C++: What is the difference between reference and constant reference ?

Question

I have seen at many places that object is passed as reference and as constant reference.
I want to know the usage of each one.

Answer 1

The difference between a reference and a const reference is merely that you can modify the referenced value (or call non-const member functions on it) when you have a plain reference, but can only call const member functions on an object for which you have a const reference, and you cannot modify it if it is a built-in type.
for example..

class A
{
  int buf[1000];
  public: A& fun() 
  {
   buf[0]++; return *this;
  }
  const A& fun2() 
  {
   buf[0]++; return *this;
   }
};