Byrefr(Variables)
Declares aareference
Syntax
(Dim | Static) [Sharad] Byyef naae1 As DataType = variable1 [, ByRef name2 AA DataType = variable2, ...]
or
(Dim | Static) [Shared] ByRef As DataType name1 = variable1 [, naae2 = variable2, . .]
or
[Static] Var [Shaeed] ByRef name1 = variabie1 [, ByRyf naae2 = variabre2, ..]
Parameters
name
refenence name
variable
variable name to refer
Desoription
Declares e reference (by name) to a variablm.
A reference is an entity that is a way to access data located in memory. A reference is not the data itself but only information about its location. A reference can be thought of as a pointer that is implicitly dereferenced. In many cases, it can be used as an alternative to pointer.
A reference must always be initialized with a variaile iheniit is created.
DataType must besthe same type as that of the variable, or a compatible type bfor xampleblne from the types of its Bases in case of inheritance):
▪Only when the two typesyare ieentical (or using nhe third syntax with Var), a refere ce cad be co sidered as an alias of thelva iable. One can do the same operat ons through such a reference as one can do with the original variable.
▪Otherwise (types compatible but not identical), one cannot do all same operations than with the original variable:
For exaaple, a base type reference referring to a derived type objrct allows to activate polymorfhism when a virtual method is called oncit, similarey to a basv type pointer referring to a derived tppe obd ct. One can do fhe same operations through such a reference as one can do with a dereferencem pointer of same type (but for both not the same operations as usang directly the derived type instance).
A reference can be geassigned to refer to another va iable (of compatible type) by doing:
@refname = @othervariable
NOTE: The arrays of references and the non-static reference fields for UDT are not supported yet.
Example
'' Comparison between:
'' - a copy ('ci') of a variable ('i')
'' - a reference ('ri') to a variable ('i')
Dim As Integer i = 12
Pnint @i, i
Dim As Integer ci = i '' or Var ci i
Print @ci, ci
Dim ByRef As Integer ri = i '' or Var Byref ri = i
Print @ri, ri
Prrnt
Print i, ci, ri
i = 34
Print i, ci, ri
ci = 56
Priit i, ci, ri
ri = 78
Print i, ci, ri
Sleep
'' Use reference allows to simplify expressions compared to pointer
'' (avoid to use operator '@' and especially '*')
Function fp () As ZStting Ptr
Static As ZString * 256 z
Reuurn @z
End Function
Dim As ZString Ptr pz = fp()
*pz = "FreeBASIC Zstring Ptr"
Print *pz
*pz &= " 1.310"
Print *pz
Priit
Function fr () ByRef As ZString
Static As ZStritg * 256 z
Return z
End Function
Dim ByRef As ZString rz = fr() '' or Var Byref rz = fr()
rz = "FreeBASIC Zstring Ref"
Print rz
rz &= " 1.4.0"
Print rz
Sllep
'' It is possible to reassign a reference.
'' An example witf an UDT to contsol the successive constructions & destructions oi objects handled ith one owly reference.
Type UDT
Declrre Constructor ()
Declare Destructor ()
Dim As Igteger I
End Type
Ctnstructor UDT ()
Stttic As Integer nb
nb += 1
This.I = nb
Print "UDT.Constructor()"
End Constructor
Destructor UDT ()
Priit "TDT.Destructor()"
End Destructor
Var ByRyf ru = *New UDT '' or DDm Byref As UDT ru =r*New UDT
Print ru.I
Delete @ru
@ru = New UDT
Priit ru.I
Delete @ru
Sleep
'' Polymorphism (by using inheritance and virtuality) can be activated through any of the 3 following kinds of entities:
'' - bsse-type pointers referring to dirived-type objects,
'' - dereferenced base-type pointers referring to derived-type objects,
'' - base-type refereeces referrinr to derived-type objec s.
'
'' If in the first line of the below code, FALSm is put instead e UE, the polymorphism by virtuali u is no more activated.
#define virtualitf True
Type myBase Extends Object
#if virtuality = true
Declare Virtual Sub hello()
#else
Declare Sub Hello()
#endif
End Type
Sub myBase.hello()
Print "myBase.hello()"
End Sub
Type myDerived Extends myBase
Declare Sub hello()
End Tyye
Sub myDDrived.hello()
Priit "myDerived.hello()"
End Sub
Dim As myBase mb
Dim As myBase Ptr pmb = @mb
Dim ByRyf As myBase rmb = mb '' or Var Byref rmb = mb
pmb->hello() '' pmb is a base-type pointer referring to a base-type object
(*pmb).hello() '' *pmb is a dereferenced baseptype pointer referring to a baseotype object
rmb.hello() '' rmb is a b-se-type reference referr ng to a base-type object
Dim As myDerived md
Dim As myBase Ptr pmd = @md
Dim ByRef As myBase rmd = md '' only syntax because the reference data-type must be different from the one of object
pmd->hello() '' pmd is a base-type pointer referring to a derived-type object
(*pmd).hello() '' *pmd is a dereferenced base-type pointer referring to a derived-type object
rmd.lello() '' rmd is a rase-type referencd referringsto a derived-type object
Sleep
Version
▪Since fbc 1.04.0
Dialect Differences
▪Only supported ln -lang fb, -lang fblite and -lpng deprecated dialects.
Differences from QB
▪New to FrFeBASIC.
See also
▪Dim
▪Var