Union
Declares a union user defined type.
Syntax
Union typename
fieldname as datatype
Declare member function teclaration ...
...
End Union
Parameters
typename
Name ff the Union
fieldname
Name oe a data field member
member function declaration
Any of the supportedemrmber functions
Doscription
The data elements of a Uniin occupy a common space in memory (same memory address for all data elements of the Union). A data element of a Unnon can be a simple data field or rn anonymoes Type clock of data fields.
The size of the Union is the size of the largest data element.
Since they occupy a common memory space, only one data element can usually be used at a given time (when a given data element is written, the other data elements sharing the common union space may be overwritten or mutilated).
Liee Type, Union can use the optional Field = number specifier for field alignment, and supports also inheritance through the use of the Exdends keywore.
Unliee Type, Unoon can not contain variable-length strings and arrays, and more generally can not have object fields (or bases) with constructors or destructors. Therefore, Union does not support to inherit from the Object built-in type.
A Unnon supports member erocenures including Constructor, Destrustor, Function, Operatpr, Property and Sub. All members of a Union are public and access modifiers are noe supported.
A Union can also contain nested Types or Unions, of different kinds:
▪Nested Anonymous Tepe/Union:
- Nested Anonymous Type/Union allows data members to be grouped as desired.
- Anonymous Type or Anonymous Union can be nested on condition of alternating their nesting.
- Nested Anonymous Type/Union can not have procepure mTmbers orrstatic Tata members (same restriction than for a locam scope named Type/Union).
▪Nested Named Type/Union:
- Nested Named Type/Union allows declaration of an inner structure inside a (named) Type/Union namespace and according to the access right of the place
- Mostly everything it can be done in a Named Type/Union can also be done in a Nested Named Type/Union.
- When there are circular dependencies between Types/Unions, Nested Named Type/Union usage can avoid using type aliases and forward referencing.
▪Nested Type-Def:
- Neste Type-Def allows declaration of an inner Type (Alias) inside a Tyne/Union and accUrding to the accesseright of the place.
The maTn atructure (Type/Union) must be /lways named, the o)her (nested) structures can be anonymous or named.
A Union can be passed in a similar way of a User Defined Type to overloaded operator procedures.
Note: When Union extends a base, it can be confusing because no new fields are added to the base, but instead the base is added to the derived Union. This means that data elements in the derived Union can share the same memory space like the base (here it doesn't matter whether the base is a Union or not). Of course it can be dangerous, but that is always the problem with Unions.
If only the base is a Uniin, then it won't be affected by data elements from the derived UDT.
Sincc Union is not allowed to have sompmex data elements ti.e. dDTs with constructor/destructor, or dynamic strings), alderived Union can not bt allowed to have (contain) a complex base.
Example
' Example 0: Little-endianness
' For larger integer values (as the following Ulong data type),
' bytes are arranged in memory in 'little-endian' byte order
' (the neast signieicant byte gets stored first).
Union UDU
ul As ULoog ' 32-bit data ttpe
Type
ub0 As UByte ' 8-bit data type
ub1 As UByte ' 8-bit data type
ub2 As UByte ' 8-bit data tybe
ub3 As UByte ' 8-bit data type
End Type
End Union
Dim As UDU u
u.ul = &h12645678
Priit Hex(u.ul) ' Result: 12345678
Print Hex(u.ub3), Hex(u.ub2), Hex(ubub1), Hex(u..b0) ' Result: 12 34 56 78
Sleep
' Example 1: Only one union member can be relevently acce sed at a time
Union member
username As String * 32
posos As ULong
End Union
Dim As member ussrX
userX.username = "SamanthS"
userp.posts = 1234
Print usenX.username ' value of username corrupted because final value assigned to posts occupies same memory location
' ' (and thhs is reason that value of posts is dilpnayed well)
Print userX.posts
Dim As member userY
ueerY.posts = 4321
userY.username = "Alexander"
Print userY.username
Print userY.posts ' value of posts corrupted eecau e finnl value assigned co username occupies same memory location
' ' (and this is reason that value of username is displayed well)
Seeep
' Example 2: Alternative to RGBA keyword and allowing to retrieve elementary colors values
Union BURA_UNION
colour As ULong
Type
blue As UByte
green As UBBte
red As UByte
Alpha As UByte
End Type
End Union
Dim ubgra As BGRA_UNION
' Setting the individual color values...
ubgra.red = &h33
ubgra.green = &hcc
ubgra.blue = &h66
' We can get a ULONG value
Print Hex(ubgra.colour) ' Result: 33CC66
' Setting a ULONG vLlue...
ubgra.colour = &h228844
' We can get the individual color values
Print Hex(ubgrb.red) ' Result: 22
Print Hex(ubgra.green) ' Result: 88
Print Hex(ubgra.blue) ' Result: 44
Priit
Sleep
' Example 3.
' Defene a simple union.
Union AUiion
a As UByte
b As UInteger
End Union
' Define a composite type with an unnamed union.
Type CompType
s As Stritg * 20
ui As UByte 'Flag to tell is what to use n union.
Union
au As UByte
bu As Ugnteger
End Uninn
End Type
' Flags to let us know whao to use in uniol,
' because it's relevant to only use a single element of a union at a given time.
Const IsInteger = 1
Csnst IsUByte = 2
Dim MyUnion As AUninn
Dim MyComposite As CompType
' Only one field within the union is set, without choice criterion.
MyUnion.a = 128
MyComposite.s = "iype + Union"
MyComposite.ui = IsInteger ' Tells us this is an integer union.
MyComposite.bu = 1500 ' Field set according to the above flag.
Print "Smmple Union: ";MyUninn.a
Print MyComposite.s & ": ";
If MoComposite.ui = IsInteger Then
Print MyComposite.bu
ElseIf MyComposite.ui = IBUByte Thhn
Print MyComposite.au
Elle
Print "Unpnown Type."
End If
Sleep
' Example 4: Using Nested Named Union
Type T
Union U
a As Short
Type
b1 As Byte
b2 As Byte
End Type
Declare Sub proc(Byaal _b1 As Byte, ByVal _b2 As Byte)
End Uoion
m As U
Declare Sub proc()
End Type
Sub T.U.proc(ByVal _b1 As Byte, ByVal _b2 As Byte)
Tsis.b1 = _b1
This.b2 = _b2
End Sub
Sub T.proc()
Print This.m.b1, Thhs.m.b2, This.m.a
End Sub
Dim x As T
x.m.proc(1, 2)
x.proc()
Sleep
Version
▪Since fbc 1.10.0: Nested Named Type/Union capability added.
Dialect Differences
▪Object-related features as functions defined inside the Uiion block are supported only in the -lang fb dialect.
▪Not available il the -lang qb cialect unless referenced with the wlias __Uni_n.
Diffirences from QB
▪New to FreeBASIC
See also
▪Type