Question

I understand the need for virtual inheritance when using multiple inheritance — it solves the Dreaded Diamond Problem.

But what if I’m not using multiple inheritance? Is there a need for virtual inheritance at all?

I seem to recall hearing that it was important for exceptions (throw a derived class, catch by base class reference). But shouldn’t virtual destructors be sufficient for that?

I’ve tried searching for the reference page I once saw on this, but I can’t seem to find it.

Answer 1

You’re probably thinking about this Boost.Exception guideline, which I’ll copy here for completeness:

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Using Virtual Inheritance in Exception Types

Exception types should use virtual inheritance when deriving from other exception types. This insight is due to Andrew Koenig. Using virtual inheritance prevents ambiguity problems in the exception handler:

#include <iostream>
struct my_exc1 : std::exception { char const* what() const throw(); };
struct my_exc2 : std::exception { char const* what() const throw(); };
struct your_exc3 : my_exc1, my_exc2 {};

int
main()
    {
    try { throw your_exc3(); }
    catch(std::exception const& e) {}
    catch(...) { std::cout << "whoops!" << std::endl; }
    }

The program above outputs "whoops!" because the conversion to std::exception is ambiguous.

The overhead introduced by virtual inheritance is always negligible in the context of exception handling. Note that virtual bases are initialized directly by the constructor of the most-derived-type (the type passed to the throw statement, in case of exceptions.) However, typically this detail is of no concern when boost::exception is used, because it enables exception types to be trivial structs with no members (there’s nothing to initialize.) See Exception Types as Simple Semantic Tags.