Reference variables in C++

In C language, we have call by value and call by pointer of functions. Both have their own drawbacks. C++ provides a better solution - call by reference.

A reference variable in C++  is an alias for another variable.

Such a variable looks like an ordinary variable but behaves like a pointer.  It always refers to another variable.

In function calls, reference parameter modifies the actual argument, without the hassle of addresses and indirections.

 Defining a reference variable 

A reference variable is defined with & along with its name. It should always be initialized during definition. 

 int &b = a;/*definition of a reference*/

b is a reference variable and it refers to a. That is b is another name for a.

Functions with reference parameters

 A function with reference parameter changes the actual argument because the parameter is an alias for argument. In case of large structs and classes, it is better to send a reference parameter, as it avoids copying of data.  

void triple(int &a)
{
    a *=3;
}
int main()
{
  int m = 10;
  triple(m);/*changes m to 30*/
  cout<<m;
}

In the above example, m is sent as a reference parameter. So m is changed by the function triple(). 

If the function needs a reference parameter for efficiency,  but the function should not change the argument, you should define parameter as constant reference. 

Notes

1.  A reference should always be initialized to another variable while defining it.
2.   A reference variable can not refer to a different variable later.
• i.e. b = c ;
• will not make b refer to c. Instead it assigns c to a.
3.   A reference variable should always be initialized when defining.
•    float &m1 = m2; /*m1 is initialized to m2*/
4.   An array of reference variables is not possible
5.   If a function has a reference parameter, changes to this reference parameter will change the actual argument.
6.  If a function returns a reference value, that function can be used in LHS of an assignment statement.

Returning a reference

A function returning a reference has the advantage that it can be used in LHS of an expression. But you have to make sure that, the return value of such a function is within the scope even when function exits.

int &fn1(int m,int n)
{
   if(m>n) return m; return n;
}
int main()
{
  int a1 = 3,b1=12;
  fn1(a1,b1) = 100;
}  

Here main fn1() returns a reference to b1. So b1 will be set to 100 in the assignment statement of main().