Extends Zstringtype

Specifies a type which inherits a Zstring behavior

Syntax

Type|Union typename Extends Zstring [, base_typename]

...

End Type|Union

Description

Extends Zstring declares typename to inherit properties and behaviors of a ZString. Purpose is to allow users to create custom string types (with i.e. dynamic memory management) that can integrate well in to existing fbc compiler built ins (good interoperability with fbc's ZString type).

This declaration of such a UDT with a suitable Cast operator will instruct compiler to convert the UDT to a ZString (in addition, other suitable operators as Let, [] (Pointer Index), Len, ..., can be also declared).

ZString behaviour can be inherited directly, or indirectly and singly from a base-type.
ZString behaviour can be inherited by a UDT also extending base_typename (a kind of pseudo multiple-inheritance).

By declaring a type (directly or indirectly) as Extends Zstring (in addition to defining a suitable Cast operator only), this promotes it fully ZString type compatible, even with StrPtr/SAdd, LSet/RSet, and Select Case.

Remarks

Version

Before fbc 1.09.0, this promotion was not yet fully ZString type compatible with the built in functions Val/ValInt/ValLng/ValUInt/Valunlg and Left/Right.
Since fbc 1.07.0

Dialect Differences

Not available in the -lang qb dialect unless referenced with the alias __Extends __Zstring, but unusable because no member procedure is allowed in this dialect.

Differences from QB

New to FreeBASIC

Example

Type myZstring Extends ZString

  Public:

    Declare Constructor (ByRef z As Const ZString = "")

    Declare Operator Cast () ByRef As Const ZString

    Declare Operator Let (ByRef z As Const ZString)

  Private:

    Dim As String s

End Type



Constructor myZstring (ByRef z As Const ZString = "")

  This.s = z

End Constructor



Operator myZstring.Cast () ByRef As Const ZString

  Return *StrPtr(This.s)

End Operator



Operator myZstring.Let (ByRef z As Const ZString)

  This.s = z

End Operator



Dim As myZstring z = "FreeBASIC"

Print "'" & z & "'"



z &= " compiler"

Print "'" & z & "'"



Sleep

Type vZstring Extends ZString

  Public:

    Declare Constructor (ByVal pz As Const ZString Ptr = 0)

    Declare Operator Cast () ByRef As ZString

    Declare Operator Let (ByVal pz As Const ZString Ptr)

    Declare Operator [] (ByVal index As Integer) ByRef As UByte

    Declare Destructor ()

  Private:

    Dim As ZString Ptr p

    Dim As UInteger l

End Type



Constructor vZstring (ByVal pz As Const ZString Ptr = 0)

  This.l = Len(*pz)

  This.p = CAllocate(This.l + 1, SizeOf(ZString))

  *This.p = *pz

End Constructor



Operator vZstring.Cast () ByRef As ZString

  Return *This.p

End Operator



Operator vZstring.Let (ByVal pz As Const ZString Ptr)

  If This.l < Len(*pz) Then

    Deallocate(This.p)

    This.l = Len(*pz)

    This.p = CAllocate(This.l + 1, SizeOf(ZString))

  End If

  *This.p = *pz

End Operator



Operator vZstring.[] (ByVal index As Integer) ByRef As UByte

  Return This.p[index]

End Operator



Destructor vZstring ()

  Deallocate(This.p)

End Destructor



Operator Len (ByRef v As vZstring) As Integer

  Return Len(Type<String>(v))        '' found nothing better than this

End Operator                         ''     (or: 'Return Len(Str(v))')



Dim As vZstring v = "FreeBASIC"

Print "'" & v & "'", Len(v)



Dim As ZString * 256 z

z = *StrPtr(v)                       '' 'error 24: Invalid data types' without 'Extends Zstring'

Print "'" & z & "'", Len(z)



v &= Space(2)

Print "'" & v & "'", Len(v)

RSet v, "FreeBASIC"                  '' 'error 24: Invalid data types' without 'Extends Zstring'

Print "'" & v & "'", Len(v)          ''     ('Cast' must return a modifiable reference)



Select Case v                        '' 'error 24: Invalid data types' without 'Extends Zstring'

Case Type<vZstring>(Trim(v) & "  ")

  Print "Left justified"

Case Type<vZstring>("  " & Trim(v))

  Print "Right justified"

End Select



v[0] = Asc("-")

Print "'" & v & "'", Len(v)



Print "'" & Right(v, 5) & "'"        '' since fbc 1.09.0, 'Right' supports types with 'Extends Zstring'

'Print "'" & Right(Str(v), 5) & "'"  '' before fbc 1.09.0, use this workaround (or: 'Right(Type<String>(v), 5)')



Sleep

Reference

Documented in KeyPgExtendsZstring.html