Re: enable_shared_from_this fails at runtime when inherited privately

[Christian Schneider <cschneider at radiodata dot biz>]

Am 29.08.19 um 12:07 schrieb Jonathan Wakely:
> On Thu, 29 Aug 2019 at 10:15, Christian Schneider
> <cschneider@radiodata.biz> wrote:
> > Hello,
> > I just discovered, that, when using enable_shared_from_this and
> > inheriting it privately, this fails at runtime.
> > I made a small example:
> >
> > #include <memory>
> > #include <boost/shared_ptr.hpp>
> > #include <boost/make_shared.hpp>
> > #include <boost/enable_shared_from_this.hpp>
> >
> > #ifndef prefix
> > #define prefix std
> > #endif
> >
> > class foo:
> >       prefix::enable_shared_from_this<foo>
> > {
> > public:
> >       prefix::shared_ptr<foo> get_sptr()
> >       {
> >           return shared_from_this();
> >       }
> > };
> >
> > int main()
> > {
> >       auto a = prefix::make_shared<foo>();
> >       auto b = a->get_sptr();
> >       return 0;
> > }
> >
> > This compiles fine, but throws a weak_ptr exception at runtime.
> > I'm aware, that the implementation requires, that
> > enable_shared_from_this needs to be publicly inherited, but as a first
> > time user, I had to find this out the hard way, as documentations (I
> > use, ie. cppreference.com) don't mention it, probably because it's not a
> > requirement of the standard.
>
> It definitely is a requirement of the standard. The new wording we
> added via http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/p0033r1.html#spec
> says that the base's weak_ptr is only initialized when the base class
> is "unambiguous and accessible". It doesn't say that an ambiguous or
> inaccessible base class makes the program ill-formed, so we're not
> allowed to reject such a program. >
I see. As far as I understand, this sentence was removed:
Requires: enable_shared_from_this<T> shall be an accessible base class 
of T. *this shall be a subobject of an object t of type T. There shall 
be at least one shared_ptr instance p that owns &t.

As far as I read it, this required enable_shared_from_this to be public 
accessible. Do you know (or someone else), why it was removed?
I find it a little, umm..., inconvenient, that the compiler happily 
accepts it when it is clear that it never ever can work...

> > On the other hand, if you compile the code with additional
> > -Dprefix=boost (and needed boost stuff installed, of course), it gives a
> > compiler error (
> > gcc: 'boost::enable_shared_from_this<foo>' is an inaccessible base of 'foo';
> > clang: error: cannot cast 'boost::shared_ptr<foo>::element_type' (aka
> > 'foo') to its private base class 'boost::enable_shared_from_this<foo>')
>
> That seems like a bug in Boost.
When Boost wants to follow the standard, then yes. If not i would see it 
as a feature, see above :)

> > I'm think, it would be helpful, if the std implemntions also would fail
> > at compile time already, and wanted to ask if this would be
> > possible/feasible.
>
> No, that would not conform to the standard.
Clear.