question about records

[virtual]

hi all

i still don't understand this :

TColor =packed record
case integer of
0 : (R, G, B, A : Byte);
1 : (C : Cardinal);
end;

i mean 0 and 1 , whats the meaning of theme

[vgo]

You can use the case statement to map different types of variables in the same memory space.The types used must be equal size, ie. Cardinal = 4 bytes in this case.

MyColor.R := $FF;
MyColor.G := $00;
MyColor.B := $FF;
MyColor.A := $00;
// Is equal to
MyColor.C :=  $00FF00FF;

Another example

TVector = record
   case Integer of
      0: (x, y, z: Single);
      1: (v: array [0..2] of Single);
end;

MyVector.x := 5.75;
MyVector.y := 1.75;
MyVector.z := 0.00;
// Is equal to
MyVector.v[0] := 5.75;
MyVector.v[1] := 1.75;
MyVector.v[2] := 0.00;

[chronozphere]

The 1 and 0 don't really mean anything. They just show that there are two different ways to access that piece of memory. During program execution, you can access either R, G, B and A or C to change the whole color at once.

[WILL]

Yeah I found it funky that you can even stick functions and procedures into records as well.

Actually what I've never been able to figure out what 'packed' really does, or maybe I simply forgot. What is the purpose of this keyword and what does it do?

[Brainer]

This is called a variant record. It's similar to C++'s union, but it's not the same, so you can't confuse them and put them two in the same basket. Compare Wikipedia article about unions and variant records.

[Brainer]

@WILL: Look here http://www.delphibasics.co.uk/RTL.asp?Name=Packed

[WILL]

@WILL: Look here http://www.delphibasics.co.uk/RTL.asp?Name=Packed

I see now. Thanks!

[farcodev]

embedded case in a record is a very useful thing

[virtual]

thank you so much

[Bpascal]

I see now. Thanks!

About the "packed" clause in record defination, the most important think is to know is that if you do not use "packed" then delphi/pascal is free to add "invisibles" fields to the record to adjust the total size of the structure for best perfomance; so take note that using for example "sizeof(myrecord)," could give you a record size different at what you think if you sum indididually every field size;

Normaly there is not any problem using or not a packed record if you are going to use its members in traditional way. but if you are going to update/read data from records using move(), or blockread()/blockwrite() or similar function & procedure where you pass the record for read or write his members from a block memory then is adviced to use packed records so you as programmer are completly sure you record is aligned as you defined.