Re: Calling a non-virtual member function for a NULL instance

"kanze" <>
25 Apr 2006 18:20:00 -0400
Ivan Kolev wrote:

Matt Curtis wrote:

As a programmer I agree that undefined behaviour should be
avoided, however I also believe that this mechanism would be
quite useful if the standard allowed it.

I have a simple example where this "trick" is useful:

class IPlugin
    virtual IClassDesc& classDesc() const = 0;

template <class T>
IClassDesc& GetClassDesc()
    return static_cast<T*>(0)->T::classDesc();

This allows to retrieve the IClassDesc of a class known at
compile time without forcing all IPlugin implementations to
implement both the virtual method classDesc() and an
additional static method ClassDesc(). If there are compilers
where the above will crash, is there a proper solution which
still avoids the double definition in each IPlugin-derived

I don't think so. I also doubt that you'll find a compiler
where it will crash. On the other hand, I would definitly
prefer the two function solution, with the virtual function
forwarding to the static one. In the static function, I don't
have a this pointer, and so I cannot possibly accidentally use

As for the extra boilerplate... if it is really a problem, I'd
use a macro. There are also tricks using an intermediate
template class, e.g.:

    template< typename Derived >
    class ConcretePlugin : public IPlugin
        // ...
        virtual IClassDesc& classDesc() const
            assert( typeid( *this ) == typeid( Derived ) ) ;
            return Derived::staticClassDesc() ;
    } ;

Your derived classes then derive from ConcretePlugin<ClassName>,
rather than IPlugin.

IMHO: in this case, the added virtual function is simple enough
that it is not worth the bother to implement the more complex
solutions. In the case where you have a whole series of similar
functions (e.g. classDesc, className, clone, another...),
however, it might be.

James Kanze GABI Software
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