Constructor

For the derived class, it doesn't have access to the base class's private members, so it need use some public method to help access them.

When a program constructs a derived-class object, it first constructs the base-class object. Conceptually, that means the base-class object should be constructed before the program enters the body of the derived-class constructor. C++ uses the member initializer list syntax to accomplish this.

For example:

RatedPlayer::RatedPlayer (unsigned int r, const string & fn, const string & ln, bool ht) : TableTennisPlayer(fn, ln, ht) {
  rating = r; 
}

Here, TableTennisPlayer(fn, ln, ht) is the member initializer list. And should be called before RatedPlayer's constructor. If you miss the list initializer, it will call the default constructor for the base class.

Copy constructor

Accept one argument const MyClass& t a reference. And the compiler will also generate a default copy constructor.

Key Points

These are the key points about constructors for derived classes:

  • The base-class object is constructed first.
  • The derived-class constructor should pass base-class information to a base-class constructor via a member initializer list.
  • The derived-class constructor should initialize the data members that were added to the derived class.

Destructor

Destroying an object occurs in the opposite order used to construct an object.That is, the body of the derived-class destructor is executed first, and then the base-class destructor is called automatically.

A program first calls the base-class constructor and then calls the derived-class constructor. The base-class constructor is responsible for initializing the inherited data members. The derived-class constructor is responsible for initializing any added data members.

Relationship between derived and base class

  • a derived-class can use base-class's method which is not private
  • a base-class pointer can point to a derived-class object without an explicit type cast (since a derived-class contains members of the base-class, but the opposite is not true) Just in one direction, use base pointer to derived object.
Base-class* p = &derived-obj  // OK
Derived-class* q = &base-obj  // Wrong, the base class may not have member of the derived class

But in default, the base class pointer or reference can only invoke just base-class methods.

Let's see another example:

Derived-class d1(...);
Base-class d2(d1);

Why this is acceptable? Since we know that for base-class, it has a default copy constructor, which accept a reference of Base-class&, but when we pass d1(Derived-class) into it, it's translated into Base-class, which can be explained.

Therefore, you can assign a Base-class with a Derived-class object;

Base-class d2 = d1;   // same as calling implicit copy constructor
  • a base-class reference can refer to a derived-class object without an explicit type cast