VIRTUAL


Declare virtual methods

Syntax:
Type typename Extends base_typename
End Type

Description:
Virtual methods are methods that can be overridden by data types derived from the type they were declared in, allowing for dynamic polymorphism. In contrast to Abstract methods, virtual methods must have an implementation, which is used when the virtual is not overridden.

A derived type can override virtual methods declared in its base type by declaring a method with the same identifier and signature, meaning same number and type of parameters (invariant parameters), same calling convention, and if any, same return type (or a covariant return type for return by reference or by pointer):
  • if that differs only in parameter passing mode or calling convention or return type, then an overriding error is returned at compile time,
  • otherwise shadowing only is permitted for any other signature difference, corresponding to case where both methods would be overloadable.
The property of being a virtual method is not implicitly inherited by the overriding method in the derived type. If this overriding method must be overridden in turn in a lower level derived type, it must also be declared as virtual.

When calling virtual methods, the compiler may need to do a vtable lookup in order to find out which method must be called for a given object. This requires an extra hidden vtable pointer field to be added at the top of each type with virtual methods. This hidden vptr is provided by the built-in Object type. Because of that, virtual methods can only be declared in a type that directly or indirectly extends Object.

Dynamic polymorphism by using override procedures:
  • Normally only a typename procedure (or upper in hierarchy) is accessible through a base-typename reference/pointer even if this one refers to an object derived from typename.
  • But when the procedure is virtual, this tells the running program to resolve the override procedure the most derived relating to the real object type by vtable lookup (dynamic binding at runtime), rather than procedure normally accessible from the raw base-type of the reference/pointer (static binding at compile time).

Constructors cannot be virtual because they create objects, while virtual methods require an already-existing object with a specific type. The type of the constructor to call is determined at compile-time from the code.
In addition, when calling a virtual method inside a constructor, only the version of the method corresponding to an object of type of this constructor is used. That is because the vptr has not yet been set up by the derived type constructor, but only by the local type constructor.

Destructors often must be virtual when deleting an object manipulated through a pointer to its base type, so that the destruction starts at the most derived type and works its way down to the base type. To do this, it may be necessary to add virtual destructors with an empty body anywhere an explicit destruction was not yet required, in order to supersede each non-virtual implicit destructor induced by the destructor in its base.
On the other hand, when calling a virtual (or abstract) method inside a destructor (virtual or not), only the version of the method corresponding to an object of type of this destructor is used because the vptr is reset at the top of the destructor according to its own type's vtable. This avoids to access child methods and so to refer to child members previously destroyed by the child destructor execution.

For member methods with Virtual in their declaration, Virtual can also be specified on the corresponding method bodies, for improved code readability.

Note: In a multi-level inheritance, a same named method (same identifier and signature) can be declared Abstract, Virtual or normal (without specifier) at each inheritance hierarchy level. When there is mixing of specifiers, the usual order is abstract -> virtual -> normal, from top to bottom of the inheritance hierarchy.
The access control (Public/Protected/Private) of an overriding method is not taken into account by the internal polymorphism process, but only for the initial call at compile-time.
Base.method() calls always the base's own method, never the overriding method.
A derived static method cannot override a base virtual/abstract method, but can shadow any base method (including virtual/abstract).

Examples:

'' Example with overriding subroutines


Type Hello Extends Object
    Declare Virtual Sub hi( )
End Type

Type HelloEnglish Extends Hello
    Declare Sub hi( )            '' overriding subroutine
End Type

Type HelloFrench Extends Hello
    Declare Sub hi( )            '' overriding subroutine
End Type

Type HelloGerman Extends Hello
    Declare Sub hi( )            '' overriding subroutine
End Type


Sub Hello.hi( )
    Print "hi!"
End Sub

Sub HelloEnglish.hi( )           '' overriding subroutine
    Print "hello!"
End Sub

Sub HelloFrench.hi( )            '' overriding subroutine
    Print "Salut!"
End Sub

Sub HelloGerman.hi( )            '' overriding subroutine
    Print "Hallo!"
End Sub


Randomize( Timer( ) )

Dim As Hello Ptr h

For i As Integer = 0 To 9
    Select Case( Int( Rnd( ) * 4 ) + 1 )
    Case 1
        h = New HelloEnglish
    Case 2
        h = New HelloFrench
    Case 3
        h = New HelloGerman
    Case Else
        h = New Hello
    End Select

    h->hi( )
    Delete h
Next

Sleep


'' Example with overriding destructor and
''              overriding function with covariant return


Type myBase Extends Object
  Declare Virtual Function clone () As myBase Ptr
  Declare Virtual Sub Destroy ()
End Type

Function myBase.clone () As myBase Ptr
  Dim As myBase Ptr pp = New myBase(This)
  Print "myBase.clone() As myBase Ptr", pp
  Function = pp
End Function

Sub myBase.Destroy ()
  Print "myBase.Destroy()", , @This
  Delete @This
End Sub


Type myDerived Extends myBase
  Declare Function clone () As myDerived Ptr     '' overriding member function with covariant return
  Declare Sub Destroy ()                         '' overriding member subroutine
End Type

Function myDerived.clone () As myDerived Ptr     '' overriding member function with covariant return
  Dim As myDerived Ptr pc = New myDerived(This)
  Print "myDerived.clone() As myDerived Ptr", pc
  Function = pc
End Function

Sub myDerived.Destroy ()                         '' overriding member subroutine
  Print "myDerived.Destroy()", , @This
  Delete @This
End Sub


Dim As myDerived c

Dim As myBase Ptr ppc = @c
Dim As myDerived Ptr pcc = @c

Dim As myBase Ptr ppc1 = ppc->clone()            '' using base pointers and polymorphism
Dim As myDerived Ptr pcc1 = pcc->clone()         '' using derived pointers and covariance of return value
Print
ppc1->Destroy()                                  '' using base pointer and polymorphism
pcc1->Destroy()                                  '' using derived pointer

Sleep

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