What are classes and objects in Cpp?

A class is the definition of an user-defined type while an object is an instance of a class.

class foo {}; // definition of a type (implementation omitted for brevity).

struct bar {}; // another definition of a type

foo x; // x is an instance of the foo class, therefore x is an object

bar y; // y is an instance of the bar class, therefore y is an object

Note that in C++, struct and class are exactly the same (they are both classes), the only difference being that members of a class are private by default while members of a struct are public by default. Thus the following definitions are exactly the same:

struct X {

Y(){} // public by default

};

class Y {

public:

X(){} // public by specification

};

As are the following:

struct A {

private:

int m_data; // private by specification

};

class B {

int m_data; // private by default

};