Explanation of the code snippet , keyword operator in , in c++ , use of objects in classes

Question

class Vector { 
          public: 
           Vector(int s) :elem{new double[s]}, sz{s} { }
           double& operator[](int i) { return elem[i]; } //function 2
           int size() { return sz; } 

            private: 
                double∗ elem;  
                int sz; 
         };

Code snippet from : the c++ prog. language 4th edition , Bjarne IDE : Microsoft Visual Studio Professional 2013 My experience : Newbie , have learned a little using turbo c++ before.

I noticed while typing function2 that operator is a keyword(shown in blue color) , but when whole function2 is declared , operator turns white , that is it doesn't remain an keyword anymore.

If i try using any other name other than "operator" , an error pops up , which says Error: array of function is not allowed

It would be very helpful if someone can explain what is happening in function2, I want to know all that there is about the keyword operator , why,how and when to use this.Ofcourse from a beginner's perspective.

Please refrain from overly complex answers , I am complete newbie

I tried searching on internet , couldn't find anything else apart from operator overloading , which is not my question.

double read_and_sum(int s) { 
                        Vector v(s);                     //line 1
                        for (int i=0; i!=v.size(); ++i) 
                        cin>>v[i];                        //line 3
                        double sum = 0; 
                        for (int i=0; i!=v.size(); ++i) 
                        sum+=v[i];
                         return sum;
                        }

Here line1 passes the argument "s" of int type needed by constructor of class vector , that's fine.

But in line3 how can the statement "cin>>v[i]" be valid ? , since the object v isn't declared as an array of objects. Even if it is valid where does the value go..?

Basically it is out of my knowledge , please explain line3 in simplest possible manner, and at the same time as exhaustively as possible , so that I can learn.

PS: Do not tell me , that i shouldn't be learning c++ , or i shouldn't be using that IDE or anything else like that. If you can , then answer the question, I shall be very thankful.

Answer 1

What this is is called operator overloading. You are overloading the operator [] with a new method. So basically, operator is a conditional keyword here. You can't use it as a method or variable name, but you can reference it for defining an operator overload. So its reserved in some situations, but not others, like the above example.

And to explain operator overloading: You are adding operators to a class so that it can do that functionality when you use the specific syntax associated with that operator.

So, in this case, we're adding the array access operator to the class, so we can use that functionality as if it were an array.

Examples of operators you can overload:

• addition (+)
• subtraction (-)
• multiplication (*)

and so on.

In essence, operator overloading allows a class to act like another type of object. Any sort of operation, like addition, assignment, increment, decrement, comparisons(<, >, !=, ==, etc), and so on all are operators that can be overloaded. This allows you to write a class and be able to use that syntax, such as newObj > oldObj, instead of calling a method, like newObj->IsGreaterThan(oldObj).