Occasionaly, the linker generates an error like the following:
 hi(undefined reference to vtable) hi(vtable: undefined reference)
        verb(    In function `Derived::Derived()':
        : undefined reference to `vtable for Derived')

This error is generated when a virtual function's implementation is
missing in a derived class, but the function is mentioned in the derived
class's interface.

    Such a situation is easily encountered:
    itemization(
    it() Construct a (complete) base class defining a virtual member function;
    it() Construct a Derived class mentioning the virtual function in its
        interface;
    it() The Derived class's virtual function is not implemented. Of course,
the compiler doesn't know that the derived class's function is not implemented
and will, when asked, generate code to create a derived class object;
    it() Eventually, the linker is unable to find the derived class's virtual
member function. Therefore, it is unable to construct the derived class's
vtable;
    it() The linker complains with the message:
    verb(undefined reference to `vtable for Derived')

)
    Here is an example producing the error:
        verbinclude(-a examples/vtable.cc)
    It's of course easy to correct the error: implement the derived class's
missing virtual member function.

    Virtual functions should em(never) be implemented inline. Since the vtable
contains the addresses of the class's virtual functions, these functions must
have addresses and so they must have been compiled as real (out-of-line)
functions. By defining virtual functions inline you run the risk that the
compiler simply overlooks those functions as they may very well never be
explicitly called (but only polymorphically, from a base class pointer or
reference). As a result their addresses may never enter their class's vtables
(and even the vtable itself might remain undefined), causing linkage problems
or resulting in programs showing unexpected behavior. All these kinds of
problems are simply avoided: em(never) define virtual members inline (see also
section ref(NOTINLINE)).
