Re: pimpl and const correctness

From:
Ulrich Eckhardt <eckhardt@satorlaser.com>
Newsgroups:
comp.lang.c++.moderated
Date:
Wed, 27 Jun 2007 09:19:20 CST
Message-ID:
<926al4-ugv.ln1@satorlaser.homedns.org>
ThosRTanner wrote:

Question: Why does this:
class splog
{
    int *wibble;
    public:
    void bork() const
    {
        * wibble = 1;
    }
};

void jim(splog const&will) { will.bork(); }

compile? It seems a pretty dangerous sort of thing to do.


This is partially answered by the FAQ, it lies in the difference between
a 'T* const' and a 'T const*'. In your case, the point is simply that the
const isn't transitive from a 'T* const' to a 'T const*'.

And - what is the recommended method of not getting caught like this
(apart from running lint).


You can easily write a wrapper that makes the const transitive, for use with
PIMPLs. Otherwise, I'd suggest simply not using pointers. If it is just an
optional single object, I'd suggest using boost::optional<> for that. If
the pointer is in fact to an array, it would be better to use a container
of the standard library.

Further, since the internal const-ness of the PIMPL pretty much doesn't
matter to the user of the enclosing class this isn't a top priority. Some
even welcome the fact that they can structure the outer interface like they
want without affecting the PIMPL interface, e.g. a pure read operation on a
DB (logically const) might require handles being changed. Further, it is
just too easy to get right and is rarely affects usability.

Uli

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