Operator Castoperator

Operator to convert a UDT (User Defined Type) variable into a specified data type

Operatorsoperatordocumented

Syntax

{ Type | Class | Union } typename

Declare Operator Cast () [ ByRef ] As datatype

...

End { Type | Class | Union }

Operator typename.Cast () [ ByRef ] As datatype [ Export ]

...

Parameters

Name		Description
`typename`		The name of the `Type`, `Class`, or `Union`
`datatype`		The name of the type for conversion into it, a built-in data type (standard data type) or a UDT different from
`typename`
`expression`		The expression to convert, an instance of
`typename`

Description

Converts an expression (a typename variable) into a different datatype.

Cast Operator must be declared inside the Type, Class, or Union.
As all non-static member procedures, it has passed a hidden This parameter.

The Cast Operator is the only operator (or function) that can be declared multiple times when only the return type differs, but not the same as the Type, Class, or Union they are declared in.

The Cast Operator allows conversions that can also be used on the right-hand side of expressions of construction (with initializer) and assignment (see example 1 below).
In addition to its explicit form usage Cast( datatype, expression ), the Cast Operator allows also implicit conversion into datatype (see same example 1 below). For this not explicit usage, the compiler may decide which cast overload to call based on how the object is used (for example on the Print keyword, the compiler calls the Cast() As String operator if defined).

Note: The overloaded Cast Operator can convert only a UDT instance (not a built-in type variable) because it can be overloaded only as a member Operator (not as a global operator).
To convert a built-in type variable into a UDT (not working syntax: Cast( UDT, built_in_type_variable )), the best way into the UDT is to define a constructor or/and a Let operator, otherwise to use a Cast Operator but with return by reference (by an assignment: Cast( built_in_datatype, UDT_instance ) = built_in_type_variable).
(see example 2 below)

For implicit conversion from one UDT into another UDT, a Cast Operator in the first UDT can replace (in second priority) a constructor and a Let operator in the second UDT (see examples 3 and 4 below).

Warning: For a Cast operator that returns by value (no byref), do not use generally an exit code like Return expression (or Operator = expression) if expression is an instance of typename.
Such a Cast operator code will induce an infinite loop when called, unless an implicit conversion from typename to datatype already exists through a matched constructor (or a let operator) for datatype, so with a higher priority (see example 5 below).

Remarks

Usage



Cast( datatype, expression )

Dialect Differences

Only available in the -lang fb dialect.

Differences from QB

New to FreeBASIC.

Example

Very simple syntaxic example highlighting the conversion capabilities (explicit and implicit) by using Cast operators:

Type UDT

  Dim As Integer I

  Declare Operator Cast () As Integer

  Declare Operator Cast () As String

End Type



Operator UDT.Cast () As Integer

  Print "UDT.Cast() As Integer",

  Return This.I

End Operator



Operator UDT.Cast () As String

  Print "UDT.Cast() As String",

  Return Str(This.I)

End Operator





Dim As UDT u



u.I = 12

Print Cast(Integer, u)               '' explicit conversion using the defined "Cast() As Integer" operator

Print Cast(String, u)                '' explicit conversion using the defined "Cast() As String" operator

Print u                              '' implicit conversion by compiler using the defined "Cast() As String" operator

Print



u.I = 34

Dim As Integer J = Cast(Integer, u)  '' construction with explicit initialization using the defined "Cast() As Integer" operator

Print J

Dim As Integer K = u                 '' construction with implicit initialization by compiler using the defined "Cast() As Integer" operator

Print K

Print



u.I = 56

J = Cast(Integer, u)                 '' explicit assignment using the defined "Cast() As Integer" operator

Print J

K = u                                '' implicit assignment by compiler using the defined "Cast() As Integer" operator

Print K

Print



u.I = 78

Dim As String S = Cast(String, u)    '' construction with explicit initialization using the defined "Cast() As String" operator

Print S

Dim As String G = u                  '' construction with implicit initialization by compiler using the defined "Cast() As String" operator

Print G

Print



u.I = 90

S = Cast(String, u)                  '' explicit assignment using the defined "Cast() As String" operator

Print S

G = u                                '' implicit assignment by compiler using the defined "Cast() As String" operator

Print G

Print



Sleep

Workarounds for the not working syntax: UDT_instance = Cast( UDT, built_in_type_variable ):
(using constructor, Let operator, and a Cast operator which returns by reference)

Type UDT

  As Integer I

  Declare Constructor ()

  Declare Constructor (ByVal I0 As Integer)

  Declare Operator Let (ByVal I0 As Integer)

  Declare Operator Cast () ByRef As Integer

End Type



Constructor UDT ()

End Constructor



Constructor UDT (ByVal I0 As Integer)

  Print "UDT.Constructor(Byval As Integer)",

  This.I = I0

End Constructor



Operator UDT.Let (ByVal I0 As Integer)

  Print "UDT.Let(Byval As Integer)",,

  This.I = I0

End Operator



Operator UDT.Cast () ByRef As Integer

  Print "UDT.Cast() Byref As Integer",,

  Return This.I

End Operator





Dim As UDT u



'u = Cast(UDT, 12)     '' unsupported - error 20: Type mismatch

u = UDT(34)            '' explicit conversion using the defined "Constructor(Byval As Integer)"

Print u.I

Print



u = 56                 '' implicit conversion by compiler using the defined "Let(Byval As Integer)" operator

Print u.I

Print



Cast(Integer, u) = 78  '' explicit conversion using the defined "Cast() Byref As Integer" operator with byref return

Print u.I

Print



Sleep

Conversion from UDT1 into UDT2, by using a constructor and a Let operator in UDT2:

Type _UDT1 As UDT1



Type UDT2

  Dim As Integer I2

  Declare Constructor ()

  Declare Constructor (ByRef u As _UDT1)

  Declare Operator Let (ByRef u As _UDT1)

End Type



Constructor UDT2 ()

End Constructor



Type UDT1

  Dim As Integer I1

End Type



Constructor UDT2 (ByRef u As UDT1)

  Print "UDT2.Constructor(Byref As UDT1)",

  This.I2 = u.I1

End Constructor



Operator UDT2.Let (ByRef u As UDT1)

  Print "UDT2.Let(Byref As UDT1)",,

  This.I2 = u.I1

End Operator





Dim As UDT1 u1



u1.I1 = 123

Dim As UDT2 u2 = u1  '' implicit conversion by compiler using the defined "UDT2.Constructor(Byref As UDT1)"

Print u2.I2

Print



u1.I1 = 456

u2 = u1              '' implicit conversion by compiler using the defined "UDT2.Let(Byref As UDT1)" operator

Print u2.I2

Print



Sleep

Conversion from UDT1 into UDT2, by using a Cast operator in UDT1:

Type UDT2

  Dim As Integer I2

End Type



Type UDT1

  Dim As Integer I1

  Declare Operator Cast () As UDT2

End Type



Operator UDT1.Cast () As UDT2

  Print "UDT1.Cast() As UDT2",,

  Dim As UDT2 u

  u.I2 = This.I1

  Return u

End Operator





Dim As UDT1 u1



u1.I1 = 123

Dim As UDT2 u2 = u1  '' implicit conversion by compiler using the defined "UDT1.Cast() As UDT2" operator

Print u2.I2

Print



u1.I1 = 456

u2 = u1              '' implicit conversion by compiler using the defined "UDT1.Cast() As UDT2" operator

Print u2.I2

Print



Sleep

Conversion from UDT1 into UDT2, by using in UDT1 a Cast operator exiting by Return This, but without infinite loop thanks to a matched Let operator in UDT2:

Type _UDT1 As UDT1



Type UDT2

  Dim As Integer I2

  Declare Operator Let (ByRef u As _UDT1)

End Type



Type UDT1

  Dim As Integer I1

  Declare Operator Cast () As UDT2

End Type



Operator UDT1.Cast () As UDT2

  Print "UDT1.Cast() As UDT2"

  Return This                       '' implicit conversion by compiler using the defined "UDT2.Let(Byref As UDT1)" operator

End Operator



Operator UDT2.Let (ByRef u As UDT1)

  Print "UDT2.Let(Byref As UDT1)"

  This.I2 = u.I1

End Operator





Dim As UDT1 u1

u1.I1 = 123



Print Cast(UDT2, u1).I2



Sleep

Reference

Documented in KeyPgOpCast.html