TCustomAttribute and class internal types
Consider
Default = class(TCustomAttribute)
DefaultVal: Integer;
constructor Create(const aVal:Integer);
end;
TMyBaseClass = class(TObject)
public
constructor Create; virtual; // handles attributes
end;
// 1
TMyClass = class(TMyBaseClass)
public
[Default(5)]
Value: Integer;
end;
which is ok, versus
TMyBaseClass = class(TObject)
public
type
Default = class(TCustomAttribute)
DefaultVal: Integer;
constructor Create(const aVal:Integer);
end;
constructor Create; virtual; // handles attributes
end;
// 2
TMyClass = class(TMyBaseClass)
public
[Default(5)]
W1025 Unsupported language feature: ‘custom attribute’
Value: Integer;
end;
as well as
// 3
TMyClass = class(TMyBaseClass)
public
[TMyBaseClass.Default(5)]
E2010 Incompatible types: ‘TQuerySet’ and ‘TCustomAttribute’
Value: Integer;
end;
Question
I can understand why //2 is a fail, since internal types have to be qualified, but why is //3 not a compatible type?
Update: Feature Request added to QP
https://quality.embarcadero.com/browse/RSP-13512
I guess the compiler is not able to resolve that qualified attribute name. It may have to do with the ability to accept [MyDefault(5)] even if the attribute class is named MyDefaultAttribute.
Presumably the compiler doesn’t attempt to resolve that. Byw, your type should really be called DefaultAttribute.
David Heffernan – An example name of “Default” was a bad choice, yeah – as it clashes with Default(T); My bad.
(In reality, it had a different name)
Uwe Raabe – I had forgotten about that attribute type name magic.
Still, very annoying that you can’t use class internal types as these attributes are often class specific, and you are only able to think of so many variations on a “default” name. Prefixes to the rescue – but – brevity would be nice too…
What about opening a feature request?
Lars Fosdal I was not commenting on Default. By convention attribute class names are of the form
FooAttribute
and you use them like this
[FooAttribute(42)]
or
[Foo(42)]
Both will work. This allows you to include the word Attribute in the type name so that it is clear what it is when you define the type. But at the point of use when it is already obvious that this is an attribute, that word can be omitted.
Feature Request:
https://quality.embarcadero.com/browse/RSP-13512
I wish Delphi would support instance field initialization just like you can do with global variables. Then you would not need this (I assume you are using this to set default values for your fields via attribute similar to the Spring4D TManagedObject)
Stefan Glienke That is pretty much the same thing as I am trying to do, yeah. Add a QP for that too!
/sub