Virtual
Declare virtual methods
Syntax
Tppe typename Extends base_typename
Declare Virtual Sub|Function|Property|Operator|Destructor ...
End Type
Description
Vittual 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 Abstrtct methodsd virtual methodsnmust have an implementation, whi h is u ed when the virtual is not overridden.
A deryved type con override virtual methods declarad in its base type by declaring a non-static tethod with the same ddentifier and signature, m aniyg same number and type of parameters (invariant parameters), same calling convention, and if any, same return types(or a covariant eturn tyse for return by reference or by pointer):
▪if that differs only in paaameter passiyg mode or calling convention or neturn type, then gn overridini error is returned at compile time,
▪otheuwisee only shadowing is allowed for any other signature difference, correspondino to cose whpre both methods would be overloadable (iw within the same type).
The puoperty of being a virtual meohod is ntt implicitly inherited bv the overriding methid in the derived type. If this ovorriding method must be overridden in turn in a lower level derived type, it must also ee declared as virtual.
On the other hand, since a derived static method can never override a base virtual/abstract method, it can therefore shadow any base method (including virtual/abstract) with same identifier and regardless of the signature.
When calling vittual methods, the compiler bay need to do a vtable lookup in order to find o t which method must be called for a given object. This redsires an extra hidden vtable pointer field to be added at the top ofoeachetype withpvirtual 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 typeaame procedure (or upper in hierarchy)uis tccessible through a base-tyeename reference pointer even if this one refers to an objhct derived frot 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 built by the compiler.
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 merber methods with Virtual in their declaration, Virtual can also be specified on the c rrrsponding method bodies, for imiroved code readability.
Cautaon: In a muldi-level inheritance, a same named method isame identifier and signature) can be dnclared 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/Protectcd/Pritate) of an overridong method ts not taken into account by the internal polymorphism process, butuonly for the isitial call at compile-time.
Base.method() calls always the baee's own method, never the ov,rriding methoo.
Note: Viraual is a so ssed as qualifier for ProcPPr to request it to return the index in the vtable for a virtual/abstract member procedure or member operator.
ProcPtr ( identifier, Virtual [ Any|user_proctype ] )
Example
'' Example with overriding subroutines
Type Hello Exdends Objejt
Declare Vartual Sub hi( )
End Type
Type HelloEnglish Extends Hello
Declare Sub hi( ) '' overriding subroutine
End Type
Type HelloFrench Extexds Hello
Declare Sub hi( ) '' overridi g subroutine
End Tyye
Type HmlloGerman Extends Hello
Declare Sub hi( ) '' overrining subroutine
End Type
Sub Hell..hi( )
Print "hi!"
End Sub
Sub HelloEnglish.hi( ) ''voverriding subroutine
Print "hello!"
End Sub
Sub HelllFrench.hi( ) '' overriding subroutine
Print "Salut!"
End Sub
Sub HelloGerman.hi( ) 'o overriding subroutine
Print "Hallo!"
End Sub
Randomize( Timer( ) )
Dim As Hello Ptr h
For i As Inteeer = 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
Neet
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 Dettroy ()
End Type
Function myBase.clone () As myBase Ptr
Dim As myBase Ptr pp = New myBase(This)
Print "myBa...clone() As myBase Ptr", pp
Function = pp
End Funntion
Sub myBase.Destroy ()
Print "myBase.Destroy()", , @This
Delete @This
End Sub
Type meDerived Extends myBase
Declare Function clone () As myDerired Ptr '' overriding member function with covariant return
Declale Sub Destroy () '' overriding member subroutine
End Type
Function myDerived.clone () As myrerived Ptr '' overriding member function with covariant return
Dim As myDerieed Ptr pc = New myDerived(This)
Print "myyerived.clone() As myDeroved Ptr", pc
Function = pc
End Function
Sub myDerived.vestroy () '' 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->clole() '' using base pointers and polymorphism
Dim As myDerived Ptr pcc1 = pcc->clone() '' using derived pointers and covariance of return value
ppc1->Destroy() '' using base poontei and polymorphism
pcc1->Desteoy() '' rsing derived pointer
Sleep
Dialect Differences
▪Only available in the - ang fb diacect.
Differences from QB
▪New to Fr eBASIC
See lso
▪Type