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                                                         '''Topic 6: Mixins'''
                                                         <b><center>Topic 6: Mixins</center></b>


Compare the use of Ruby mixins with how one would solve the same problem in Java or C++. In Java, you might use interfaces or the decorator pattern (p. 91 of Head-First Design Patterns). In C++, you would probably use multiple inheritance. Give code in all  three languages and compare the solutions on the basis of verbosity and elegance. 


Compare the use of Ruby mixins with how one would solve the same problem in Java or C++.  In Java, you might use interfaces or the
decorator pattern (p. 91 of Head-First Design Patterns). In C++, you would probably use multiple inheritance. Give code in all 
three languages and compare the solutions on the basis of verbosity and elegance. 


'''Inheritance and Mixins'''
 
<h2>Inheritance and Mixins</h2>


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.  
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.  
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 '''
<h3> Modules and Mixins </h3>
 
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.
A module looks very similar to class. Modules can have methods, constants and classes. Modules provide a means for grouping related methods, constants and classes. They provide a means for implementing multiple inheritance in Ruby. There are two major features that classes possess but that modules do not: instances and inheritance. Classes can have instances (objects), superclasses (parents) and subclasses (children); modules can have none of these.


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.


Sample


module SampleMixin
<h2>Sample</h2>
def do_something
<pre>
module SampleMixin
    def do_something
         puts “doing something”
         puts “doing something”
end
    end
end
end
class SampleClass
include SampleMixin
end
s=SampleClass.new
s.do_something 


class SampleClass
    include SampleMixin
end


s=SampleClass.new
s.do_something 
</pre>


<h2>Include </h2>


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.
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
<h2>Including Modules From Files</h2>


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:
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:
Line 56: Line 59:




<center><h2>EXAMPLE</h2></center>
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.




'''Example in RUBY'''


'''Implementation in Ruby '''
<pre>
module Personlike
module Personlike
     attr_accessor :name
     attr_accessor :name
Line 70: Line 79:
end
end
end
end


module Emplike
module Emplike
          attr_accessor :salary
    attr_accessor :salary
         def getsalary
         def getsalary
         return "#{salary}"
         return "#{salary}"
end
end
end
end


class Employee
class Employee
include Personlike
    include Personlike
include Emplike
    include Emplike
end
end


s=Employee.new
s=Employee.new
Line 97: Line 102:




Output:
Federer
26
100000
</pre>
The class Employee includes modules Personlike and Emplike. So we can access the methods getname(), getage() and getsalary() as if they are methods in Employee class itself. This provides the functionality of multiple inheritance using mixins.
<h3>Disadvantages of Mixins in Ruby</h3>
A mixin will not warn if the mixin and the user define the same method.(i.e if there are methods with same name in mixin and also in the class, the method in the class is executed without warning the user).


Consider the above example: Incase we include a getsalary method in both the module and also in the class, the method in the class is executed without warning.
<pre>
module Emplike
    attr_accessor :salary
        def getsalary
        return "#{salary}"
end
end


class Employee
      include Personlike
      include Emplike   
          def getsalary
            return" This is the salary of user defined getsalary"
          end
end


s=Employee.new
s.salary=100000
puts(s.getsalary)


Output:
This is the salary of user defined getsalary


'''Implementation in Java'''
</pre>


<h2>Implementation in Java</h2>
The class "emp" implements "emplike" and "personlike" packages in this example.
<pre>
package assignment;
package assignment;


public interface personlike {
public interface personlike {
String getname();
    String getname();
int getage();
    int getage();
}
}


package assignment;
package assignment;


public interface emplike {
public interface emplike {
float getsalary();
    float getsalary();
}
}


package assignment;
package assignment;
Line 124: Line 167:
public class emp implements assignment.emplike,assignment.personlike  
public class emp implements assignment.emplike,assignment.personlike  
{
{
String name1;
    String name1;
int age1;
    int age1;
float salary1;
    float salary1;
emp(String name,int age,float salary)
    emp(String name,int age,float salary)
{
    {
name1=name;age1=age;salary1=salary;
name1=name;age1=age;salary1=salary;
}
    }
public String getname()
    public String getname()
{
    {
return name1;
return name1;
    }
}
    public int getage()
public int getage()
    {
{
return age1;
return age1;
    }
}
    public float getsalary()
public float getsalary()
    {
{
return salary1;
return salary1;
    }
}
}
}


package assignment;
package assignment;
Line 165: Line 203:
}
}


</pre>




The class emp implements the interfaces emplike, personlike. So the methods in those interfaces can be implemented by emp class.This provides the functionality of multiple inheritance even thought we are not directly inheriting from a multiple classes.In this example using java,multiple inheritance is achieved by using interfaces.
<h2>Implementation in C++ </h2>


The class "emp" extends "personlike" and "employeelike" classes.


 
<pre>
''' Implementation in C++ '''
 
  #include<iostream.h>
  #include<iostream.h>
  #include<stdio.h>
  #include<stdio.h>
  #include<string.h>
  #include<string.h>
  using namespace std;
using namespace std;


  class personlike
  class personlike
Line 222: Line 263:
int main()
int main()
{
{
emp* a=new emp("suhas",22,50000);
emp* a=new emp("Federer",26,100000);
cout<<a->getname()<<endl;;
cout<<a->getname()<<endl;;
cout<<a->getage()<<endl;
cout<<a->getage()<<endl;
Line 229: Line 270:
return 0;
return 0;
}
}
</pre>
C++ provides multiple inheritance. So class emp extends personlike and employeelike classes. Now getage(), getsal() and getname() methods can be accessed by emp class object.
<b><h3>Conclusion</h3></b>
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.








     
''' <center><h2>References Used</h2></center>'''


C++ multiple inheritance can sometimes become knotted and unordered hierarchy .
Java, even though is single inheritance language can exhibit multiple inheritance using interfaces. But care has to be taken to implement all the interfaces to instantiate objects.
Ruby is unambiguous, elegant and concise.


       
''' References Used'''


1) The Little Book of Ruby
1) The Little Book of Ruby
Line 247: Line 294:
http://www.rubycentral.com/pickaxe/tut_modules.html
http://www.rubycentral.com/pickaxe/tut_modules.html


3)Bowler Ruby
3) Bowler Ruby
http://www.softwaresummit.com/2006/speakers/BowlerRubyForJavaProgrammers.pdf
http://www.softwaresummit.com/2006/speakers/BowlerRubyForJavaProgrammers.pdf

Latest revision as of 01:44, 20 September 2007

Topic 6: Mixins


Compare the use of Ruby mixins with how one would solve the same problem in Java or C++. In Java, you might use interfaces or the decorator pattern (p. 91 of Head-First Design Patterns). In C++, you would probably use multiple inheritance. Give code in all three languages and compare the solutions on the basis of verbosity and elegance.


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.

Modules and Mixins


A module looks very similar to class. Modules can have methods, constants and classes. Modules provide a means for grouping related methods, constants and classes. They provide a means for implementing multiple inheritance in Ruby. There are two major features that classes possess but that modules do not: instances and inheritance. Classes can have instances (objects), superclasses (parents) and subclasses (children); modules can have none of these.

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.


Sample

 module SampleMixin
     def do_something
        puts “doing something”
     end
 end

 class SampleClass
     include SampleMixin
 end

 s=SampleClass.new
 s.do_something  

Include

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)


Output:
Federer
26
100000

The class Employee includes modules Personlike and Emplike. So we can access the methods getname(), getage() and getsalary() as if they are methods in Employee class itself. This provides the functionality of multiple inheritance using mixins.

Disadvantages of Mixins in Ruby

A mixin will not warn if the mixin and the user define the same method.(i.e if there are methods with same name in mixin and also in the class, the method in the class is executed without warning the user).

Consider the above example: Incase we include a getsalary method in both the module and also in the class, the method in the class is executed without warning.

module Emplike
     attr_accessor :salary
        def getsalary
         return "#{salary}"
	end
end

class Employee
      include Personlike
      include Emplike    
          def getsalary
            return" This is the salary of user defined getsalary"
          end
end

s=Employee.new
s.salary=100000
puts(s.getsalary)
 

Output:
This is the salary of user defined 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);
		
	}
}


The class emp implements the interfaces emplike, personlike. So the methods in those interfaces can be implemented by emp class.This provides the functionality of multiple inheritance even thought we are not directly inheriting from a multiple classes.In this example using java,multiple inheritance is achieved by using interfaces.

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;
}

C++ provides multiple inheritance. So class emp extends personlike and employeelike classes. Now getage(), getsal() and getname() methods can be accessed by emp class object.


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.



References Used


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