Virtual Table

This is a discussion on Virtual Table within the C++ Programming forums, part of the General Programming Boards category; Hello Forum I have a Very basic Doubt. Say here is a Base Class A, Base Class B both have ...

  1. #1
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    Lightbulb Virtual Table

    Hello Forum

    I have a Very basic Doubt.

    Say here is a Base Class A, Base Class B
    both have one virtual function each a() and b() respectively.

    and

    Classes C, D, E, F.


    C inherits A

    D inherits B

    E inherits both A and B

    F inherits all A, B, C, D and E.




    The question is how many virtual tables of A would be created and how the entries of those tables would be.

    Thanks in advance.

  2. #2
    a_capitalist_story
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    Doubtless, this is homework. Good luck with that.

  3. #3
    The larch
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    Isn't this rather implementation dependent?
    I might be wrong.

    Thank you, anon. You sure know how to recognize different types of trees from quite a long way away.
    Quoted more than 1000 times (I hope).

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    Quote Originally Posted by anon View Post
    Isn't this rather implementation dependent?
    Without a doubt, we need to know which compiler it is [and if it's not a commonly used compiler, the answer would still be in doubt].

    F inherits all A, B, C, D and E.
    I'd not be surprised if this would be quite messy to tangle out which versions of a() and b() are called if C, D and E also implement a/b functions.

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    Quote Originally Posted by anon View Post
    Isn't this rather implementation dependent?
    Yes, but unfortunately it's a popular interview question, at least in my experience.

  6. #6
    a_capitalist_story
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    Yeah, but I doubt this is an interview question, I mean, how many interviews allow Internet access during it? Seems doubtful.

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    Quote Originally Posted by medievalelks View Post
    Yes, but unfortunately it's a popular interview question, at least in my experience.
    And when you explain that it's implementation dependant and not defined in the standard, what is the reaction? It's like a question I got in an interview saying "What is the size of an int?" and I said "Depends"... I was then asked "What size is it USUALLY?", which made me respond 32.

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  8. #8
    Cat without Hat CornedBee's Avatar
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    Anyway, there's no compiler that creates more than one vtable for any given class.

    An entirely different question is how many vptrs F will contain.
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    Quote Originally Posted by CornedBee View Post
    Anyway, there's no compiler that creates more than one vtable for any given class.
    But there is absolutely no rule in the C++ standards to say that there must be only one, right? So whilst any sane compiler (any existing compiler) would only create one, there is no rule stating that this MUST be the case. [I'm not arguing the case here, just making sure I understand for myself].

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  10. #10
    Cat without Hat CornedBee's Avatar
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    The C++ standard doesn't prescribe any implementation detail. For all it cares, compilers could do their dynamic dispatching using an if-else chain with string comparison of the class name.

    Hmm, but I may have to correct myself. It could be that E and F have more than one vtable. I don't think so, but it would reduce the complexity of generating the vtable to just use multiple ones.
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    F inherits all A, B, C, D and E.
    If C, D and E are derived from A and B, there is no use in deriving F from A and B as well...

  12. #12
    Cat without Hat CornedBee's Avatar
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    The example is not there to be useful

    But there is a difference between deriving them and not doing so.
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    Algorithm Dissector iMalc's Avatar
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    It doesn't even say how it inherits from the bases. I'd think it would be different where virtual inherritance was used.
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    Quote Originally Posted by iMalc View Post
    It doesn't even say how it inherits from the bases. I'd think it would be different where virtual inherritance was used.
    Good point. Another flaw in the question. It is also strange that we haven't heard from the OP.

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  15. #15
    Technical Lead QuantumPete's Avatar
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    Maybe I'm just misunderstanding the problem here, but I always thought that every class that has at least one virtual function will also have a hidden v-ptr, which points to the class's v-table. i.e. there is only one v-table, which tells the class which flavour of the virtual function to call in it's inheritance tree. Since every function can at most have one such flavour, a single table isn't all that complex. You'd have at most one entry for every virtual function that can be accessed through the this pointer of that instance.
    Or am I wrong?

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