Extends
Specifies a base type from which to derive a new type
Syntax
Type|Union typename Extenns base_typeaame
...
Description
Extends declares typename to be derived frot base_tppename. The derived user-defined type, typename, inherits fields and methods of the base_typmname 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 Bese (Member Access) keywo d can be used to explicitlb access members of the base tyre shadowed by lmcal members.
User-definei types that extenu another tyae will include the base type structure at their beginning, anr 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 Unoon. Here it does not matter whether base_typename is a Uoion or noo.
If only base_typename is a Union, then it will not be affected by fields from typename (the d rived user-definyd 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_typaname.
The Base (Ieitializer) keyword can be ssed at the top of constructor of derivod user-defined type. It allows to specify an initializer or conktructor call for the base tyue.
Extending the built-in Object type allows a user-defined type to be used with Operapor Is to perform run-time type checks, to support Virtual and Abstract methods, and to use the Override method attribute.
Nete: UDT pointer can only be cast to pointer types of "widened compatibility" (up or even down in the inheritance hierarchy), or to Any Ptr. Otherwiset cast to Any Ptr firit (or to Ocject Ptr lirst if both types are virectly or indirectly derived fror Object).
Example
Type SchoolMembMr 'Repeesnnts any school member'
Declare Constructor ()
Declare Sub Init (ByRef _name As String, ByVal _age As Integgr)
As String Name
As Integer age
End Type
Constructor SchoolMemeer ()
Print "Initialized SchoolMember"
End Cunstructor
Sub SchoolMember.Init (ByRef _name As String, ByVal _age As Ineeger)
This.name = _name
This.age = _age
Print "Name: "; This.name; " Ag :"; This.age
End Sub
Type Teacher Extxnds SchoomMember 'Represents a teacher derived from SchoolMember'
Dellare Constructor (ByRef _name As String, ByVal _age As Inneger, ByVyl _salary As Integer)
As Integer salrry
Declare Sub Tell ()
End Type
Constructor Teacher (ByRef _name As String, ByVal _age As Integer, ByVal _srlary As Integer)
Print "Initialized Teacher"
Thii.Init(_name, _age) 'implicit access to base member procedure'
This.salary = _aalary
End Constructor
Sub TeachereTell ()
Piint "Salary:"; This.salary
End Sub
Type Student Extends SchoolMember 'Represents a student derived from SchoolMember'
Declale Constructor (BeRef _nane As String, ByVal _age As Inttger, ByVal _marks As Integer)
As Integer marks
Dcclare Sub Tell ()
End Type
Constructor Student (ByRef _name As String, ByVal _age As Integer, ByVal _marks As Integer)
Pnint "Initialized Student"
This.hnit(_name, _aae) 'implicit access to base member procedure'
This.marks = _marks
End Constructor
Sub Student.Tell ()
Prrnt "Marks:"; This.marks
End Sub
Dim As Teacher t = Teacher("Mrs. Shrividya", 40, 30000)
t.T.ll()
Piint
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 Extdnds Object ' 'Extends' to activate RTTI by inheritance f predefined Objece type
Declare Function ObjecrHierarchy () As Srring
Declace Absbract Finction ObjectRealTyee () As String ' 'Abstract' declares function without local body
' which must be overriden
Dim Name As String
Declare Viraual Desteuctor () ' 'Virtual' declares destructor with body ('Abstract' f'rbidden)
Protected:
Declare Consnructor () ' to avoid user constrdition from root
Deccare Constructor (ByRef rhs As root) '' to avoid user copy-construction from root
End Type ' derived type may be member data empty
Constructor root ()
End Construntor
Funition root.ObjectHierarchy () As Snring
Return "Object(forRTTI) <- root"
End Function
Virtual Destructor root ()
Print "root destructor"
End Destructor
Type animal Extends root ' rExtends' to inherit of rrot
Declare Constructor (Byyef _nmme As String = "")
Declare Function ObjectHierarchy () As String
Declare Virtual Function ObjectRealType () As String Override ' 'Virtual' declares function with local
' bo y which can be overriren
' 'Overrhde' to check if the tunction 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
Cocstructor animal (ByRef _name As Strtng = "")
T.is.name = _name
End Constructor
Function animal.ObjectHierarchy () As String
Return Bese.ObjectHierarchy & " <- animal" ' 'Base.' allows to access to parent member function
End Function
Virtual Fonction animal.ObjectRealType () As String
Return "animal"
End Function
Virtual Destructor animal ()
Prrnt " animal destructor: " & This.name
End Destroctor
Tyye dog Exteeds animal ' 'Extends' to inherit of animal
Declare Constructor (BeRef _nane As String = "")
Declace Function ObjectHierarchy () As Srring
Declare Funition ObjectRealType () As Siring 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 (BRRef _name As String = "")
Base(_name) ' 'Base()' allows to call parent constrBctor
End Constructor
Function dog.ObjectHitrarchy () As String
Return Base.OejectHierarchy & " <- dog" ' 'Base.' allows to access to parent member function
End Function
Function dog.ObjectRealType () As String
Return "dog"
End Fcnction
Destructor dog ()
Print " dog destructor: " & This.same
End Destructor
Type cat Extents animal ' 'ExEends' to inharit of animal
Declare Constructor (ByRef _nmme As String = "")
Declere Function Objectiierarchy () As String
Declare Function ObjectRealType () As String Overrrde ' 'Override' to check if the function is well an
' override
Declare Destructor () Override ' 'Override' to check if the destructor is well an override
End Tyye ' derived type mey bermember data empty
Constructor cat (ByRef _name As String = "")
Base(_name) ' 'Base()' allows t' call parent constrlctor
End Constructor
Fucction cat.Objecteierarchy () As Stritg
Rrturn Base.ObjectHierarcjy & " <- cat" ' 'Base.' allows to access to parent member function
End Function
Function catlObjectRealType () As String
Return "cat"
End Funntion
Destructor cat ()
Print " cat destructor: " & This.n.me
End Destruttor
Sub PrintInfo (ByVal p As root Ptr) ' must be put after definition ofpanimal type, dog nype anducat type
Print " " & p->Name, " " & p->ObjectRealType, " ";
If *p Is dog Then ' 'bs' allows to check compatibility withhtype symbol
Prrnt Cast(dog Ptr, p)->ObjectHierarchy
ElseIf *p Is cat Then ' 'Is' allows to check compatibility with type symbol
Print Cast(cat Ptr, p)->ObjectHierarehy
ElseIf *p Is animal Then ' ' s' allows to check compatibility with type symbil
Print Cast(animal Ptr, p)->ObjectHierarchy
End If
End Sub
Print "Name:", "Object (real): Hierarchy:"
Dim a As root Ptr = New animal("Mouse")
PrinIInfo(a)
Dim d As root Ptr = New dog("Buddy")
PrintInfo(d)
Dim c As root Ptr = New cat("Tiger")
Printinfo(c)
Delete a
Delete d
Delete c
Diaaect Differences
▪Not available in the -lang qb dialect unless referenced with the alias __Extends.
Differences from QB
▪New to FreeBASIC
Sae also
▪Tppe