Declare a TDateTime as a Const in Delphi

Question

As far as I know there is no way to do this, but I am going to ask just in case someone else knows how to do this. How can I declare a date as a const in Delphi?

The only solution I have found is to use the numeric equivalent, which is kind of a pain to maintain because it is not human readable.

const   Expire : TDateTime = 39895; // Is actually 3/23/2009 

What I would like to be able to do is something like this:

const   Expire : TDateTime = TDateTime ('3/23/2009'); 

or

const   Expire : TDateTime = StrToDate('3/23/2009'); 

So let me know if this is a feature request or if I just missed how to do this (yeah, I know it seems like an odd thing to want . . . .)

Answer 1

Ok, my reaction is a bit late, but here's a solution for the newer Delphi's.

It uses implicit class overloaders so that records of this type can be used as if they are TDateTime variables.

  TDateRec = record     year,month,day,hour,minute,second,millisecond:word;     class operator implicit(aDateRec:TDateRec):TDateTime;     class operator implicit(aDateTime:TDateTime):TDateRec; // not needed     class operator implicit(aDateRec:TDateRec):String; // not needed     class operator implicit(aDateRec:String):TDateRec; // not needed   end; 

Implementation:

uses DateUtils;  class operator TDateRec.Implicit(aDateRec:TDateRec):TDateTime; begin   with aDateRec do // Yeah that's right you wankers. I like "with" :)     Result := encodeDateTime(Year,Month,Day,Hour,Minute,Second,Millisecond); end;  class operator TDateRec.Implicit(aDateTime:TDateTime):TDateRec; begin   with Result do     DecodeDateTime(aDateTime,Year,Month,Day,Hour,Minute,Second,Millisecond); end;  class operator TDateRec.Implicit(aDateRec:TDateRec):String; begin   Result := DateTimeToStr(aDateRec) end;  class operator TDateRec.Implicit(aDateRec:String):TDateRec; begin   Result := StrToDateTime(aDateRec) end; 

Now you can declare your dates like this:

const   Date1:TDateRec=(Year:2009;month:05;day:11);   Date2:TDateRec=(Year:2009;month:05;day:11;hour:05);   Date3:TDateRec=(Year:2009;month:05;day:11;hour:05;minute:00); 

To see if it works, execute the following:

ShowMessage(Date1); // it can act like a string ShowMessage(DateToStr(Date1)); // it can act like a date 

If you really want to replace all your TdateTime variables with this, you probably need to overload some other operators too (Add, subtract, explicit, ...).

Answer 2

The only? possible way, but probably not what you are looking for:

const {$J+}   Expire: TDateTime = 0; {$J-}  initialization   Expire := EncodeDate(2009, 3, 23);