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copied!<p>To answer the question about which objects (instances from now on) have vtables and where, it's helpful to think about when you need a vtable pointer.</p> <p>For any inheritance hierarchy, you need a vtable for each set of virtual functions defined by a particular class in that hierarchy. In other words, given the following:</p> <pre><code>class A { virtual void f(); int a; }; class B: public A { virtual void f(); virtual void g(); int b; }; class C: public B { virtual void f(); virtual void g(); virtual void h(); int c; }; class D: public A { virtual void f(); int d; }; class E: public B { virtual void f(); int e; }; </code></pre> <p>As a result, you need five vtables: A, B, C, D, and E all need their own vtables.</p> <p>Next, you need to know what vtable to use given a pointer or reference to a particular class. E.g., given a pointer to A, you need to know enough about the layout of A such that you can get a vtable that tells you where to dispatch A::f(). Given a pointer to B, you need to know enough about the layout of B to dispatch B::f() and B::g(). And so on and so on.</p> <p>One possible implementation could put a vtable pointer as the first member of any class. That would mean the layout of an instance of A would be:</p> <pre><code>A's vtable; int a; </code></pre> <p>And an instance of B would be:</p> <pre><code>A's vtable; int a; B's vtable; int b; </code></pre> <p>And you could generate correct virtual dispatching code from this layout.</p> <p>You can also optimize the layout by combining vtable pointers of vtables that have the same layout or if one is a subset of the other. So in the above example, you could also layout B as:</p> <pre><code>B's vtable; int a; int b; </code></pre> <p>Because B's vtable is a superset of A's. B's vtable has entries for A::f and B::g, and A's vtable has entries for A::f.</p> <p>For completeness, this is how you would layout all the vtables we've seen so far:</p> <pre><code>A's vtable: A::f B's vtable: A::f, B::g C's vtable: A::f, B::g, C::h D's vtable: A::f E's vtable: A::f, B::g </code></pre> <p>And the actual entries would be:</p> <pre><code>A's vtable: A::f B's vtable: B::f, B::g C's vtable: C::f, C::g, C::h D's vtable: D::f E's vtable: E::f, B::g </code></pre> <p>For multiple inheritance, you do the same analysis:</p> <pre><code>class A { virtual void f(); int a; }; class B { virtual void g(); int b; }; class C: public A, public B { virtual void f(); virtual void g(); int c; }; </code></pre> <p>And the resultant layouts would be:</p> <pre><code>A: A's vtable; int a; B: B's vtable; int b; C: C's A vtable; int a; C's B vtable; int b; int c; </code></pre> <p>You need a pointer to a vtable compatible with A and a pointer to a vtable compatible with B because a reference to C can be converted to a reference of A or B and you need to dispatch virtual functions to C.</p> <p>From this you can see that the number of vtable pointers a particular class has is at least the number of root classes it derives from (either directly or due to a superclass). A root class is a class that has a vtable that does not inherit from a class that also has a vtable.</p> <p>Virtual inheritance throws another bit of indirection into the mix, but you can use the same metric to determine the number of vtable pointers.</p>

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