
2. Create a base class (or maybe call it abstract) that implements the protocol. For all the methods you want \"abstract\" implement them in the .m file, but not the .h file.
3. Have your child class inherit from the base class AND implement the protocol.

This way the compiler will give you a warning for any method in the protocol that isn\'t implemented by your child class.

It\'s not as succinct as in Java, but you do get the desired compiler warning.

回答5:

From the Omni Group mailing list:

Objective-C doesn\'t have the abstract compiler construct like Java at this time.

So all you do is define the abstract class as any other normal class and implement methods stubs for the abstract methods that either are empty or report non-support for selector. For example...

- (id)someMethod:(SomeObject*)blah
{
     [self doesNotRecognizeSelector:_cmd];
     return nil;
}

I also do the following to prevent the initialization of the abstract class via the default initializer.

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

回答6:

Instead of trying to create an abstract base class, consider using a protocol (similar to a Java interface). This allows you to define a set of methods, and then accept all objects that conform to the protocol and implement the methods. For example, I can define an Operation protocol, and then have a function like this:

- (void)performOperation:(id<Operation>)op
{
   // do something with operation
}

Where op can be any object implementing the Operation protocol.

If you need your abstract base class to do more than simply define methods, you can create a regular Objective-C class and prevent it from being instantiated. Just override the - (id)init function and make it return nil or assert(false). It\'s not a very clean solution, but since Objective-C is fully dynamic, there\'s really no direct equivalent to an abstract base class.

回答7:

This thread is kind of old, and most of what I want to share is already here.

However, my favorite method is not mentioned, and AFAIK there’s no native support in the current Clang, so here I go…

First, and foremost (as others have pointed out already) abstract classes are something very uncommon in Objective-C — we usually use composition (sometimes through delegation) instead. This is probably the reason why such a feature doesn’t already exist in the language/compiler — apart from @dynamic properties, which IIRC have been added in ObjC 2.0 accompanying the introduction of CoreData.

But given that (after careful assessment of your situation!) you have come to the conclusion that delegation (or composition in general) isn’t well suited to solving your problem, here’s how I do it:

1. Implement every abstract method in the base class.
2. Make that implementation [self doesNotRecognizeSelector:_cmd];…
3. …followed by __builtin_unreachable(); to silence the warning you’ll get for non-void methods, telling you “control reached end of non-void function without a return”.
4. Either combine steps 2. and 3. in a macro, or annotate -[NSObject doesNotRecognizeSelector:] using __attribute__((__noreturn__)) in a category without implementation so as not to replace the original implementation of that method, and include the header for that category in your project’s PCH.

I personally prefer the macro version as that allows me to reduce the boilerplate as much as possible.

Here it is:

// Definition:
#define D12_ABSTRACT_METHOD {\\
 [self doesNotRecognizeSelector:_cmd]; \\
 __builtin_unreachable(); \\
}

// Usage (assuming we were Apple, implementing the abstract base class NSString):
@implementation NSString

#pragma mark - Abstract Primitives
- (unichar)characterAtIndex:(NSUInteger)index D12_ABSTRACT_METHOD
- (NSUInteger)length D12_ABSTRACT_METHOD
- (void)getCharacters:(unichar *)buffer range:(NSRange)aRange D12_ABSTRACT_METHOD

#pragma mark - Concrete Methods
- (NSString *)substringWithRange:(NSRange)aRange
{
    if (aRange.location + aRange.length >= [self length])
        [NSException raise:NSInvalidArgumentException format:@\"Range %@ exceeds the length of %@ (%lu)\", NSStringFromRange(aRange), [super description], (unsigned long)[self length]];

    unichar *buffer = (unichar *)malloc(aRange.length * sizeof(unichar));
    [self getCharacters:buffer range:aRange];

    return [[[NSString alloc] initWithCharactersNoCopy:buffer length:aRange.length freeWhenDone:YES] autorelease];
}
// and so forth…

@end

As you can see, the macro provides the full implementation of the abstract methods, reducing the necessary amount of boilerplate to an absolute minimum.

An even better option would be to lobby the Clang team to providing a compiler attribute for this case, via feature requests. (Better, because this would also enable compile-time diagnostics for those scenarios where you subclass e.g. NSIncrementalStore.)

Why I Choose This Method

1. It get’s the job done efficiently, and somewhat conveniently.
2. It’s fairly easy to understand. (Okay, that __builtin_unreachable() may surprise people, but it’s easy enough to understand, too.)
3. It cannot be stripped in release builds without generating other compiler warnings, or errors — unlike an approach that’s based on one of the assertion macros.

That last point needs some explanation, I guess:

Some (most?) people strip assertions in release builds. (I disagree with that habit, but that’s another story…) Failing to implement a required method — however — is bad, terrible, wrong, and basically the end of the universe for your program. Your program cannot work correctly in this regard because it is undefined, and undefined behavior is the worst thing ever. Hence, being able to strip those diagnostics without generating new diagnostics would be completely unacceptable.

It’s bad enough that you cannot obtain proper compile-time diagnostics for such programmer errors, and have to resort to at-run-time discovery for these, but if you can plaster over it in release builds, why try having an abstract class in the first place?

回答8:

Using @property and @dynamic could also work. If you declare a dynamic property and don\'t give a matching method implementation, everything will still compile without warnings, and you\'ll get an unrecognized selector error at runtime if you try to access it. This essentially the same thing as calling [self doesNotRecognizeSelector:_cmd], but with far less typing.

回答9:

In Xcode (using clang etc) I like to use __attribute__((unavailable(...))) to tag the abstract classes so you get an error/warning if you try and use it.

It provides some protection against accidentally using the method.

Example

In the base class @interface tag the \"abstract\" methods:

- (void)myAbstractMethod:(id)param1 __attribute__((unavailable(\"You should always override this\")));

Taking this one-step further, I create a macro:

#define UnavailableMacro(msg) __attribute__((unavailable(msg)))

This lets you do this:

- (void)myAbstractMethod:(id)param1 UnavailableMacro(@\"You should always override this\");

Like I said, this is not real compiler protection but it\'s about as good as your going to get in a language that doesn\'t support abstract methods.

回答10:

The answer to the question is scattered around in the comments under the already given answers. So, I am just summarising and simplifying here.

Option1: Protocols

If you want to create an abstract class with no implementation use \'Protocols\'. The classes inheriting a protocol are obliged to implement the methods in the protocol.

@protocol ProtocolName
// list of methods and properties
@end

Option2: Template Method Pattern

If you want to create an abstract class with partial implementation like \"Template Method Pattern\" then this is the solution. Objective-C - Template methods pattern?

回答11:

Another alternative

Just check the class in the Abstract class and Assert or Exception, whatever you fancy.

@implementation Orange
- (instancetype)init
{
    self = [super init];
    NSAssert([self class] != [Orange class], @\"This is an abstract class\");
    if (self) {
    }
    return self;
}
@end

This removes the necessity to override init

回答12:

(more of a related suggestion)

I wanted to have a way of letting the programmer know \"do not call from child\" and to override completely (in my case still offer some default functionality on behalf of the parent when not extended):

typedef void override_void;
typedef id override_id;

@implementation myBaseClass

// some limited default behavior (undesired by subclasses)
- (override_void) doSomething;
- (override_id) makeSomeObject;

// some internally required default behavior
- (void) doesSomethingImportant;

@end

The advantage is that the programmer will SEE the \"override\" in the declaration and will know they should not be calling [super ..].

Granted, it is ugly having to define individual return types for this, but it serves as a good enough visual hint and you can easily not use the \"override_\" part in a subclass definition.

Of course a class can still have a default implementation when an extension is optional. But like the other answers say, implement a run-time exception when appropriate, like for abstract (virtual) classes.

It would be nice to have built in compiler hints like this one, even hints for when it is best to pre/post call the super\'s implement, instead of having to dig through comments/documentation or... assume.

回答13:

If you are used to the compiler catching abstract instantiation violations in other languages, then the Objective-C behavior is disappointing.

As a late binding language it is clear that Objective-C cannot make static decisions on whether a class truly is abstract or not (you might be adding functions at runtime...), but for typical use cases this seems like a shortcoming. I would prefer the compiler flat-out prevented instantiations of abstract classes instead of throwing an error at runtime.

Here is a pattern we are using to get this type of static checking using a couple of techniques to hide initializers:

//
//  Base.h
#define UNAVAILABLE __attribute__((unavailable(\"Default initializer not available.\")));

@protocol MyProtocol <NSObject>
-(void) dependentFunction;
@end

@interface Base : NSObject {
    @protected
    __weak id<MyProtocol> _protocolHelper; // Weak to prevent retain cycles!
}

- (instancetype) init UNAVAILABLE; // Prevent the user from calling this
- (void) doStuffUsingDependentFunction;
@end

---

//
//  Base.m
#import \"Base.h\"

// We know that Base has a hidden initializer method.
// Declare it here for readability.
@interface Base (Private)
- (instancetype)initFromDerived;
@end

@implementation Base
- (instancetype)initFromDerived {
    // It is unlikely that this becomes incorrect, but assert
    // just in case.
    NSAssert(![self isMemberOfClass:[Base class]],
             @\"To be called only from derived classes!\");
    self = [super init];
    return self;
}

- (void) doStuffUsingDependentFunction {
    [_protocolHelper dependentFunction]; // Use it
}
@end

---

//
//  Derived.h
#import \"Base.h\"

@interface Derived : Base
-(instancetype) initDerived; // We cannot use init here :(
@end

---

//
//  Derived.m
#import \"Derived.h\"

// We know that Base has a hidden initializer method.
// Declare it here.
@interface Base (Private)
- (instancetype) initFromDerived;
@end

// Privately inherit protocol
@interface Derived () <MyProtocol>
@end

@implementation Derived
-(instancetype) initDerived {
    self= [super initFromDerived];
    if (self) {
        self->_protocolHelper= self;
    }
    return self;
}

// Implement the missing function
-(void)dependentFunction {
}
@end

回答14:

Probably this kind of situations should only happen at development time, so this might work:

- (id)myMethodWithVar:(id)var {
   NSAssert(NO, @\"You most override myMethodWithVar:\");
   return nil;
}

回答15:

You can use a method proposed by @Yar (with some modification):

#define mustOverride() @throw [NSException exceptionWithName:NSInvalidArgumentException reason:[NSString stringWithFormat:@\"%s must be overridden in a subclass/category\", __PRETTY_FUNCTION__] userInfo:nil]
#define setMustOverride() NSLog(@\"%@ - method not implemented\", NSStringFromClass([self class])); mustOverride()

Here you will get a message like:

<Date> ProjectName[7921:1967092] <Class where method not implemented> - method not implemented
<Date> ProjectName[7921:1967092] *** Terminating app due to uncaught exception \'NSInvalidArgumentException\', reason: \'-[<Base class (if inherited or same if not> <Method name>] must be overridden in a subclass/category\'

Or assertion:

NSAssert(![self respondsToSelector:@selector(<MethodName>)], @\"Not implemented\");

In this case you will get:

<Date> ProjectName[7926:1967491] *** Assertion failure in -[<Class Name> <Method name>], /Users/kirill/Documents/Projects/root/<ProjectName> Services/Classes/ViewControllers/YourClass:53

Also you can use protocols and other solutions - but this is one of the simplest ones.

回答16:

Cocoa doesn’t provide anything called abstract. We can create a class abstract which gets checked only at runtime, and at compile time this is not checked.

回答17:

I usually just disable the init method in a class that I want to abstract:

- (instancetype)__unavailable init; // This is an abstract class.

This will generate an error at compile time whenever you call init on that class. I then use class methods for everything else.

Objective-C has no built-in way for declaring abstract classes.

回答18:

Changing a little what @redfood suggested by applying @dotToString\'s comment, you actually have the solution adopted by Instagram\'s IGListKit.

1. Create a protocol for all the methods that make no sense to be defined in the base (abstract) class i.e. they need specific implementations in the children.
2. Create a base (abstract) class that does not implement this protocol. You can add to this class any other methods that make sense to have a common implementation.
3. Everywhere in your project, if a child from AbstractClass must be input to or output by some method, type it as AbstractClass<Protocol> instead.

Because AbstractClass does not implement Protocol, the only way to have an AbstractClass<Protocol> instance is by subclassing. As AbstractClass alone can\'t be used anywhere in the project, it becomes abstract.

Of course, this doesn\'t prevent unadvised developers from adding new methods referring simply to AbstractClass, which would end up allowing an instance of the (not anymore) abstract class.

Real world example: IGListKit has a base class IGListSectionController which doesn\'t implement the protocol IGListSectionType, however every method that requires an instance of that class, actually asks for the type IGListSectionController<IGListSectionType>. Therefore there\'s no way to use an object of type IGListSectionController for anything useful in their framework.

回答19:

In fact, Objective-C doesn\'t have abstract classes, but you can use Protocols to achieve the same effect. Here is the sample:

CustomProtocol.h

#import <Foundation/Foundation.h>

@protocol CustomProtocol <NSObject>
@required
- (void)methodA;
@optional
- (void)methodB;
@end

TestProtocol.h

#import <Foundation/Foundation.h>
#import \"CustomProtocol.h\"

@interface TestProtocol : NSObject <CustomProtocol>

@end

TestProtocol.m

#import \"TestProtocol.h\"

@implementation TestProtocol

- (void)methodA
{
  NSLog(@\"methodA...\");
}

- (void)methodB
{
  NSLog(@\"methodB...\");
}
@end

回答20:

A simple example of creating an abstract class

// Declare a protocol
@protocol AbcProtocol <NSObject>

-(void)fnOne;
-(void)fnTwo;

@optional

-(void)fnThree;

@end

// Abstract class
@interface AbstractAbc : NSObject<AbcProtocol>

@end

@implementation AbstractAbc

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

-(void)fnOne{
// Code
}

-(void)fnTwo{
// Code
}

@end

// Implementation class
@interface ImpAbc : AbstractAbc

@end

@implementation ImpAbc

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

// You may override it    
-(void)fnOne{
// Code
}
// You may override it
-(void)fnTwo{
// Code
}

-(void)fnThree{
// Code
}

@end

回答21:

Can\'t you just create a delegate?

A delegate is like an abstract base class in the sense that you say what functions need to be defined, but you don\'t actually define them.

Then whenever you implement your delegate (i.e abstract class) you are warned by the compiler of what optional and mandatory functions you need to define behavior for.

This sounds like an abstract base class to me.