RTTI and detecting virtual method overrides

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RTTI and detecting virtual method overrides
What is the best way of programmatically detecting at runtime whether a virtual method in the base class has been overridden for a descendant type?
Calling the method is not an option, as an instance does not exist as of yet, and I only have a type reference.

Comments

  1. Stefan GlienkeJune 16, 2014 at 3:13 AM

    So you have 2 class refs and want to check if a particular method has been overridden by the 2nd class?

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  2. Eric GrangeJune 16, 2014 at 3:15 AM

    Are you checking the constructor? If not you can create the instance then check, or create a set of "blank" instances which you'll use for the purpose of checking.

    Once you have instances, you can just compare the address of the method, ie. if base.Method<>@sub.Method then it's overridden.

    This is fast, foolproof, can also work for private methods without involving RTTI (just perform the check in a method of the base class), and it provides a degree of compiler support.

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  3. Lars FosdalJune 16, 2014 at 3:34 AM

    Stefan Glienke - Correct.

    Eric Grange - The base class is an abstract class, so not instantiable. The methods that I need to check are not virtual abstract, but the base class implementation simply tosses an exception if they happen to be called in the default implementation.  They are also declared in the protected section.

    I guess the easiest way around is to add a public function in the base class that does the call and checks for an exception of the expected type.

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  4. Stefan GlienkeJune 16, 2014 at 5:05 AM

    This should work: http://pastebin.com/gb2mE0A1

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  5. Nicholas RingJune 16, 2014 at 5:07 AM

    I don't know if this is any use: http://hallvards.blogspot.com.au/2006/09/extended-class-rtti.html

    Since you have type reference, maybe you could iterate the methods in the VMT in both classes (references) and see if the addresses are the same...? A bit messy but it might work (or not) ;-)

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  6. Stefan GlienkeJune 16, 2014 at 5:12 AM

    Nicholas Ring Not this article but probably another one from Hallvard might help. That one is about the extended class rtti (when you set $M+) and provides information about arguments.

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  7. Nicholas RingJune 16, 2014 at 5:21 AM

    Stefan Glienke  I choose that one because of the source but I will not say that your are wrong - just hopefully pointing people in the right direction :-)

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  8. Hallvard VassbotnJune 16, 2014 at 9:47 AM

    This probably covers what you are looking for

    http://hallvards.blogspot.com/2006/03/hack-8-explicit-vmt-calls.html?m=1

    You can find each method in the vmt table then check if the methods of two classes (where one inherits from the other) have the same vmt index.

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  9. Stefan GlienkeJune 16, 2014 at 9:59 AM

    I already posted the solution I think.
    P.S.: I think there is still one little bug with the skipping of the TVmtMethodEntry array but that only matters for a class with published methods. I will fix that later.

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  10. Hallvard VassbotnJune 16, 2014 at 10:34 AM

    Indeed you did, Stefan Glienke :)

    I'm on a phone so I didn't follow all the links.

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  11. Nick HodgesJune 16, 2014 at 10:42 AM

    Hallvard! Great to see you. I hope all is well.

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  12. Lars FosdalJune 16, 2014 at 10:51 AM

    I ended up with virtual methods in the base class instead.  Less magic.

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  13. Hallvard VassbotnJune 16, 2014 at 11:49 AM

    Magic is fun! :)

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  14. Stefan GlienkeJune 16, 2014 at 12:02 PM

    Hallvard Vassbotn Could not agree more! ;)

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  15. Nicholas RingJune 16, 2014 at 1:11 PM

    Hallvard Vassbotn Stefan Glienke Could not agree more and it is even more fun when you understand some-one else's magic.

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