Rule of three in C++ is a fundamental principle of the design and the development of three requirements that if there is clear definition in one of the following member function, then the programmer should define the other two members functions together. Namely the following three member functions are indispensable: destructor, copy constructor, copy assignment operator.
Copy constructor in C++ is a special constructor. It is used to build a new object, which is the new object equivalent to a copy of an existing object.
Copy assignment operator is a special assignment operator that is usually used to specify an existing object to others of the same type of object.
There are quick examples:
// default constructor
My_Class a;
// copy constructor
My_Class b(a);
// copy constructor
My_Class c = a;
// copy assignment operator
b = a;