It is possible to declare a REFERENCE to another variable

NOTE : -

THIS CONTENT IS NOT ORIGINAL

8. It is possible to declare a REFERENCE to another variable

It is possible to make one variable be another:

using namespace std;
#include 

int main ()
{
  double a = 3.1415927;

  double &b = a;                            // b is a

  b = 89;

  cout << "a contains: " << a << endl;     // Displays 89.

  return 0;
}

Output

a contains: 89

(If you are used to pointers and absolutely want to know what happens, simply think double &b = a is translated to double *b = &a and all subsequent b are replaced by *b.)

The value of REFERENCE b cannot be changed after its declaration. For example you cannot write, a few lines further, &b = c expecting that b is now c. It won't work. Everything is said on the declaration line of b. Reference b and variable a are married on that line and nothing will separate them.

References can be used to allow a function to modify a calling variable:

using namespace std;
#include 

void change (double &r, double s)
{
  r = 100;
  s = 200;
}

int main ()
{
  double k, m;

  k = 3;
  m = 4;

  change (k, m);

  cout << k << ", " << m << endl;        // Displays 100, 4.

  return 0;
}

Output

100, 4

If you are used to pointers in C and wonder how exactly the program above works, here is how the C++ compiler would translate it to C:

using namespace std;
#include 

void change (double *r, double s)
{
  *r = 100;
  s = 200;
}

int main ()
{
  double k, m;

  k = 3;
  m = 4;

  change (&k, m);

  cout << k << ", " << m << endl;        // Displays 100, 4.

  return 0;
}

Output

100, 4

A reference can be used to let a function return a variable:

using namespace std;
#include 

double &biggest (double &r, double &s)
{
  if (r > s) return r;
  else       return s;
}

int main ()
{
  double k = 3;
  double m = 7;

  cout << "k: " << k << endl;       // Displays  3
  cout << "m: " << m << endl;       // Displays  7
  cout << endl;

  biggest (k, m) = 10;

  cout << "k: " << k << endl;       // Displays  3
  cout << "m: " << m << endl;       // Displays 10
  cout << endl;

  biggest (k, m) ++;

  cout << "k: " << k << endl;       // Displays  3
  cout << "m: " << m << endl;       // Displays 11
  cout << endl;

  return 0;
}

Output

k: 3 m: 7 k: 3 m: 10 k: 3 m: 11

Again, provided you're used to pointer arithmetic and if you wonder how the program above works, just imagine that the compiler translated it into the following standard C program:

using namespace std;
#include 

double *biggest (double *r, double *s)
{
  if (*r > *s) return r;
  else         return s;
}

int main ()
{
  double k = 3;
  double m = 7;

  cout << "k: " << k << endl;
  cout << "m: " << m << endl;
  cout << endl;

  (*(biggest (&k, &m))) = 10;

  cout << "k: " << k << endl;
  cout << "m: " << m << endl;
  cout << endl;

  (*(biggest (&k, &m))) ++;

  cout << "k: " << k << endl;
  cout << "m: " << m << endl;
  cout << endl;

  return 0;
}

Output

k: 3 m: 7 k: 3 m: 10 k: 3 m: 11

To end with, for people who have to deal with pointers yet do not like it, references are useful to effectively un-pointer variables. Beware this is considered bad practice. You can get into trouble. See for example http://www.embedded.com/story/OEG20010311S0024.

using namespace std;
#include 

double *silly_function ()    // This function returns a pointer to a double
{
  static double r = 342;
  return &r;
}

int main ()
{
  double *a;

  a = silly_function();

  double &b = *a;          // Now b is the double towards which a points!

  b += 1;                  // Great!
  b = b * b;               // No need to write *a everywhere!
  b += 4;

  cout << "Content of *a, b and r: " << b << endl;

  return 0;
}

Output

Content of *a, b and r: 117653

BIBILOGRAPHY / REFERENCE : - http://www.4p8.com/eric.brasseur/cppcen.html