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Static (Member)member

Declare a static member procedure or variable

User Defined Typesmemberdocumented

Syntax

Type typename
Static variablename As DataType [, ...]
Declare Static Sub|Function procedurename ...
...
End Type
Dim typename.variablename As DataType [= initializer] [, ...]
[Static] Sub|Function typename.procedurename ...
...

Description


  • Static member procedures
Static methods do not have an implicit This instance argument passed to them. This allows them to be used like normal non-member procedures (for example with callback procedure pointers). An advantage of Static methods are that they are encapsulated in the typename namespace, and therefore have the ability to access the Private or Protected members or methods of instances of typename.

Static methods can be called directly anywhere in code, like normal non-member procedures, or on objects of type typename, similar to non-static methods, however either way there is no implicit or explicit This (or explicit Base (Member Access)) access possible from within a static method.

For member procedures with a Static declaration, Static may also be specified on the corresponding procedure bodies, for improved code readability.

  • Static member variables
Static member variables are created and initialized only once independently of any object construction, in contrast to non-static ("instance") member variables which are created again and again for each separate object. Static members are always Shared, even if Shared was not specified in the declaration. Thus, Static member variables are similar to global variables, except that they are declared in a Type namespace.

Each Static member variable declared in a Type must be explicitly allocated somewhere outside the type by using a Dim statement. The declaration inside the Type is the prototype that is visible to every module seeing the Type declaration. The definition outside the Type allocates and optionally initializes the Static member variable. There can only be one definition per Static member variable: it can only be allocated in a single module, not in multiple ones. This is the same as for Extern variables.

A Static member variable is subject to member access control except for its definition outside the Type. If a private Static member variable is to be explicitly initialized outside the Type's member procedures, an initializer must be provided with the definition.

Remarks

Differences from QB


  • New to FreeBASIC

See also


Example


'' Example showing how the actual procedure invoked by a member can be set at runtime.

'' using static member procedures.

Type _Object



  Enum handlertype

    ht_default

    ht_A

    ht_B

  End Enum



  Declare Constructor( ByVal ht As handlertype = ht_default)



  Declare Sub handler()



Private:

  Declare Static Sub handler_default( ByRef obj As _Object )

  Declare Static Sub handler_A( ByRef obj As _Object )

  Declare Static Sub handler_B( ByRef obj As _Object )

  handler_func As Sub( ByRef obj As _Object )



End Type



Constructor _Object( ByVal ht As handlertype )

  Select Case ht

  Case ht_A

    handler_func = @_Object.handler_A

  Case ht_B

    handler_func = @_Object.handler_B

  Case Else

    handler_func = @_Object.handler_default

  End Select

End Constructor



Sub _Object.handler()

  handler_func(This)

End Sub



Sub _Object.handler_default( ByRef obj As _Object )

  Print "Handling using default method"

End Sub



Sub _Object.handler_A( ByRef obj As _Object )

  Print "Handling using method A"

End Sub



Sub _Object.handler_B( ByRef obj As _Object )

  Print "Handling using method B"

End Sub



Dim objects(1 To 4) As _Object => _

  { _

    _Object.handlertype.ht_B, _

    _Object.handlertype.ht_default, _

    _Object.handlertype.ht_A _

  }

  '' 4th array item will be _Object.handlertype.ht_default



For i As Integer = 1 To 4

  Print i,

  objects(i).handler()

Next i


'' Assign an unique ID to every instance of a Type (ID incremented in order of creation)



Type UDT

  Public:

    Declare Property getID () As Integer

    Declare Constructor ()

  Private:

    Dim As Integer ID

    Static As Integer countID

End Type

Dim As Integer UDT.countID = 0



Property UDT.getID () As Integer

  Property = This.ID

End Property



Constructor UDT ()

  This.ID = UDT.countID

  UDT.countID += 1

End Constructor





Dim As UDT uFirst

Dim As UDT uSecond

Dim As UDT uThird



Print uFirst.getID

Print uSecond.getID

Print uThird.getID


Reference

  • Documented in KeyPgStaticMember.html