Base (Member Access)keyword

Provides explicit access to base type members in non-static methods of a Type

Syntax

Base.member

Base [ .Base ... ] .member

Description

Base provides a way to explicitly access members of a specific base type, in the context of non-static methods of a user-defined type derived from another type using Extends.

By using Base repeatedly, as in base.base.base.member, it is possible to access any desired base type, in case there are multiple levels of inheritance.

Base is especially useful when a base type's member is shadowed by a local variable or member of a derived type using the same identifier. Base then allows unambiguous access to the base type.

For virtual methods, base.method() always calls the base method and never the overriding method.

Note: There is no specific syntax with Base to access a member operator of a specific base type. The only way is to apply the operator on the instance beforehand up-casted to the right type (but for virtual operators, this workaround does not allow to call the base operator when overridden, because that does not modify the run-time type of the instance but only its compile-time type).

Remarks

Dialect Differences

Methods are only supported in the -lang fb dialect, hence Base has no function in other dialects.

Differences from QB

New to FreeBASIC

Example

Type Parent

    As Integer a

    Declare Constructor(ByVal As Integer = 0)

    Declare Sub show()

End Type



Constructor Parent(ByVal a As Integer = 0)

    This.a = a

End Constructor



Sub Parent.show()

    Print "parent", a

End Sub



Type Child Extends Parent

    As Integer a

    Declare Constructor(ByVal As Integer = 0)

    Declare Sub show()

End Type



Constructor Child(ByVal a As Integer = 0)

    '' Call base type's constructor

    Base(a * 3)

    This.a = a

End Constructor



Sub Child.show()

    '' Call base type's show() method, not ours

    Base.show()

   

    '' Show both a fields, the base type's and ours'

    Print "child", Base.a, a

End Sub



Type GrandChild Extends Child

    As Integer a

    Declare Constructor(ByVal As Integer = 0)

    Declare Sub show()

End Type



Constructor GrandChild(ByVal a As Integer = 0)

    '' Call base type's constructor

    Base(a * 2)

    This.a = a

End Constructor



Sub GrandChild.show()

    '' Call base type's show() method, not ours

    Base.show()

   

    '' Show both a fields, the base.base type's, the base type's and ours'

    Print "grandchild", Base.Base.a, Base.a, a

End Sub



Dim As GrandChild x = GrandChild(3)

x.show()

Reference

Documented in KeyPgBase.html