Is gcc warning about non-virtual destructor useless?

Hi!

I'm using gcc 4.1.1 on linux. For the following class "Visitor" it
generates a warning (enable all warnings using g++ option "-Wall").

struct A;
struct Visitor {
    virtual void visit(A*) =0;
protected:
    ~Visitor() {}
};

The warning reads "warning: 'struct Visitor' has virtual functions but
non-virtual destructor". It is warning not to try polymorphic deletion
through a base pointer.

I don't see how one is supposed to "delete" a Visitor*. The destructor
is protected and therefore can only be called from derived classes. The
Visitor itself should know not to do "delete this;". And I think this is
enough protection against the "base pointer deletion bug". (With enough
effort one can always shoot oneself in the foot, I know)

I'm raising this issue because I'm trying to compile my code cleanly at
high warning levels and this issue keeps anoying me. My current solution
is to make the dtor virtual because it doesn't impact runtime
performance significantly. But I'd like to see gcc improving on this if
there is consensus about not issuing the warning for protected dtors.

What is your opinion?

Frank

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