Vtables riddle.

I've come across this riddle:
What will this code do -

Code:

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(***someClass)();

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What will these 3 asterisks do?
This relates to the virtual table and the __vptr.

(someClass is AN INSTANCE OF an inherited/the base class; only this detail is known about the classes' inheritance)

Any suggestions?

Hopefully someClass is something that, when dereferenced three times, is something that can take () (a function or similar).

Probably a function which is overloaded in classes that inherit from someClass, what with all of the references to the vtable. http://en.wikipedia.org/wiki/Virtual_table

Quote:

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Originally Posted by tabstop

Hopefully someClass is something that, when dereferenced three times, is something that can take () (a function or similar).

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Well it seems like it, but you can't be sure because you don't know if someClass is an instance of another class (which is the derived/base class) and it might that the () operator was overloaded or w/e. Personally, I don't think this is the point of this riddle, plus, it doesn't relate to virtual tables/the __vptr (which points to the virtual table)

Quote:

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Originally Posted by eXeCuTeR

Well it seems like it, but you can't be sure because you don't know if someClass is an object of another class (which is the derived/base class) and it might that the () operator was overloaded or w/e. Personally, I don't think this is the point of this riddle, plus, it doesn't relate to virtual tables/the __vptr (which points to the virtual table)

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But the () overloading/function call won't happen until the three *** are taken care of. It doesn't matter what the () would mean in someClass().

Quote:

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(someClass is inherited/is the base class; only this detail is known about the classes' inheritance)

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But with your example, someClass would have to be an instance of a class, wouldn't it?

Quote:

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Originally Posted by dwks

But with your example, someClass would have to be an instance of a class, wouldn't it?

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Oh, I'm sorry, you're right, I'll edit.
And again, I assure you these 3 asterisks relate to the __vptr.

Well, here's the only thing I can think of right now.

Perhaps the * operator is overloaded in a base class, and overridden in inherited classes. The code that you have would then have to look at the virtual table to figure out which * operator to call. *shrugs*

I think you're probably mistaken, and that it's the () that relates to the virtual table.

Even ignoring all type issues that the compiler will complain about for the dereferencing, and ignoring that even the existence of a vtable is an implementation detail (also known as "none of your business"), unless someClass is a pointer, you've got an asterisk too many.

Not necessarily. What if the * operator is overloaded? For example, Dinkumware doesn't seem to mind this:

Code:

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#include <iostream>

class c {
public:
    c &operator *() { return *this; }
    void operator()() { std::cout << "Greetings.\n"; }
};

int main() {
    c someClass;

    (***someClass)();

    return 0;
}

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That has nothing to do with vtables, though ;)

I think the OP's goal is about the fact that most compilers implement virtual dispatch using a vptr as the first member of the class, so if you "dereference" the object, you get the vtable, which is an array of function pointers.