A class variable must be initialized within the class definition as shown below. A class method is defined using def self. Ruby gives you three levels of protection at instance methods level, which may be public, private, or protected. Ruby does not apply any access control over instance and class variables. Methods are public by default except for initialize, which is always private. Only the class methods can access private members. Access is kept within the family. When the above code is executed, it produces the following result. Here, first method is called successfully but second method gave a problem.
Ruby by Example: Concepts and Code by Kevin Baird
One of the most important concepts in object-oriented programming is that of inheritance. Inheritance allows us to define a class in terms of another class, which makes it easier to create and maintain an application.
Inheritance also provides an opportunity to reuse the code functionality and fast implementation time but unfortunately Ruby does not support multiple levels of inheritances but Ruby supports mixins. A mixin is like a specialized implementation of multiple inheritance in which only the interface portion is inherited. When creating a class, instead of writing completely new data members and member functions, the programmer can designate that the new class should inherit the members of an existing class.
This existing class is called the base class or superclass , and the new class is referred to as the derived class or sub-class. Ruby also supports the concept of subclassing, i. The syntax for extending a class is simple. Though you can add new functionality in a derived class, but sometimes you would like to change the behavior of already defined method in a parent class.
Ruby - Object Oriented
Sometimes, we want to prevent an object from being changed. The freeze method in Object allows us to do this, effectively turning an object into a constant.
Any object can be frozen by invoking Object. A frozen object may not be modified: you can't change its instance variables. You can check if a given object is already frozen or not using Object. By convention, we keep constant names in upper case. Once a constant is defined, you cannot change its value but you can access a constant directly inside a class much like a variable but if you want to access a constant outside of the class then you would have to use classname::constant as shown in the below example.
There may be a situation when you want to create an object without calling its constructor initialize i. If class definitions are executable code, this implies that they execute in the context of some object: self must reference something. Let's find out what it is. Interestingly the class keyword does not create the C class twice, which is what allows our earlier example of monkey patching to work.
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Take a look at the following example:. In the first case, class B does not exist, so Ruby defines the class and method1.
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The second time class B is referenced, the class exists, so instead of redefining class B, it opens the existing class and defines method2 there. The method1 is still accessible because it is still the same class. When opening a class we can even reference class level instance variables. Now its time to see some examples of metaprogramming in action. Lets take a look at the Awesome Print gem. Awesome Print is a gem for pretty printing Ruby objects. To use AwesomePrint, first install the gem. Lets take a look at the irb!
Define Ruby Objects
If we monkey patch it like so, check what happens:. Returning back to the inspector irb!
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This allows us to open up classes without the class keyword. Lets take a look at another metaprogramming trick used in AwesomePrint. The format method is the main entry point to format an object.
After determining the class of the object it is about to format, the method dynamically calls the corresponding formatter using send. This file contains extensions on the core Kernel class. This example comes from the book Metaprogramming Ruby by Paolo Perrota. Here we can see that AwesomePrint has opened up the Kernel module, and is defining two methods ai and ap. Why define them on the Kernel module? Since Kernel is included in Object, and object is in the ancestor chain of nearly every Ruby class, defining a method within Kernel is a handy way of making it available nearly everywhere.