Extendstype

Specifies a base type from which to derive a new type

Syntax

Type|Union typename Extends base_typename

...

End Type|Union

Description

Extends declares typename to be derived from base_typename. The derived user-defined type, typename, inherits fields and methods of the base_typename base type. typename objects may be used in place of base_typename objects. Fields and methods inherited from base_typename will be implicitly accessible like members of typename.
However, a member will shadow an inherited member if they have the same identifier. The Base (Member Access) keyword can be used to explicitly access members of the base type shadowed by local members.

User-defined types that extend another type will include the base type structure at their beginning, and their size as reported by Sizeof() is the sum of their base type's size plus the size needed for any regular data fields (non-static data fields). Since the inherited members make sure that the structure is not empty, a derived type is not required to have regular data fields of its own.

In typename (the derived user-defined type), the fields can share the same memory space than the base_typename only if typename is a Union. Here it does not matter whether base_typename is a Union or not.
If only base_typename is a Union, then it will not be affected by fields from typename (the derived user-defined type).
As a Union is not allowed to have complex fields (i.e. user-defined types with constructor/destructor, or dynamic strings), a derived Union cannot be allowed to have (contain) a complex base_typename.

The Base (Initializer) keyword can be used at the top of constructor of derived user-defined type. It allows to specify an initializer or constructor call for the base type.

Extending the built-in Object type allows a user-defined type to be used with Operator Is to perform run-time type checks, to support Virtual and Abstract methods, and to use the Override method attribute.

Note: UDT pointer can only be cast to pointer types of "widened compatibility" (up or even down in the inheritance hierarchy), or to Any Ptr. Otherwise, cast to Any Ptr first (or to Object Ptr first if both types are directly or indirectly derived from Object).

Remarks

Dialect Differences

Not available in the -lang qb dialect unless referenced with the alias __Extends.

Differences from QB

New to FreeBASIC

Example

Type SchoolMember 'Represents any school member'

    Declare Constructor ()

    Declare Sub Init (ByRef _name As String, ByVal _age As Integer)

    As String Name

    As Integer age

End Type



Constructor SchoolMember ()

    Print "Initialized SchoolMember"

End Constructor



Sub SchoolMember.Init (ByRef _name As String, ByVal _age As Integer)

    This.name = _name

    This.age = _age

    Print "Name: "; This.name; "   Age:"; This.age

End Sub





Type Teacher Extends SchoolMember 'Represents a teacher derived from SchoolMember'

    Declare Constructor (ByRef _name As String, ByVal _age As Integer, ByVal _salary As Integer)

    As Integer salary

    Declare Sub Tell ()

End Type



Constructor Teacher (ByRef _name As String, ByVal _age As Integer, ByVal _salary As Integer)

    Print "Initialized Teacher"

    This.Init(_name, _age) 'implicit access to base member procedure'

    This.salary = _salary

End Constructor



Sub Teacher.Tell ()

    Print "Salary:"; This.salary

End Sub





Type Student Extends SchoolMember 'Represents a student derived from SchoolMember'

    Declare Constructor (ByRef _name As String, ByVal _age As Integer, ByVal _marks As Integer)

    As Integer marks

    Declare Sub Tell ()

End Type



Constructor Student (ByRef _name As String, ByVal _age As Integer, ByVal _marks As Integer)

    Print "Initialized Student"

    This.Init(_name, _age) 'implicit access to base member procedure'

    This.marks = _marks

End Constructor

    

Sub Student.Tell ()

    Print "Marks:"; This.marks

End Sub





Dim As Teacher t = Teacher("Mrs. Shrividya", 40, 30000)

t.Tell()

Print

Dim As Student s = Student("Swaroop", 22, 75)

s.Tell()

' Example using all eight keywords of inheritance:

'   'Extends', 'Base.', 'Base()', 'Object', 'Is' operator, 'Virtual', 'Abstract', 'Override'



Type root Extends Object ' 'Extends' to activate RTTI by inheritance of predefined Object type

  Declare Function ObjectHierarchy () As String

  Declare Abstract Function ObjectRealType () As String ' 'Abstract' declares function without local body

                                                        '    which must be overriden

  Dim Name As String

  Declare Virtual Destructor () ' 'Virtual' declares destructor with body ('Abstract' forbidden)

Protected:

  Declare Constructor () ' to avoid user construction from root

  Declare Constructor (ByRef rhs As root) '' to avoid user copy-construction from root

End Type ' derived type may be member data empty



Constructor root ()

End Constructor



Function root.ObjectHierarchy () As String

  Return "Object(forRTTI) <- root"

End Function



Virtual Destructor root ()

  Print "root destructor"

End Destructor





Type animal Extends root ' 'Extends' to inherit of root

  Declare Constructor (ByRef _name As String = "")

  Declare Function ObjectHierarchy () As String

  Declare Virtual Function ObjectRealType () As String Override ' 'Virtual' declares function with local

                                                                '    body which can be overriden

                                                                ' 'Override' to check if the function is

                                                                '    well an override

  Declare Virtual Destructor () Override ' 'Virtual' declares destructor with local body

                                         ' 'Override' to check if the destructor is well an override

End Type



Constructor animal (ByRef _name As String = "")

  This.name = _name

End Constructor



Function animal.ObjectHierarchy () As String

  Return Base.ObjectHierarchy & " <- animal" ' 'Base.' allows to access to parent member function

End Function



Virtual Function animal.ObjectRealType () As String

  Return "animal"

End Function



Virtual Destructor animal ()

  Print "  animal destructor: " & This.name

End Destructor





Type dog Extends animal ' 'Extends' to inherit of animal

  Declare Constructor (ByRef _name As String = "")

  Declare Function ObjectHierarchy () As String

  Declare Function ObjectRealType () As String Override ' 'Override' to check if the function is well an

                                                        '    override

  Declare Destructor () Override ' 'Override' to check if the destructor is well an override

End Type ' derived type may be member data empty



Constructor dog (ByRef _name As String = "")

  Base(_name) ' 'Base()' allows to call parent constructor

End Constructor



Function dog.ObjectHierarchy () As String

  Return Base.ObjectHierarchy & " <- dog" ' 'Base.' allows to access to parent member function

End Function



Function dog.ObjectRealType () As String

  Return "dog"

End Function



Destructor dog ()

  Print "    dog destructor: " & This.name

End Destructor





Type cat Extends animal ' 'Extends' to inherit of animal

  Declare Constructor (ByRef _name As String = "")

  Declare Function ObjectHierarchy () As String

  Declare Function ObjectRealType () As String Override ' 'Override' to check if the function is well an

                                                        '    override

  Declare Destructor () Override ' 'Override' to check if the destructor is well an override

End Type ' derived type may be member data empty



Constructor cat (ByRef _name As String = "")

  Base(_name) ' 'Base()' allows to call parent constructor

End Constructor



Function cat.ObjectHierarchy () As String

  Return Base.ObjectHierarchy & " <- cat" ' 'Base.' allows to access to parent member function

End Function



Function cat.ObjectRealType () As String

  Return "cat"

End Function



Destructor cat ()

  Print "    cat destructor: " & This.name

End Destructor





Sub PrintInfo (ByVal p As root Ptr) ' must be put after definition of animal type, dog type and cat type

  Print "  " & p->Name, "  " & p->ObjectRealType, "           ";

  If *p Is dog Then ' 'Is' allows to check compatibility with type symbol

    Print  Cast(dog Ptr, p)->ObjectHierarchy

  ElseIf *p Is cat Then ' 'Is' allows to check compatibility with type symbol

    Print Cast(cat Ptr, p)->ObjectHierarchy

  ElseIf *p Is animal Then ' 'Is' allows to check compatibility with type symbol

    Print Cast(animal Ptr, p)->ObjectHierarchy

  End If

End Sub





Print "Name:", "Object (real):         Hierarchy:"

Dim a As root Ptr = New animal("Mouse")

PrintInfo(a)

Dim d As root Ptr = New dog("Buddy")

PrintInfo(d)

Dim c As root Ptr = New cat("Tiger")

Printinfo(c)

Print

Delete a

Delete d

Delete c

Reference

Documented in KeyPgExtends.html