I'm looking for a way to get a static methods list for a certain class. I only get a list of instance methods with the runtime function class_copyMethodList().
Is there a way to list static methods?
Each Class is itself an Objective-C object, and in turn has an object which is (sort of) its class. You need to get this metaclass object (see also: "[objc explain]: Classes and Metaclasses"), and then ask that for its methods (which will be the class methods* you are after).
From the class_copyMethodList docs:
###Discussion
To get the class methods of a class, use class_copyMethodList(object_getClass(cls), &count)
*There's no such thing as static methods in Obj-C.
Related
Is there any way to get only the declared members of a class (not inherited) with Kotlin Reflection?
Something equivalent to getDeclaredMethods(), or ...Fields(), in Java, but for members and JVM free, which:
Returns an array containing Method objects reflecting all the declared methods of the class ... but excluding inherited methods.
Or like a binding flag, such as BindingFlags.DeclaredOnly of dotnet.
Because the reflection is based on the class, So the following is only for the kotlin/JVM, not suitable for the Kotlin/JS or Kotlin/Native.
For the Kotlin/JS it supports limit, for detail, you can see this
document
The only supported parts of the API are: (::class),KType and typeOf
Firstly, you can use the SomeClass::class.java.declaredMethods to get the
getDeclaredMethods. That is the java method. Because the Kotlin file after compiled it is still a class. so you can directly use it.
You can also add the kotlin reflect to get the KClass, then use the declaredFunctions to get. Here is the Document
Returns all functions declared in this class. If this is a Java class, it includes all non-static methods (both extensions and non-extensions) declared in the class and the superclasses, as well as static methods declared in the class
For how to get the KClass, you can use the following code
Class.forName("mypackage.MyClass").kotlin.declaredFunctions
Besides the method, the other property you can also get. such as
declaredMembers
Returns all functions and properties declared in this class. Does
not include members declared in supertypes.
allSuperclasses
functions
Returns all functions declared in this class, including all non-static methods declared in the class and the superclasses, as well as static methods declared in the class.
you can read the document using it.
While reading THIS question and accepted answer for the question, I was unable to get the difference between these two types of methods. Actually got the point by reading the example, but then, I was not able to write my own static method.
I tried googling create static method in objective c static methods
Which returned me links to THIS and THIS question. But, the example here are CLASS methods as per the first link in the question. Which is confusing me.
Can anyone here show me how do I create a static method which is not a class method ?
Any light on this would be appreciated.
The problem you are having is the following - there are no static methods in Obj-C, that's why you cannot create them.
The difference between static and class methods is a difference between language concepts. You can find static methods in languages like Java or C++, you will find class methods in languages like Obj-C and Ruby.
The principal difference is that
Static methods are shared between all instances (this doesn't exist in Obj-C). They are dispatched statically (at compile time) depending on the type of the variable.
Class method is a method on a class. In languages like Obj-C and Ruby a class itself is an instance of another class (metaclass). Using + before a method declaration means the method will be defined on the class. Technically, it's just an instance method, just on a different object.
Don't worry if you don't understand the concept of class method perfectly, it takes time. To simplify, you can consider it as a method shared between instances, but it can be overriden in subclasses.
I was reading about private constructor and found a few points that I couldn't understand. It said, if you declare a constructor as private:
That class cannot be explicitly instantiated from another class
That class cannot be inherited
Should be used in classes containing only static utility methods
My first question: Point 2 says the class cannot be inherited. Well, if you declare a class private then it would still satisfy this property. Is it because, if a class is private, it can still be explicitly instantiated from outside by another class?
My second question: I don't understand point 3. If I have a helper class which is full of static methods, I would never have to instantiate that class to use the methods. So, what is the purpose of a constructor in that class which you are never going to instantiate?
Answer for Java
Question 1 You're confusing a private class, with a class that has a private constructor. Private constructors are used mainly for static classes that are not meant to be instatiated (i.e. they just have a bunch of static methods on them).
Question 2 Exactly there is no need for a constructor so you have to explicitly create a private constructor so that it does not get a default constructer that the JVM will provide if none is defined
An empty class with no methods defined will always be given a no argument constructor by the JVM by default
I take java and c++ as an examples (not the best OO languages known, but very popular) - since you are not defining which languge do you mean.
Ad.2. In these languages you must either call superclass constructor explicitly or it is implicitly called for you. From a subclass you cannot call private methods (only public and protected) - this rule applies to constructors as well. This means if the class has only private constructors, there is no way to call one in subclass constructor. So you cannot subclass such class.
Ad. 3. It is just to avoid confusion - since this class is only a container for utility methods, there is no point in instantiating it. This way you can enforce this rule at compile time.
It is my understanding that it is difficult to create constructors of classes in Smalltalk and Objective-C. This is because the constructor can't be a message of a class instance because the class Class is not yet defined.
As far as I can tell, the solution is to create a new class whose only instance is itself a class. But how does the constructor work in this situation? I don't understand the process.
I'm talking in terms of Smalltalk. There are two types of things that you could reasonably call constructors here. One is the method that initializes a new instance of a class. The other is the things that initializes a class. Neither of them is difficult.
For instance initialization, the convention is that you implement the "new" method on the class as
new
^super new initialize
(do the superclass's implementation of new, and then send the "initialize" message to the result and return it)
Lots of classes may already inherit this implementation, so you just need to write an initialize method as
initialize
super initialize.
foo := 1.
etc.
To initialize a new class, the mechanism is similar. You implement a class method called "initialize", and it will automatically get sent when the class is loaded into a new Smalltalk environment (image).
The solution is to create a method on the class which does all the necessary initialisation of the instance.
I have concept of static variables but what are the benefits of static methods in a class. I have worked on some projects but I did not make a method static. Whenever I need to call a method of a class, I create an object of that class and call the desired method.
Q: Static variable in a method holds it's value even when method is executed but accessible only in its containing method but what is the best definition of static method?
Q: Is calling the static method without creating object of that class is the only benefit of static method?
Q: What is the accessible range for static method?
Thanks
Your description of a static variable is more fitting to that found in C. The concept of a static variable in Object Oriented terms is conceptually different. I'm drawing from Java experience here. Static methods and fields are useful when they conceptually don't belong to an instance of something.
Consider a Math class that contains some common values like Pi or e, and some useful functions like sin and cos. It really does not make sense to create separate instances to use this kind of functionality, thus they are better as statics:
// This makes little sense
Math m = new Math();
float answer = m.sin(45);
// This would make more sense
float answer = Math.sin(45);
In OO languages (again, from a Java perspective) functions, or better known as methods, cannot have static local variables. Only classes can have static members, which as I've said, resemble little compared to the idea of static in C.
Static methods don't pass a "this" pointer to an object, so they can't reference non-static variables or methods, but may consequently be more efficient at runtime (fewer parameters and no overhead to create and destroy an object).
They can be used to group cohesive methods into a single class, or to act upon objects of their class, such as in the factory pattern.
Syntax (php) for static methods:
<?php
class Number {
public static function multiply($a, $b) {
return $a * $b;
}
}
?>
Client code:
echo Number::multiply(1, 2);
Which makes more sense than:
$number = new Number();
echo $number->multiply(1, 2);
As the multiply() method does not use any class variables and as such does not require an instance of Number.
Essentially, static methods let you write procedural code in an object oriented language. It lets you call methods without having to create an object first.
The only time you want to use a static method in a class is when a given method does not require an instance of a class to be created. This could be when trying to return a shared data source (eg a Singleton) or performing an operation that doesn't modify the internal state of the object (String.format for example).
This wikipedia entry explains static methods pretty well: http://en.wikipedia.org/wiki/Method_(computer_science)#Static_methods
Static variables and static methods are bound to the class, and not an instance of the class.
Static methods should not contain a "state". Anything related to a state, should be bound to an instantiated object, and not the class.
One common usage of static methods is in the named constructor idiom. See: http://www.parashift.com/c++-faq-lite/ctors.html#faq-10.8.
Static Methods in PHP:
Can be called without creating a class object.
Can only call on static methods and function.
Static variable is used when you want to share some info between different objects of the class.As variable is shared each object can update it and the updated value be available for all other objects as well.
As static variable can be shared,these are often called as class variable.
static elements are accessible from any context (i.e. anywhere in your script), so you can access these methods without needing to pass an instance of the class from object to object.
Static elements are available in every instance of a class, so you can set values that you want to be available to all members of a type.
for further reading a link!