Static (Member)
 
Declare a static member procedure or variable

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 ...
...
End Sub|Function

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), the only difference being that they are encapsulated in the typename namespace and have access to typename's Private or Protected members.

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) access possible from within a static method.

For member procedures with Static in their prototype, Static can 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 member variables which are created again and again for each separate object. They always are 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.

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


Differences from QB

  • New to FreeBASIC

See also