[QOD] Guess the output (Union)

Guess the output (Union)

Giving the following UNION and relative variable ...

Code:

Union MyUnion b(12) As Byte st As String * 12 End Union Dim MyU As MyUnion

Giving one of the following two methods to fill the union array b ...

Code:

'---Method 1 to fill UDT array With MyU .b( 1) = 116 .b( 2) = 104 .b( 3) = 105 .b( 4) = 110 .b( 5) = 66 .b( 6) = 97 .b( 7) = 115 .b( 8) = 105 .b( 9) = 99 .b(10) = 32 .b(11) = 33 End With '---Method 2 to fill UDT array With MyU .b( 1) = 116, 104, 105, 110, 66, 97, 115, 105, 99, 32, 33 End With

What will be the output of:

Code:

MsgBox 0, MyU.st

Hehe,

since PB/DOS I have the ASCII table in my head, so I am quite sure the result will be "thinBASIC !" :)

UNIONs are cool feature when porting from other languages, but in practice I use more the DIM .. AT syntax to achieve same result in most projects.

I think in this special case it is enough to have the array dimensioned to 11 elements and the string to 11 characters.

Why?

Because STRING * n is not null terminated, which is the difference compared to ASCIIZ * n.

So the code could be in theory this (added one method for array fill):

Code:

Union MyUnion b(11) As Byte st As String * 11 End Union Dim MyU As MyUnion '---Method 1 to fill UDT array With MyU .b( 1) = 116 .b( 2) = 104 .b( 3) = 105 .b( 4) = 110 .b( 5) = 66 .b( 6) = 97 .b( 7) = 115 .b( 8) = 105 .b( 9) = 99 .b(10) = 32 .b(11) = 33 End With '---Method 2 to fill UDT array With MyU .b( 1) = 116, 104, 105, 110, 66, 97, 115, 105, 99, 32, 33 End With '---Method 3 to fill UDT array MyU.b( 1) = 116, 104, 105, 110, 66, 97, 115, 105, 99, 32, 33 MsgBox 0, MyU.st

Petr

I always wanted to know what that "Union" does.

Now I think I've learned from this thread that st and b of myU refer to the same adress in memory and both display the same values stored as different types. Just for interest: I could replace some "cast"-Methods (cstr, cbyte etc.) by using a Union, for example I could use

Code:

Union someUnion l as long b(4) as Byte End Union dim myU as someUnion myU.l = 123456

' now the b()-bytes show the value 123456 split up to &H00, &H01, &HE2 and &H40 ? Cool.

@Petr: right you are.
ASCIIZ * n needs 12 bytes (due to NULL at the end) while for STRING * n 11 bytes are enough.

@ReneMiner

Quote:

Originally Posted by ReneMiner

... st and b of myU refer to the same address in memory and both display the same values stored as different types.

Exactly: every element into an UNION shares the same memory area with all the other elements.
The total memory area occupied by an UNION is the size of the longest element present into the UNION.

Here a little more interesting UNION example

Code:

Uses "console" '---3 different structures each of which representing a thing: dog, orange, coin Type Dog lBreed As Long lAge As Byte sName As String * 16 End Type Type Orange lSize As Byte End Type Type Coin lCountry As Long End Type '---Put all the things together into an union so they all share the same memory area Union ThingProperties tDog As Dog tOrange As Orange tCoin As Coin End Union '---Now creates a general purpose thing able to represent all the possible things Type Thing lThingType As Long uThingProp As ThingProperties End Type '---Possible things type %ORANGE = 1 %COIN = 2 %DOG = 3 '---Allocate few things Dim tThing(3) As Thing Dim lCurrentThing As Long '---Fill things with a thing type and thing data tThing(1).lThingType = %ORANGE tThing(1).uThingProp.tOrange.lSize = 25 tThing(2).lThingType = %COIN tThing(2).uThingProp.tCoin.lCountry = 1234 tThing(3).lThingType = %DOG tThing(3).uThingProp.tDog.sName = "FUFFY" '---Show something a bout the things For lCurrentThing = 1 To UBound(tThing) Print "Thing is ... " Select Case tThing(lCurrentThing).lThingType Case %ORANGE PrintL "an Orange of size" , tThing(lCurrentThing).uThingProp.tOrange.lSize Case %COIN PrintL "a Coin by country" , tThing(lCurrentThing).uThingProp.tCoin.lCountry Case %DOG PrintL "a Dog whose name is" , tThing(lCurrentThing).uThingProp.tDog.sName End Select Next WaitKey