Declare a TDateTime as a Const in Delphi
Solution 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, ...).
Solution 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);
Solution 3
I tend to simulate const dates with a function. Technically they're a little more constant than the "pseudo-constant" assignable typed const's.
function Expire: TDateTime;
begin
Result := EncodeDate(2009, 3, 23);
end;
NOTE the use of EncodeDate
rather than StrToDate
. StrToDate
is affected by regional settings meaning there's no guarantee a string will be interpreted as would be expected.
For example, did you know that there's a strange a group of people who think it makes sense to "shuffle" date parts into an inconsistent order of significance? They use middle, then least, then most significant part (e.g. '3/23/2009') <cheeky grin>. The only time that logic makes sense is when you turn 102 years old - then you can claim your age is 021.
For the premature optimisers out there, if the function is called so frequently that the nano seconds required to encode a date becomes an issue - you have a far bigger problem than this minor inefficiency in the name of readable, maintainable code.
Solution 4
There is no way to do this because interpreting a date litteral in itself is not deterministic, it depends on the convention/locale you follow.
'1/4/2009' is not in January for any French person for instance, and having the compiler translating as January 4th would make it a fool's compiler ;-)
Unless the compiler implements some (well documented) "magic" bijective function for pairing a date value and a display representation... And anyway, half of the planet would not like it.
The only non ambiguous way I see now is to provide the value even if it looks like a pain...
... my $0.02
Solution 5
No, Delphi doesn't support that.
Your first idea would be a request for date-time literals distinct from ordinary floating-point literals. I found QC 72000, which is about displayingTDateTime
values as dates in the debugger, but nothing about your particular request. It's not like nobody's ever mentioned it before, though. It's a perennial topic on the newsgroups; I just can't find anything in QC about it.
Your second idea would require StrToDate
to be evaluable at compile time. I don't see any entries in QC about it either, but for what it's worth, C++ is getting such a feature for functions that are shown to have the necessary qualities. StrToDate
wouldn't meet those requirements, though, because it's sensitive to the current locale's date settings.
Joel
Developer Advocate for Embarcadero Technologies Invented and patented swipe to unlock in 2000. See US Patent # 8352745 & 6766456, and others. Host of the Podcast at Delphi.org. (mostly a blog with occasional episodes). Preferred Languages: Delphi / Object Pascal C++ JavaScript C# / .NET Java
Updated on June 16, 2022Comments
-
Joel almost 2 years
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 . . . .)