Delphi – Declare a TDateTime as a Const in Delphi

constantsdatedelphioperator-overloadingtdatetime

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 . . . .)

Best Solution

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, ...).