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Delphi grammar quirk of the day: sealed and abstract classes

Delphi’s syntax for sealed and abstract classes is a bit bizarre. The following all compile:

TFoo = class sealed sealed sealed sealed

TBar = class abstract abstract abstract abstract

If you put the sealed keyword more than once, does that make the class somehow extra-sealed?

Interestingly, sealed and abstract are both what I’m calling “semikeywords” — they only have a special meaning in a particular context. Outside that particular context, they can be used as plain old identifiers. So you could actually have a field in the class called sealed or abstract… as long as it’s not the first thing after the class keyword. Add another field first, or a visibility specifier, and you’re fine:

TFoo = class sealed
Index: Integer;
Abstract: Boolean;

TBar = class abstract
strict private
Sealed: string;

But there is a bit of sense amidst all this oddity. They at least don’t let you mix and match sealed (cannot descend from this class) with abstract (must descend from this class):

TBaz = class abstract sealed

[Pascal Error] Project3.dpr(17): E2383 ABSTRACT and SEALED cannot be used together

Now, if they would just make it so that class abstract actually did something… (see QC#24662)

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