[fpc-pascal] IS operator and library created objects

Michael Van Canneyt michael at freepascal.org
Thu Aug 20 08:50:25 CEST 2015

On Thu, 20 Aug 2015, Torsten Bonde Christiansen wrote:

> On 2015-08-20 08:06, Sven Barth wrote:
>> Am 20.08.2015 03:42 schrieb "Fabio Luis Girardi" <fluisgirardi at gmail.com 
>> <mailto:fluisgirardi at gmail.com>>:
>> > Currently, I'm doing a small program that uses libraries, objects and the 
>> operator IS, that is know that this operator fails because of duplication 
>> of VMT.
>> And /that/ is one of the reasons why dynamic packages were invented...
>> >
>> > Everyone know some alternative (or more elegant) method to test if a 
>> object created on a library is a class descendant without use 
>> object.ClassName? Or in case of use String to check the class name, how 
>> retrieve the class name of all descendant classes from a object?
>> >
>> Nope, there is no more elegant way. And you'll probably need to walk the 
>> class tree using ParentClass and retrieve each name.
>> Note: an except-handler will suffer from the same problem if it needs to 
>> catch an exception that was raised from the library (or the other way 
>> round).
>> You are going to a extreme fragile direction there.
> What about TObject.InheritsFrom(aclass : tclass) - Is that not the equivalent 
> of the *IS* operator

It is, but will suffer from the same problem.
The library has a completely separate VMT and inheritance tree from the calling program.

You need dynamically loadable packages for this to work.


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