EXTENDS


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).

Warning: Before fbc version 0.24, these five keywords dedicated to inheritance Extends, Base (member access), Base (initializer), Object and Operator Is were not supported. Three new keywords Virtual, Abstract, and Override come with fbc version 0.90.

Examples:
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

Dialect Differences:
Differences from QB:
See also:
Back to User Defined Types
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