I know usually, when you want to call a method on another object, you do:
NewObject *object = [NewObject alloc]init];
[object callMethod];
But I created a class that isn't an object itself meaning it doesn't have properties or memory management. It has a couple methods that calculate some stuff.
From any other class, all I have to do is import the header for this class and do:
#import "MyClass.h"
[MyClass callMethod];
Why in this case do I not have to alloc init? It works just fine.
It sounds like you are trying to call a class method. These are methods which have been defined as:
+(void) myStaticMethod;
instead of
-(void) myMethod;
The plus sign indicates that the method does not use any fields, and thereby does not need to instantiate the object.
In your example, "object" is an instance of a class "NewObject" which has been allocated memory and initialized. Where-as your example, "MyClass" is only a class which because it has static members declared as above, does not need to be instantiated.
Class methods provide a nice way to combine a bunch of related functions into one place, rather than having them spread out in the regular namespace, as would usually be done in straight C. You can also have both class methods and instance methods in the same class, using the class ones when needed, and instantiating the class to use the instance ones when needed.
EDIT: Changed terminology to refer to class methods instead of static methods.
because you are calling a class method. You only need to alloc init objects. Classes only need to be included but not alloc inited. So you don't need to init an NSString class, say.
Edit:
Let's just have some nonsense examples:
+ (void)classMethod {
NSLog("Hi!");
}
[SomeClass classMethod]; // prints Hi!
- (void)instanceMethod { // (say it's an instance method of NSString)
NSLog(self);
}
[#"someNSString" instanceMethod]; // prints someNSString. But you need to have a string first, otherwise you cannot use this method.
There is a difference between "instance" methods (normal ones), that have to be called on an object and have access to self, and "class" methods (called static, in many programming languages), that are invoked on the class and thus do not have a self.
Class methods are similar to C++ static methods, in that they can be invoked without creating a concrete instance of the class. The usefulness of this is you can call a class's specialized factory methods to create a new instance; or, you can define a utility library under the scope of a class that may or may not provide concrete instances depending on the task.
Look at NSDate and NSNumber are good examples of this in the Foundation framework.
Related
I'm new to coding, apologies for dumb question.
Am following a tutorial to build a note taking app using Swift in Xcode.
Within a class definition I have been defining methods using the keyword func myMethod etc. At one point the instructor decides to define a Class method (within the existing class) using class func myMethod.
Why would you do this?
Thanks in advance for any feedback.
By defining a class method it means that you don't need an instance of that class to use the method. So instead of:
var myInstance: MyClass = MyClass()
myInstance.myMethod()
You can simply use:
MyClass.myMethod()
The static (class) function is callable without needing an instance of the class available; it may be called without having to instantiate an object.
This can be useful for encapsulation (avoiding placing the function in the global namespace), or for operations that apply to all objects of a given class, such as tracking the total number of objects currently instantiated.
Static functions can be used to define a namespaces collection of related utility functions:
aDate = Utils.getDate()
aTime = Utils.getTime()
Another common use is for the singleton pattern, where a static function is used to provide access to an object that is limited to being instantiate only once:
obj = MySingleton.getInstance()
obj.whatever()
One answer is namespacing. If a function is only relevant to a certain class there is no need to declare the function globally.
This is Swift's take on static methods:
Static methods are meant to be relevant to all the instances of a class (or no instances) rather than to any specific instance.
An example of these are the animation functions in UIView, or the canSendMail function from MFMailComposeViewController.
I have created a class which contain only static method in iPhone. The class was mainly to do my core data operations. But suddenly, I had a need to make a method call in a view controller, when an insertion of data in to a table got completed.
At first, I decided to send an NSNotification, once the loop finishes iteration. But then, since I need to use this only for single time, I decided not to go for NSNotificationCenter, instead to use delegation.
Now I have many static method, and two instance methods:
//1
-(id)initWithDelegate:(id)delegate;
//2
-(void)insertContentsInToTheTableFromArray:(NSArray *)contentArray;
Is this a good design pattern, to have both instance methods and class methods in this class. Please share your thoughts.
We can use both method in one class. We know that instance methods use an instance of a class, whereas a static method can be used with just the class name. But the static method is a convenience method that use on many foundation classes.
I have two classes, both are subclasses of CCLayer,
I want to call a method of first class into second class, what should I code?
Your question is not providing much detail, but from my understanding of what you say, you need the following:
a selector in the public interface of your first class;
a pointer ivar in the second class that you will properly initialize so that it points to an instance of the first class;
In this way you will be able to call the first class' method from the second class.
I have built a class which has a few methods in it, once of which returns an array, lets call this class A.
I have a second class, class B, which I would like to use to call the method from class A.
But, now how do I call that method from class A and store what is returned in a var in class B? Do I have to initiate the class? I have made sure to include the .h file from class A into class B.
Thanks for helping a newbie.
UPDATE:
Here is how I thought I could do this (DataStore is my class A and pushRideData is my method that returns an array):
DataStore *store = [[DataStore alloc] init];
trailsArray = [store pushRideData];
Assuming you have files A.h, A.m B.h and B.m to define your two classes, then you need to do the following:
Make sure A.h and B.h are #imported into your projects PCH file (this is the easiest/fastest way, but you could also choose to import the files into all the .m files, instead).
If you refer to a class -- say, B *something in A.h -- before that class's header file is imported, then use a forward class declaration to shut up the compiler. I.e. #class B; before the #interface A:NSObject in A.h
If you want to call an instance method of a class, you need to instantiate the class as you describe. Or, if the instance is created somewhere else, you'll need some mechanism to retrieve it. A class method, perhaps, or a global variable or a controller or something like it.
None of this is really that much different than straight C save for a formal notion of Objects (as opposed to malloc'ing a bunch of memory and passing around pointers).
That's a fairly abstract question. Yes, you need an instance to be able to store instance variables in it. You will need to allocate and init the instance, assinging it to an instance or local (pointer) variable in the calling class unless it is one of the several in the Cocoa Touch frameworks which use the singleton pattern, such as the application delegate. Such singletons have special case-specific class methods for obtaining the singleton instance.
I have a class which is intended to be abstract. This means: When someone subclasses it, a few methods MUST be overwritten.
But on the other hand, those methods are not intended to be called manually from anywhere except inside the abstract class (the superclass of the subclass).
Must I declare these methods in .h anyways or can I just add comments in .h which say "you must overwrite -foo and -bar"? Or is there a better pattern to make abstract methods?
Related: Is there a way to create an abstract class in Objective C?
Objective-C doesn't actually have a way to declare a class as abstract. From Apple's Docs:
Abstract Classes
Some classes are designed only or
primarily so that other classes can
inherit from them. These abstract
classes group methods and instance
variables that can be used by a number
of different subclasses into a common
definition. The abstract class is
typically incomplete by itself, but
contains useful code that reduces the
implementation burden of its
subclasses. (Because abstract classes
must have subclasses to be useful,
they’re sometimes also called abstract
superclasses.)
Unlike some other languages,
Objective-C does not have syntax to
mark classes as abstract, nor does it
prevent you from creating an instance
of an abstract class.
The NSObject class is the canonical
example of an abstract class in Cocoa.
You never use instances of the
NSObject class in an application—it
wouldn’t be good for anything; it
would be a generic object with the
ability to do nothing in particular.
The NSView class, on the other hand,
provides an example of an abstract
class instances of which you might
occasionally use directly.
Abstract classes often contain code
that helps define the structure of an
application. When you create
subclasses of these classes, instances
of your new classes fit effortlessly
into the application structure and
work automatically with other objects.
So to answer your question, yes, you need to place the method signature in the header, and should implement the method in the base class such that it generates an error if called, like the related question's answer states.
You can also use a protocol to force classes to implement certain methods.
However you choose to implement the base class, clearly document in the header, as well as in your documentation, exactly what the class assumes and how to go about sub-classing it correctly.
Whenever possible write your code so that improper implementations fail to compile. If you cannot do that then you should try to generate a runtime error (at the very least in a debug build) if the subclass is not written correctly. Do not rely on comments because people will not read them.
You must declare your "protected" and "abstract" methods in a header file, but you can use separate categories to clearly indicate their purpose and intended use.
#interface MyBaseClass : NSObject {
}
- (void)foo;
#end
#interface MyBaseClass(ProtectedMethods)
- (void)bar;
#end
#interface MyBaseClass(AbstractMethods) // Subclasses must implement
- (void)internalBar;
#end
You can put everything in a single header, or you could put your protected and abstract declarations in a separate "protected" header, say MyClassProtected.h, meant to be included only by your subclass implementations. It depends on how badly you want "hide" your protected methods.
Your base class can log, assert, or throw when an abstract/pure-virtual method is called.
As other people have said, Objective-C does not support pure virtual classes.
You can enforce pure virtual behaviour at runtime though. The cleanest way to do this is by using the Objective-C runtime's _cmd and NSObject's -doesNotRecognizeSelector:
- (void)iMustBeImplementedInaSubclass;
{
[self doesNotRecognizeSelector:_cmd]; // Pure virtual
}
As ben says you are probably better served by using a protocol to get your API design right.