Re: [boost] Review: Base from Member

From: Ed Brey (edbrey_at_[hidden])
Date: 2001-08-07 08:26:37

From: "Douglas Gregor" <gregod_at_[hidden]>

> Without the ability for a user to specify the name of the data member
(which
> would require either macro tricks or a core language change), I think
that
> this particular idiom does not lend itself well to being translated
into a
> reusable, generic library.

It certainly would be nice to have a nicely named member.  For some
small classes it is not important, but in general a meaningful name is
needed.  Fortunately, a meaningful name can be attached to the member by
typedefing the base class and using the qualified access to the member.
The user code would look like:

class foo: base_from_member<bar_type> {
    typedef base_from_member<bar_type> meaningful_name;

    void fn() {
        meaningful_name::member.whatever();
    }
};

The cost is an extra line of code, but that is still a nice savings over
an entirely separate class.  Using base_from_member also avoids the
problem of the obfuscation that the hand-coded extra class causes: it
appears that there is a an is-implemented-in-terms-of relation, which
isn't really the intent.  Base_from_member makes the intent clear, which
is just to work around an initialization ordering limitation.

So overall, even though a perfect solution isn't available, I think that
a base_from_member class would be the best of the alternatives, and
would be a useful addition to boost.