I'm implementing an application where both instances of a class as well as the class itself have "children" (placed inside a NSMutableArray). It's a pretty complicated application, but thanks to Objective-C, it's a breeze: classes are themselves objects and can have methods and "variables" (with the help of static variables and such).
To make it myself easy, I implemented accessor/setter method on both my instances (using -) and classes (using +) to fetch and manipulate the "children" without having direct access to the arrays. I like to have my objects as much as possible closed and the methods do some data validation.
I also implemented a fast enumeration instance method from the NSFastEnumeration protocol. And here comes the question: can I implement a fast enumeration class method and use it with Objective-C's for...in construct? In order words, can I implement this:
+ (NSUInteger)countByEnumeratingWithState: (NSFastEnumerationState *)state objects: (__unsafe_unretained id [])buffer count: (NSUInteger)len;
And then use it somewhere like that:
for (id child in [MyClass class]) {
// Do magic stuff…
}
I peeked in GNUStep's GSFastEnumeration.h file which implements fast enumeration as a macro, which affirms the above is possible, but I'm not sure if Apple does the same.
Even if I can't associate the NSFastEnumeration protocol to my class object, is fast enumeration without that protocol possible (and future-proof)?
The method -countByEnumeratingWithState:objects:count: is the whole of fast enumeration—the protocol is, I believe, mostly there for description (it's easier to implement a protocol than declare the method with the correct signature). I would expect it to work just fine, but I don't have a reference for that. You would probably want to loop over [MyClass class], though.
I would probably consider it future-proof. Note that it'd be really trivial to make a tiny wrapper class around your class object that does nothing but implement NSFastEnumeration and forward the instance method -countByEnumeratingWithState:objects:count: to your class's method +countByEnumeratingWithState:objects:count:.
I would recommend creating a protocol with a class method that is identical to the NSFastEnumeration method. You could then iterate over the [MyClass class] as John Calsbeek mentioned.
//Protocol implementation
#protocol FastClassEnumeration <NSObject>
#required
+ (NSUInteger)countByEnumeratingWithState:(NSFastEnumerationState *)state objects:(id [])buffer count:(NSUInteger)len;
#end
//Class implementation
#interface EnumeratedClass : NSObject<FastClassEnumeration>
#end
#implementation EnumeratedClass
+ (NSUInteger)countByEnumeratingWithState:(NSFastEnumerationState *)state objects:(id [])buffer count:(NSUInteger)len
{
static const unsigned long items_length = 4;
static NSString * items[items_length] = { #"item1", #"item2", #"item3", #"item4" };
if(state->state >= items_length)
{
return 0;
}
state->itemsPtr = items;
state->state = items_length;
state->mutationsPtr = (unsigned long *)self;
return items_length;
}
#end
//Usage
...
for(NSString *item in [EnumeratedClass class])
{
NSLog(#"%#", item);
}
...
can I ... ?
Well, did you try it? Does it work? If you've tried it, you would notice that it does indeed compile and work.
And why shouldn't it? Class objects are objects just like other objects. Class methods are just methods that happen to be on the class object. If you send a message to a class object, it will call a class method; whereas if you send a message to a non-class object, it will call an instance method. So pretty much, you can put class methods on a class and use the class object the same way you can use a normal object by putting instance methods on its class.
The only possible difference is that the class object won't explicitly conform to the NSFastEnumeration protocol, similar to if you loop over a normal object whose class does not explicitly specify that it conforms to the NSFastEnumeration protocol. So the question is, do they check that an object explicitly conforms to the protocol before using it (as opposed to checking if it responds to the selector)?
In my experience, for pretty much all of Cocoa, for APIs that say they require an object that conforms to a protocol, you can give an object that does not explicitly conform to the protocol, but implements all of the protocol's methods, and it will work fine. (How would they check it anyway? If they use conformsToProtocol:, that won't work for class objects since there's a +conformsToProtocol:, which has a different meaning. They would have to use runtime functions or special-case class objects probably.) For example, the NSDictionary documentation says its keys are required to conform to NSCopying, but if you have an object that does not conform to NSCopying, but does implement copyWithZone:, it works fine. (In fact, there is a +copyWithZone: method, whose stated purpose is to allow class objects to be used as dictionary keys, so obviously it is intended that keys don't need to explicitly conform to NSCopying.)
The top voted answer to this SA question ( Objective C Static Class Level variables ) outlines my question quite well but to it, I'd like to add one more criteria:
Issue Description
You want your ClassA to have a ClassB class variable.
You are using Objective-C as programming language.
Objective-C does not support class variables as C++ does.
I want to access ClassA's class variable from subclass ClassASub
or even better
4a. I want ClassA's method to access the class variable as it is, overridden in ClassASub
Any ideas? Or is this just bending Objective-C one step too far?
Just make a normal getter method for your class variable, and you can override it in the subclass. Just remember to access it through the method.
static SomeClass *gClassVar;
#implementation ClassA
+ (SomeClass *)classVar {
if (!gClassVar)
gClassVar = ...;
return gClassVar;
}
+ (...)someMethod {
[[self classVar] doSomething];
}
#end
Then,
static SomeClass *gClassVar;
#implementation ClassASubclass
+ (SomeClass *)classVar {
if (!gClassVar)
gClassVar = ...;
return gClassVar;
}
#end
So, when you call [ClassA someMethod], it will operate on the ClassA instance of classVar. When you call [ClassASubclass someMethod], it will operate on the ClassASubclass instance.
The idea of having variables of any sort attached to an object (class or instance) is a feature that is kind of "stapled on" to Objective C. Any time you want to do anything object-oriented using Objective C, start by working with methods. (Almost) everything else is just syntactic sugar for things you can do with methods.
The concept of private / protected / public is somewhat alien to Objective C, even though access control is supported for member variables. The best you can do for methods is to define them in a separate header (and this applies to class variables and properties, if we implement both using methods).
I'd like to specify an Objective-C protocol with an optional routine. When the routine is not implemented by a class conforming to the protocol I'd like to use a default implementation in its place. Is there a place in the protocol itself where I can define this default implementation? If not, what is the best practice to reduce copying and pasting this default implementation all over the place?
Objective-C protocols have no affordance for default implementations. They are purely collections of method declarations that can be implemented by other classes. The standard practice in Objective-C is to test an object at runtime to see if it responds to the given selector before calling that method on it, using -[NSObject respondsToSelector:]. If e object does not respond to the given selector, the method isn't called.
One way you could achieve the result you're looking for would be to define a method encapsulating the default behavior you're looking for in the calling class, and call that method if the object doesn't pass the test.
Another approach would be to make the method be required in the protocol, and provide default implementations in the superclasses of any classes wherein you may not want to provide a specific implementation.
There are probably other options as well, but generally speaking there isn't a particular standard practice in Objective-C, except perhaps to just not call the given method if it hasn't been implement by the object, per my first paragraph, above.
There is no standard way for doing that as protocols should not define any implementations.
Since Objective-C comes with a neat runtime, you can of course add such a behavior if you really think you need to do it that way (and there's no possibility by achieving the same with inheritance).
Say you declared MyProtocol, then just add an interface with the same name in the .h file under your protocol declaration:
#interface MyProtocol : NSObject <MyProtocol>
+ (void)addDefaultImplementationForClass:(Class)conformingClass;
#end
And create a corresponding implementation file (using MAObjCRuntime for readability here, but the standard runtime functions wouldn't be much more code):
#implementation MyProtocol
+ (void)addDefaultImplementationForClass:(Class)conformingClass {
RTProtocol *protocol = [RTProtocol protocolWithName:#"MyProtocol"];
// get all optional instance methods
NSArray *optionalMethods = [protocol methodsRequired:NO instance:YES];
for (RTMethod *method in optionalMethods) {
if (![conformingClass rt_methodForSelector:[method selector]]) {
RTMethod *myMethod = [self rt_methodForSelector:[method selector]];
// add the default implementation from this class
[conformingClass rt_addMethod:myMethod];
}
}
}
- (void)someOptionalProtocolMethod {
// default implementation
// will be added to any class that calls addDefault...: on itself
}
Then you just have to call
[MyProtocol addDefaultImplementationForClass:[self class]];
in the initializer of your class conforming to the protocol and all default methods will be added.
A truly fascinating way is to use the runtime. At the start-up, very early in the program execution, do the following:
Enumerate all the classes, find classes which implement the protocol
Check if the class implements a method
If not, add to the class the default implementation
It can be achieved without that much trouble.
I agree with "w.m." A very nice solution is to put all the default implementations into an interface (with the same name as the protocol). In the "+initialize" method of any subclass it can simply copy any unimplemented methods from the default interface into itself.
The following helper functions worked for me
#import <objc/runtime.h>
// Get the type string of a method, such as "v#:".
// Caller must allocate sufficent space. Result is null terminated.
void getMethodTypes(Method method, char*result, int maxResultLen)
{
method_getReturnType(method, result, maxResultLen - 1);
int na = method_getNumberOfArguments(method);
for (int i = 0; i < na; ++i)
{
unsigned long x = strlen(result);
method_getArgumentType(method, i, result + x, maxResultLen - 1 - x);
}
}
// This copies all the instance methods from one class to another
// that are not already defined in the destination class.
void copyMissingMethods(Class fromClass, Class toClass)
{
// This gets the INSTANCE methods only
unsigned int numMethods;
Method* methodList = class_copyMethodList(fromClass, &numMethods);
for (int i = 0; i < numMethods; ++i)
{
Method method = methodList[i];
SEL selector = method_getName(method);
char methodTypes[50];
getMethodTypes(method, methodTypes, sizeof methodTypes);
if (![toClass respondsToSelector:selector])
{
IMP methodImplementation = class_getMethodImplementation(fromClass, selector);
class_addMethod(toClass, selector, methodImplementation, methodTypes);
}
}
free(methodList);
}
Then you call it in your class initializer such as...
#interface Foobar : NSObject<MyProtocol>
#end
#implementation Foobar
+(void)initialize
{
// Copy methods from the default
copyMissingMethods([MyProtocol class], self);
}
#end
Xcode will give you warnings about Foobar missing methods, but you can ignore them.
This technique only copies methods, not ivars. If the methods are accessing data members that do not exist, you could get strange bugs. You must ensure that the data is compatible with the code. It is as if you did a reinterpret_cast from Foobar to MyProtocol.
As Ryan mention there are no default implementations for protocols, another option to implementing in the superclass would be is to implement a "Handler" kind of class that can be contained in any class that want to provide the default implementation, the appropriate method then calls the default handlers implementation.
I ended up creating a macro that has a default implementation of the method.
I've defined it in the protocol's header file, and then it's just a one-liner in each implementation.
This way, I do not have to change the implementation several places, and it's done on compile time, so no run-time magic is necessary.
I've been searching around Apple's delegation and protocol documentation for an answer to this, but after more than a day I've decided to give up and let you guys have a shot at it. I have three classes: HTTPManager, LoginManager, and FetchManager. You can probably guess what these classes do, but to be explicit...
HTTPManager - Wraps NSURLConnection and provides a simple interface for LoginManager and FetchManager to do HTTP requests with authentication.
LoginManager / FetchManager - Basically the same class, but they respond to HTTPManager's messages differently.
HTTPManager expects a delegate to implement the HTTPManagerDelegate protocol and both LoginManager and FetchManager do this. The Login- and FetchManager classes also provide a protocol for my application delegate so that the data can make its way all the way back to the user interface.
Within my application delegate's init: method, I initialize both a login and a fetch manager and get the following warnings for both:
warning: class 'MyAppDelegate' does not implement the 'HTTPManagerDelegate' protocol
warning: incompatible Objective-C types assigning 'struct HTTPManager *', expected 'struct LoginManager *'
Neither of the two classes being initialized are derived from HTTPManager, but they do implement the HTTPManagerDelegate protocol. The line of code that produces the above warning is:
_loginMgr = [[LoginManager alloc] initWithDelegate:self];
So what on earth is making LoginManager's initWithDelegate: method return an HTTPManager*? There is no inheritance and my return types are correct, so to me this is some dark form voodoo that I cannot best.
Here is the shell of my application. There are probably typos and small inconsistencies so ask me before assuming a syntactical problem:
// HTTPManager.h
#protocol HTTPManagerDelegate
...
#end
#interface HTTPManager : NSObject
{
id <HTTPManagerDelegate> _delegate;
...
}
- (HTTPManager *) initWithDelegate:(id <HTTPManagerDelegate>)delegate;
...
#end
// LoginManager.h
#protocol LoginManagerDelegate
...
#end
#interface LoginManager : NSObject <HTTPManagerDelegate>
{
id <LoginManagerDelegate> _delegate;
...
}
- (LoginManager *) initWithDelegate:(id <LoginManagerDelegate>)delegate;
...
#end
// MyAppDelegate.h
#interface MyAppDelegate : NSObject <NSApplicationDelegate, LoginManagerDelegate, FetchManagerDelegate>
{
LoginManager *_loginMgr;
...
}
...
#end
// MyAppDelegate.m
...
- (MyAppDelegate *) init
{
self = [super init];
if (self)
{
// WARNING HAPPENS HERE
_loginMgr = [[LoginManager alloc] initWithDelegate:self];
...
}
return self;
}
...
Thanks in advance.
The problem is that you have two methods with the same method signature -initWithDelegate: but with different types in their arguments and/or return types. The compiler cannot handle this case very well and in certain cases, it could also lead to errors at runtime (not in your case because the types in your methods do not differ in size, they're all pointers).
The reason for this (AFAIK) is that the runtime has no straightforward access to the types used in a method. It just reads a selector (which contains no type information) and decides based on this selector what method to call. To help the runtime pack the method arguments onto the stack, the compiler creates a table at compile time that maps selectors to the argument and return value types. This table has just one entry per selector. So if two methods exist that have the same selector but different types in arguments or return value, this system can fail.
In your case:
-init... methods should always return id and not a specific type.
This solves the problem of different return types. The other problem (different argument types) is harder to solve. You can either omit the protocol specification from your method declaration (initWithDelegate:(id)delegate) or give the two methods different names:
- (id) initWithHttpMgrDelegate:(id <HTTPManagerDelegate>)delegate;
- (id) initWithLoginMgrDelegate:(id <LoginManagerDelegate>)delegate;
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?
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.
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.
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).
The solution I came up with is:
Create a protocol for everything you want in your "abstract" class
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.
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.
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;
}
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.
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:
Implement every abstract method in the base class.
Make that implementation [self doesNotRecognizeSelector:_cmd];…
…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”.
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
It get’s the job done efficiently, and somewhat conveniently.
It’s fairly easy to understand. (Okay, that __builtin_unreachable() may surprise people, but it’s easy enough to understand, too.)
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?
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.
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.
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?
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
(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.
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
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;
}
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.
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.
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.
Changing a little what #redfood suggested by applying #dotToString's comment, you actually have the solution adopted by Instagram's IGListKit.
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.
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.
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.
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
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
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.