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Saturday, June 23, 2007

Reference Variables in Detail

A reference is an alias or alternate name for an object. It is just like having two variables having the same memory address, if one of them changes the other will also change.

Actually, a reference is an implicit pointer. Suppose, we have an int(eger) variable num and its reference refnum, then both will always have the same value no matter which one gets alerted.

We can have independent references, reference parameters in functions, functions returning references etc. that are illustrated below with the help of example programs:-

EXAMPLE 1: Independent Reference to a variable

  // Example Program in C++
  #include<iostream.h>

  void main(void)
  {
   int num;
   // notice the declaration of a reference
   // & has nothing to do with the 'address
   // of' operator (&amp;)
   int &refnum=num;

   cout<<"enter value for num: ";
   cin>>num;
   cout<<"now refnum="<<refnum;

   cout<<"\nenter value for refnum: ";
   cin>>refnum;
   cout<<"now num="<<num;
   cout<<endl;
  }

OUTPUT:

   enter value for num: 10
   now refnum=10
   enter value for refnum: 20
   now num=20
   Press any key to continue

EXAMPLE 2: Functions having reference parameters

  // Example Program in C++
  #include<iostream.h>

  void myfunc(int &);//notice the prototype

  void main(void)
  {
   int a=10;
   cout<<a<<endl;
   myfunc(a);
   // the variable itself has been changed
   cout<<a;

   cout<<endl;
  }

  void myfunc(int &var)
  {
   var=-var;
   // no need to return anything
   // the reference of the variable
   // passed is altered, so that the
   // actual variable gets changed
  }

OUTPUT:

   10
   -10
   Press any key to continue

EXAMPLE 3: Returning Reference from a function

  // Example Program in C++
  #include<iostream.h>

  int &myfunc(int &, int&);//notice the prototype

  void main(void)
  {
   int a=10, b=20;
   cout<<"a="<<a<<endl;
   cout<<"b="<<b<<endl;

   // beauty of reference
   // function used at the left hand side
   myfunc(a,b)=30;
   // value of 'b' has been assigned to
   // 30 since it has greater value of
   // the two

   cout<<"\n\nnow...\n\n";
   cout<<"a="<<a<<endl;
   cout<<"b="<<b<<endl;

   cout<<endl;
  }

  // this function return the reference of the
  // variable which is greater of the two
  int &myfunc(int &a, int &b)
  {
   if(a>b) return a;
   else return b;
  }

OUTPUT:

   a=10
   b=20

   now...

   a=10
   b=30

   Press any key to continue

Notice the use of function at the left hand side of the (=) operator, it looks as if we are assigning value to a function!

Confused!

Let me explain this, what is actually happening here is that the function myfunc() is returning a reference to the variable which is greater of the two passed as arguments. Since in the example above, the value of variable ‘b’ is greater, therefore its reference is returned from the function, which is then assigned the value 30, ultimately changing the value of the actual variable passed.

Good-Bye!

5 comments:

  1. You explained that difficult topic well.

    ReplyDelete
  2. very good explination..

    ReplyDelete
  3. pallavi11:09 AM

    i could easily understand the topic . thans a lot

    ReplyDelete
  4. Hi Pallavi,

    It's my pleasure!

    ReplyDelete

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