Hi what I want to do is similar to this question here: https://en.delphipraxis.net/topic/423-rtti-determine-record-helper-type-for-a-base-type/

I am going to create multiple type helpers for enums and I would like to look up the helper and use common methods AsString AsOrdinal to get/set the values. I am recieving them as json that I dont have control over. I am using typeinfo to fill in all the properties but I would like to convert the strings I am receiving to the enum values semi-automatically.

LString := 'STATUS';
LHelper := GetHelper(LPropInfo^.PropType); // enum proptype
LToStringInterface := LHelper as IToStringInterface;
SetOrdProp(LObject, 'propname', LToStringInterface.OrdValue(LString);

Something like that. I'm not sure if it's even possible to find the helper type with typeinfo.

Thanks for your suggestions :)

Regards,

Andrew

So, I came up with a solution using Custom Attributes.

I can declare this:

[TEnumAttr('YES', 'NO', 'COULD_BE')]
   TMyEnum = (meYes, meNo, meCouldBe);

and at runtime look for the TEnumAttr attribute and use that.

TEnumAttr = class(TCustomAttribute)
   protected
     FValues: TStringArray;
     FTypeName: String;
   public
     class function FromType(ATypeInfo: PTypeInfo): TEnumAttr;
     constructor Create(Val1: String);
     constructor Create(Val1, Val2: String);

etc

Maybe this can help someone else with a similar use case.

Regards,

Andrew Haines

Since there can be multiple type helpers for a type, this will never be possible. At best you would get a list.

But I don't think there is even a list of type helpers per type. Sven will need to confirm.

In your case, I would simply register a function that does this for each known enum type.

Michael.

Michael Van Canneyt via fpc-pascal
schrieb am Fr., 23. Dez. 2022, 08:18:

As the active helper(s) for a type change(s) depending on the current scope (e.g. in unit A it might be a different helper than in unit B) this is not possible. Also helpers are not in any way usable on their own except to retrieve their RTTI information.

Regards,
Sven

But theoretically it should be possible to generate and get a list of all defined type helpers, whether they are in scope or not ?

If their RTTI could include a function that returns an interface, you could retrieve that and do as the OP suggests...

I think the OP's solution with attributes is the same as the one TMS Software's Aurelius ORM uses.

Michael.

This may be simpler:

[TEnumAttr(['YES', 'NO', 'COULD_BE'])]
   TMyEnum = (meYes, meNo, meCouldBe);
TEnumAttr = class(TCustomAttribute)
   protected
     FValues: TStringArray;
     FTypeName: String;
   public
     class function FromType(ATypeInfo: PTypeInfo): TEnumAttr;
     constructor Create(Values: array of string);

---

Or, if the enum values are valid identifiers, you can simple use scoped enums:

{$SCOPEDENUMS ON}
   TMyEnum = (YES, NO, COULD_BE);

and the usual TypeInfo methods.

Ondrej

Michael Van Canneyt via fpc-pascal
schrieb am Fr., 23. Dez. 2022, 10:48:

No, because the compiler doesn't keep track of that. It's only interested in what is in scope *right now*.

If their RTTI could include a function that returns an interface, you could retrieve that and do as the OP suggests..

Don't make helpers to something that they're simply not there for.

Regards,
Sven

I tried that first but the compiler gave an error on the "["

Error: Ordinal expression expected

[TEnumAttr(['YES', 'NO', 'COULD_BE'])]

I didn't know about those, that could also be a good solution, thanks!

Andrew

I understand such a list is not kept now.
That's why I said "theoretically" as in "it is possible to add to the compiler"

To be clear:
I don't think you should add such a list, I'm simply interested in the possibility of such a thing.

Michael.

Indeed. I would say that is a bug and therefore I reported it: https://gitlab.com/freepascal.org/fpc/source/-/issues/40058

I usually use the scoped enums but in case the enum values are not valid identifiers this is the perfect way to handle them. Therefore thanks for the idea :) I use attributes for various things but didn't realize this possibility.

Ondrej

Andrew Haines wrote

Error: Ordinal expression expected [TEnumAttr(['YES', 'NO', 'COULD_BE'])]

There is another (and cleaner) approach that uses an abstract attribute class. See attachment.

The advantage is that the compiler warns you with an error when you add a new value to the enum and you don't declare a string value for it.

Ondrej