Hello,
It looks like declaring GUIDs follow a different set of rules than declaring any other record type.
Consider the following code:
{$define USE_DATA_TYPE_IDENTICAL_TO_GUID}
program _GUID_Declarations;
uses
Windows,
ActiveX
;
type
{ EXACT copy of TGUID definition }
TDATA_TYPE = packed record
case integer of
1 : (
Data1 : DWord;
Data2 : word;
Data3 : word;
Data4 : array[0..7] of byte;
);
2 : (
D1 : DWord;
D2 : word;
D3 : word;
D4 : array[0..7] of byte;
);
3 : ( { uuid fields according to RFC4122 }
time_low : dword; // The low field of the timestamp
time_mid : word; // The middle field of the timestamp
time_hi_and_version : word; // The high field of the timestamp multiplexed with the version number
clock_seq_hi_and_reserved : byte; // The high field of the clock sequence multiplexed with the variant
clock_seq_low : byte; // The low field of the clock sequence
node : array[0..5] of byte; // The spatially unique node identifier
);
end;
type
TDATA_TYPE2 = TGUID;
const
{ this compiles }
SOMEGUID : TGUID = '{00000114-0000-0000-C000-000000000046}';
const
{ this also compiles }
SOMEGUID2 : TDATA_TYPE2 = '{00000114-0000-0000-C000-000000000046}';
{$ifdef USE_DATA_TYPE_IDENTICAL_TO_GUID}
const
{ this does NOT compile ... why ??? - TDATA_TYPE is identical to TGUID }
SOMEGUID2 : TDATA_TYPE = '{00000114-0000-0000-C000-000000000046}';
{$endif}
// in activex.pp
//
// IID_IOleWindow : TGUID = '{00000114-0000-0000-C000-000000000046}';
//
// IOleWindow = interface(IUnknown)
// ['{00000114-0000-0000-C000-000000000046}'] { <- duplicated value }
//
// ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
//
// this duplicated value opens the door to errors
const
{ define IID as plain - untyped - constant }
IID_IOleWindow = '{00000114-0000-0000-C000-000000000046}';
type
{ _IOleWindow }
IOleWindow = interface(IUnknown) [IID_IOleWindow] { <- use the plain constant }
{ methods here }
end;
begin
readln;
end.
On line 15, there is an EXACT copy of the definition of TGUID (copy/pasted.) if I try to declare a value using the copy of TGUID's definition (line 58), the compiler rejects it, that's in spite of the fact that other than the data type name being different, the definition is identical to the one in lines 46 and 51 which are both accepted.
Now a different question altogether....
In activex.pp, just about every interface id is declared as a typed constant (line 64) and then restated in full in the interface declaration (line 67.) The concern here is possible errors due to incorrect duplication of the interface id.
The compiler accepts declaring the constants without type (which as a bonus saves data storage) (line 77) and use that constant as the interface id in the interface declaration (line 82)
There are several advantages to that method, one is that no storage in the data section is necessary, it is impossible to modify the interface id since it is not stored and lastly, there is no interface id duplication.
My only concern is, does doing the way shown in lines 77 and 82 have the potential to cause problems that the other way does not ?
Thank you for your help.
Attached to the post is the code presented above. Comment/uncomment the define to see the different behavior.
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