Question

I need to prevent a class from being derived from so I thought to myself, this is something that Boost is bound to have already done. I know they have a noncopyable, they must have a nonderivable…

Imagine my surprise when I couldn’t find it….

That got me thinking.. There must be a reason. Maybe it isn’t possible to do using templates..

I’m sure if it was easy it’s be in the boost libraries.

I know how to do it without using templates, i.e. using a base class with a private constructor i.e.

class ThatCantBeDerived;  // Forward reference

class _NonDeriv
{
    _NonDeriv() {}
    friend class ThatCantBeDerived;
};

class ThatCantBeDerived : virtual public _NonDeriv
{
public:
    ThatCantBeDerived() :
      _NonDeriv()
    {
    }
};

Or something like this..

Maybe it’s the forward reference that causes the problem, or maybe there isn’t a portable way to achieve it..

Either way, I’m not sure why it isn’t in boost..

Any ideas?

Answer 1

Under the current spec, it is explicitly forbidden to “friend” a template argument, so templatizing your example would make it not standards compliant. Boost probably would not want to add something like that to its libraries. I believe this restriction is being relaxed in Ox however, and there are workarounds for compilers.