Best Places to use Const

“const” is the keyword which comes very handy saving much of our time finding bugs and aiding in writing correct code. If there is a keyword provided from the compiler which makes our life easier , why not to use it. But most of the time , we are confused about its usage . Below are the places where const “should” be used .

The behavior of const is known thing that if we make an object const , compiler puts a constraint on your code that it shouldn’t be modified.

Following is the semantics of const usage:

• const char *p = "C_PLUS_CPLUS"                 //here  pointer is not constant but the data is constant

                                                 //You can increment decrement the pointer

                                               //but you cannot change the data

     As such 

            p++ ;   p += 2;                    //Valid statement as Pointer is  not const

     //But if you do this 

     p[1] = 'D';                        //Invalid because data is constant

• 
  

  char * const p = “C_PLUS_CPLUS"               //here the pointer is constant but not the data

  p++ ;                         //Invalid statement

    p[0] = 'D'                   //Valid as data can be changed.

• 
  

  const char * const p = "C_PLUS_CPLUS"               //Here not even the data in constant but even teh pointer is constant

As we can we figure out the syntax , if const is put before variable name after the asterix (“*”) , variable in const not the memory it owns in const , but if the const is placed at the left side of asterix , then the data which the variable owns in const.

Const in the Function Parameters

Most of the time const is placed as the return value of a function as

const Complex operator+(const Complex& C1, const Complex& C2);

Const should be specified in the function parameters , which does not change the parameters, When you know that you will not change the parameters inside the function body you should specify those as const , so as to avoid any mistake of changing them.

NOTE:: Above two parameters are passed as reference is a separate topic , which wiill be discussed shortly.

Const in Function Return type

In the above function declaration , return value is returned as const , because to handle following reasons.

If it was not there , any programmer can do it like this

Complex c1,c2,c3;

c1 + c2 = c3

const is specified in return type just to avoid these kind of issues.

Const Member function

We have seen const in function parameters , in function return values , but there is third situation where const is used ,  most of us of unaware of it and beginners mostly hesitate to write it

//Consider a ArrayList containg the array of 100 chars
Class ArrayList
{
     private:
            char m_cData[100];
   public:
        const int & opeator[](int  nPosition)  const    //operator [] for const ArrayList Objects
        {
                           return  m_cData[nPosition];
         }
        int & operator[](int nPosition)                //operator [] for non-const ArrayList objects
       {
                          return m_cData[nPosition];
        }
};

//Now the Above ArrayList object can be used like this

ArrayList arrList("C_PLUS_PLUS");

std::cout<<arrList[0];                                 //calls non-const member function ArrayList::operator[]

const ArrayList arrList("C_PLUS_PLUS");

std::cout << arrList[0];                             //calls const member function ArrayList::operator[]

 

Now you would be asking why to write a const member function when there is a equivalent function for the same purpose.

Its because there will be functions which are accepting parameters as a const – reference , so if you have written a const member function , the const objects passed as a reference in the function , which when used in the function calls their own version of const member functions.