Const keyword in C++

Constant Pointer

• If we make a pointer const, Its address cannot be changed after initialization
• This means that the pointer will always point to the same address.
• Here address of const pointer cant be changed but the value that is pointed can be changed

Example:

int main()
{
  int a = 4,b=5;
  int* const ptr = &a; // const pointer p pointing to the variable a
  //ptr=&b;//invalid :const pointer address cannot be changed
  *ptr=6;//valid,value at ptrs address is non_const
}

Note that we cannot change the pointer p but can change the value of a.

Pointer to Constant Variable

Here, the value that is pointed by the pointer cannot be modified

Example:

int main()
{
  int a = 4,b=5;
  const int *p=&a;//a becomes constable variable
  //*p=5;//inavlid:a is const
  p=&b;//valid: address pointed by p is non_const
}

Here, p is a pointer which is pointing to a const int variable, this means that we cannot change the value of that int variable.

Constant objects

• We cannot modify the data members of a const object, only one initialization is allowed for all the copies of that object
• const objects are reads only objects

We make an object const by writing the const keyword at the beginning of the object declaration as shown below.

const test t(5);

• We made the object a of class test const by writing the const keyword before the class name at the time of its declaration.
• A const class object is initialized through the constructor.

Example:

class test
{
  public: int x;
  test()
  {
    x = 0;
  }
};
main()
{
  const test t; // declaring const object 'a' of class 'A'
  // t.x = 10; // compilation error
}

• The above program will give a compilation error.
• Since we declared ‘t’ as a const object of the class test, we cannot change its data members.
• When we created ‘t’, its constructor got called assigning a value 0 to its data member ‘x’. Since ‘x’ is a data member of a const object, we cannot change its value further in the program. So when we tried to change its value, we got a compilation error.

Constant Data Members

• Once initialized, a const data member may never be modified, not even in the constructor or destructor.
• The const data member must be initialized by the constructor using an initialization list.

Example:

class test
{
  const int x;//const_data member
  public:
  test(int y)
  {
    x = y;
  }
  test(float z)
  {
    //x = z;//const memeber x cannot be reassigned
   }
};
main()
{
  A a(5);
  //A b(2.3);//invalid :assignment of read_only_object
}