How use bit/bit-operator to control object state?

Question

I want to create light object data-package to pass between client and server applications.

It is a so simple task, that I can control with only 1 byte, so each bit in a byte will have a different meaning,

Using only the bit

0 = False 
1 = True

Itens I need now:

1 - Loaded from database 
2 - Persisted
3 - Changed
4 - Marked to Delete
5 -
6 - 
7 - Null Value 
8 - Read Only


1) How do I use bit operators in Delphi to check each bit value? 
2) How do I set the bit Values?

Solution

After all help, Ill use the next Set

  TStateType = (
    stLoaded    = 0,   // loaded from persistance
    stNative    = 2,   // value loaded and converted to native type
    stPersisted = 3,   // saved
    stChanged   = 4,   // object or member changed
    stToDelete  = 5,   // marked to delete
    stReadOnly  = 6,   // read only object, will not allow changes
    stNull      = 7    // value is null
  );
  TState = Set of TStateType;

And for stream -> persistance, this will be the record to be used:

  TDataPackage = record
    Data: TBytes;
    TypeInfo: TMetaInfo;
    State: Byte;
    Instance: TBuffer;
  end;

Thank you guys, for all the answers and comments.

Answer 1

I'd really use a set for this. However, I see you really want a byte. Use sets everywhere then typecast to a byte in the end.

This solution will require much less typing, has support for standard delphi operators and really carries no performance penalty as Barry Kelly has pointed out.

procedure Test;
type
  TSetValues = (
    TSetValue1   = 0,
    TSetValue2   = 1,
    TSetValue4   = 2,
    TSetValue8   = 3,
    TSetValue16  = 4,
    TSetValue32  = 5,
    TSetValue64  = 6,
    TSetValue128 = 7
  );

  TMySet = set of TSetValues;
var
  myValue: byte;
  mySet: TMySet;
begin
  mySet := [TSetValue2, TSetValue16, TSetValue128];
  myValue := byte(mySet);
  ShowMessage(IntToStr(myValue)); // <-- shows 146
end;

Answer 2

I would use a set for this:

type
    TMyDatum = (mdLoaded, mdPersisted, mdChanged, mdMarkedToDelete, ...);
    TMyData = set of TMyDatum;

var
  Foo: TMyData;
begin 
  Foo := [mdLoaded, mdChanged];
  if (mdPersisted in Foo) then ...

These are implemented as integers, so you can pass them easily. And I find the code much, much more readable than bitwise operators.

Answer 3

This page describes Delphi operators, including bitwise operators.

It sounds like you need to use the and operator. For example:

const
  LOADED_FROM_DATABASE = 1;
  PERSISTED = 2;
  CHANGED = 4;
  // etc...

//...

if (bitFlags and LOADED_FROM_DATABASE) <> 0 then
begin
  // handle LOADED FROM DATABASE
end;

if (bitFlags and PERSISTED) <> 0 then
begin
  // handle PERSISTED
end;

// etc...

In order to set the flags, you can use OR:

bitFlags := LOADED_FROM_DATABASE or PERSISTED or CHANGED;