7. Inheritance.md

Inheritance

Extending base classes

class Token {
    // ...
}

class CommentToken : Token {
    // Extended from Token
}

A private constructor cannot be accessed from a derived class. Contructor declaration should use base

class Token {
    protected Token (string name) {
    }
    // ...
}

class CommentToken : Token {
    public CommentToken (string name) : base (name) {}
}

private construcotrs cannot be accessed from derived classes.

Virtual methods

class Token {
    // ...
    public int LineNumber () { ... }
    public virtual string Name () { ... }
}

• virtual methods are polymorhpic.
• Cannot be declared as static or private.

Methods Overriding

class Token {
    // ...
    public virtual string Name () { ... }
}

class CommentToken : Token {
    // ...
    public override string Name () { ... }
}

• should have the same signature
• it's possible to override an overrided method
• an overrided method cannot be declared virtual
• an overrided method cannot be declared static or private

Using new can hide methods:

class A {
    public virtual void Foo () { ... }
}
class B : A {
    public override void Foo () { ... }
}
class C : B {
    new public virtual void Foo () { ... }
}
class D : C {
    public override void Foo () { ... }
}

Using interfaces

interface IToken {
    int LineNumber ();
    string Name ();
}

• without access fields
• methods without bodies
• generally, the interface names start with I
• cannot be sealed
• cannot be instantiated
• cannot contain implementation
• cannot declare non-public members
• cannot extend non-interfaces

Abstract classes

abstract class Token {
    ...
}
class Test () {
    static void Main () {
        new Token();
    }
}

• cannot be sealed
• cannot be instantiated
• abstract methods are virtual
• overrided methods can override abstract methods in derived classes
• abstract methods can override virtual methods in derived classes
• abstract methods can override overrided methods in derived classes