We know structures can be used to create user-defined data types in C and C++. But then why we need classes? We can have functions, constructors, etc in structures as well but what differentiates it from classes are lack of abstraction and inheritance(And actually many other things as well). We can hide certain members from outside world(abstraction) in a class but we can't do anything like this in structures. Also, we cannot overload an operator here. For example

struct complex{
    float x,y;
};
// Structure for a complex number of form x+iy(i stands for iota)
complex c1,c2;
c1.x=1;
c1.y=2;
c2.x=3;
c2.y=3;
// c1=1+2i, c2=3+3i
complex c3=c1+c2;//Illegal statement in C because we cannot add these two structures. 

class className{
   private:
       variable declaration;
       function declaration;
   public:
       variable declaration;
       function declaration;
};

1. private: Data members labelled private are only available withing the class.
2. public: Data members are available for the outside world as well.
3. protected: Data members are not available for the whole world but are available to the inherited classes.

By default, all members are private.

1. Abstraction: Data hiding. All those visibility labels are used for abstraction only. We can