Const

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The compiler won't allow a const member function to change *this or to invoke a non-const member function for this object:

class Stack {
public:
 int pop();             //Mutator
 int numElems() const;  //Inspector
private:
   int element;
};
int Stack::numElems() const
{
   ++element; // Error
   pop(); // Error: pop is not const function
   return 1;
};
void sample(const Stack *s)
{
 s->numElems();// OK: A const Stack can be inspected
 s->pop();     // Error: A const Stack cannot be mutated
}

If Fred is some type, then

• Fred* is a pointer to a Fred (the * is pronounced "pointer to a").
  

• const Fred* is a pointer to a Fred that cannot be changed via that pointer.
  

• Fred* const is a const pointer to a Fred. The Fred object can be changed via the pointer, but the pointer itself cannot be changed.
  

• const Fred* const is a const pointer to a Fred that cannot be changed via that pointer.
  

References are similar: read them right to left.

• Fred& is a reference to a Fred (the & is pronounced "reference to a").
  

• const Fred& is a reference to a Fred that cannot be changed via that reference

• Note that Fred& const and const Fred& const are not included in the second list. This is because references are inherently immutable: you can never rebind the reference so that it refers to a different object.
  

A better description is in former post. (in the middle)