{{fbdoc item="title" value="EXTENDS ZSTRING"}}----
Specifies a type which inherits a Zstring behavior

{{fbdoc item="syntax"}}##
	[[KeyPgType|Type]]|[[KeyPgUnion|Union]] //typename// **Extends Zstring** [, //base_typename//]
		...
	End [[KeyPgType|Type]]|[[KeyPgUnion|Union]]
##
{{fbdoc item="desc"}}
	##**Extends Zstring**## declares ##//typename//## to inherit properties and behaviors of a ##[[KeyPgZstring|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 ##[[KeyPgZstring|Zstring]]## type).
	
	This declaration of such a UDT with a suitable ##[[KeyPgOpCast|Cast]]## operator will instruct compiler to convert the UDT to a ##[[KeyPgZstring|Zstring]]## (in addition, other suitable operators as ##[[KeyPgOpLet|Let]]##, ##{{fbdoc item="keyword" value="KeyPgOpPtrIndex|[] (Pointer index)"}}##, ##[[KeyPgLen|Len]]##, ..., can be also declared).
	
	##[[KeyPgZstring|Zstring]]## behaviour can be inherited directly, or indirectly and singly from a base-type.
	##[[KeyPgZstring|Zstring]]## behaviour can be inherited by a UDT also extending ##//base_typename//## (a kind of pseudo multiple-inheritance).
	
	**Note:** By declaring a type (directly or indirectly) as ##**Extends Zstring**## (in addition to defining a suitable ##[[KeyPgOpCast|Cast]]## operator only), this promotes it fully compatible even with ##[[KeyPgOpStrptr|Strptr]]##/##[[KeyPgSadd|Sadd]]##, ##[[KeyPgLset|Lset]]##/##[[KeyPgRset|Rset]]##, and ##[[KeyPgSelectcase|Select Case]]##, but currently not yet with the built in functions ##[[KeyPgVal|Val]]##/##[[KeyPgValint|Valint]]##/##[[KeyPgVallng|Vallng]]##/##[[KeyPgValuint|Valuint]]##/##[[KeyPgValulng|Valunlg]]## and ##[[KeyPgLeft|Left]]##/##[[KeyPgRight|Right]]##.

{{fbdoc item="ex"}}
{{fbdoc item="filename" value="examples/manual/udt/extendszstring1.bas"}}%%(freebasic)
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
%%
{{fbdoc item="filename" value="examples/manual/udt/extendszstring2.bas"}}%%(freebasic)
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 ('vZstring.l' being private)
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) & "'"      '' 'Right' does not yet support types with 'Extends Zstring'
Print "'" & Right(Str(v), 5) & "'"  '' workaround (or: 'Right(Type<String>(v), 5)')

Sleep
%%
{{fbdoc item="lang"}}
	- Not available in the //[[CompilerOptlang|-lang qb]]// dialect unless referenced with the alias ##**""__Extends __Zstring""**##, but unusable because no member procedure is allowed in this dialect.

{{fbdoc item="diff"}}
	- New to ""FreeBASIC""

{{fbdoc item="see"}}
	- ##[[KeyPgType|Type]]##
	- ##[[KeyPgUnion|Union]]##
	- ##[[KeyPgExtends|Extends]]##
	- ##[[KeyPgZstring|Zstring]]##
	- ##[[KeyPgExtendsWstring|Extends Wstring]]##

{{fbdoc item="back" value="CatPgUserDefTypes|User Defined Types"}}