C++ Data Abstraction

* Abstract Data Type (class and object)
- Public interface
- Private implementation and private interface for derived classes

* Class
The following is very important for the abstract data type. They might be generated by compiler and could not be derived (not be virtual except destructor). (Note another one which can not be derived is the hidden data member or member function.)
- Constructor
1) Use initialization list in the constructor as much as possible to improve initialization efficiency. Const and reference must be put in the list while static and array member must not. Base class, const, reference and object members must be initialized.
2) Note the consistent order of member initialization.
3) Constructor with only one input or default inputs performs implicit casting.
4) Direct initialization instead of copy initialization, for example,
X a(10);
X a = X(10);
x a = 10;

- Destructor
1) If the class is designed for being derived, destructor must be virtual to allow the correct destructor to be called with dynamic binding and uniform data type mapping in memory. If not, do not declare it as virtual to save memory.
2) Even the destructor is pure virtual function, it still needs to be defined and it is allowed.

- Copy constructor
1) If some handle to object/memory outside the object is contained, the copy constructor must be redefined.
2) Each member must be copied explicitly in the copy constructor, esp. the base class.
3) The standard format for copy constructor
X(X const &x)
4) Put it in the private area without new definition to avoid use it illegally.

- Assignment operator
Note the difference of initialization and assignment.
1) If some handle to object/memory outside the object is contained, the assignment operator must be redefined. Delete the original memory allocation and reallocate and assign it.
2) The self assignment must be checked.
if (this != that) ...
3) Each member must be assigned explicitly in the assignment operator, esp. calling the assignment operator of base class.
B::operator =(that);
4) The standard format for assignment operator
X const &operator=(X const &x)
5) Put it in the private area without new definition to avoid use it illegally.