CSC/ECE 517 Fall 2007/wiki1 6 c9
Inheritance and Mixins
Some object-oriented languages (notably C++) support multiple inheritance, where a class can have more than one immediate parent, inheriting functionality from each. Although powerful, this technique can be dangerous, as the inheritance hierarchy can become ambiguous. Ruby offers an interesting and powerful compromise, giving you the simplicity of single inheritance and the power of multiple inheritance. A Ruby class can have only one direct parent (i.e no class can inherit from more than one class) and so Ruby is a single-inheritance language. In cases where you want numerous extra behaviors for a class’s instances, Ruby classes provide the multiple inheritance functionality by using mixins. You can use include any number of mixins (a mixin is like a partial class definition). This provides a controlled multiple-inheritance-like capability with none of the drawbacks.
Mixins
The process of including a module in a class is also called ‘mixing in’ the module – which explains why included modules are often called ‘mixins’. An object can access the instance methods of a module just by including that module using the include method.
There are two major features which classes possess but which modules do not: instances and inheritance. Classes can have instances (objects), superclasses (parents) and subclasses (children); modules can have none of these.
Sample
module SampleMixin
def do_something
puts “doing something”
end
end
class SampleClass
include SampleMixin
end
s=SampleClass.new
s.do_something
A couple of points about the include statement. It has nothing to do with files. C programmers use a preprocessor directive called #include to insert the contents of one file into another during compilation. The Ruby include statement simply makes a reference to a named module. If that module is in a separate file, you must use require to drag that file in before using include. Second, a Ruby include does not simply copy the module's instance methods into the class. Instead, it makes a reference from the class to the included module. If multiple classes include that module, they'll all point to the same thing. If you change the definition of a method within a module, even while your program is running, all classes that include that module will exhibit the new behavior.
Including Modules From Files
Often it is more useful to define modules in separate files and include them as needed. The first thing you have to do in order to use code from another file is to load that file using the require method, like this: require( "testmod.rb" )
The required file must be in the current directory, on the search path or in a folder listed in the predefined array variable $:. You can add a directory to this array variable using the usual array-append method, << in this way: $: << "C:/mydir" The require method returns a true value if the specified file is successfully loaded; otherwise it returns false. If in doubt, you can simply display the result: puts(require( "testmod.rb" ))
Pre-Defined Modules
The following modules are built in to the Ruby interpreter: Comparable, Enumerable, FileTest, GC, Kernel, Math, ObjectSpace, Precision, Process, Signal
EXAMPLE
Consider a scenario of an employee. An employee has the features of a person ( having name,age and other attributes) and also that of an employee( having salary, hours of work and other attributes). In our example, we explain multiple inheritance by showing how modules "Emplike" and "Personlike" are inherited by "Employee" class. Thus we illustrate mixins in ruby.
Implementation in Ruby
module Personlike
attr_accessor :name
attr_accessor :age
def getname
return "#{name}"
end
def getage
return "#{age}"
end
end
module Emplike
attr_accessor :salary
def getsalary
return "#{salary}"
end
end
class Employee
include Personlike
include Emplike
end
s=Employee.new
s.name="Federer"
s.age=26
s.salary=100000
puts (s.getname)
puts(s.getage)
puts(s.getsalary)
Implementation in Java
The class "emp" implements "emplike" and "personlike" packages in this example.
package assignment;
public interface personlike {
String getname();
int getage();
}
package assignment;
public interface emplike {
float getsalary();
}
package assignment;
public class emp implements assignment.emplike,assignment.personlike
{
String name1;
int age1;
float salary1;
emp(String name,int age,float salary)
{
name1=name;age1=age;salary1=salary;
}
public String getname()
{
return name1;
}
public int getage()
{
return age1;
}
public float getsalary()
{
return salary1;
}
}
package assignment;
public class emptest {
public static void main(String args[])
{
emp ob=new emp("Federer",26,100000);
String name=ob.getname();
int age=ob.getage();
float sal=ob.getsalary();
System.out.println("name is "+name+"age is "+age+"salary is "+sal);
}
}
Implementation in C++
The class "emp" extends "personlike" and "employeelike" classes.
#include<iostream.h>
#include<stdio.h>
#include<string.h>
using namespace std;
class personlike
{
public:
char* name;
int age;
personlike(char* a,int b)
{
name=new char[10];
strcpy(name,a);
age=b;
}
char* getname()
{
return name;
}
int getage()
{
return age;
}
};
class employeelike
{
public:
int salary;
employeelike(int a)
{
salary=a;
}
int getsal()
{
return salary;
}
};
class emp:public personlike,public employeelike
{
public:
emp(char* a,int b,int c):personlike(a,b),employeelike(c)
{
}
};
int main()
{
emp* a=new emp("Federer",26,100000);
cout<<a->getname()<<endl;;
cout<<a->getage()<<endl;
cout<<a->getsal()<<endl;
cout<<a->name<<"\t"<<a->age<<"\t"<<a->salary<<endl;
return 0;
}
Conclusion
C++ multiple inheritance can sometimes become knotted and lead to unordered hierarchy . Java, even though is a single inheritance language can exhibit multiple inheritance using interfaces. However, care has to be taken in Java to implement all the interfaces and it can be a lengthy procedure. Ruby is unambiguous, elegant and concise.
1) The Little Book of Ruby http://www.sapphiresteel.com/The-Little-Book-Of-Ruby
2) Programming Ruby: The Pragmatic Programmer’s guide http://www.rubycentral.com/pickaxe/tut_modules.html
3) Bowler Ruby http://www.softwaresummit.com/2006/speakers/BowlerRubyForJavaProgrammers.pdf