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Re: enable_shared_from_this fails at runtime when inherited privately

Am 29.08.19 um 12:07 schrieb Jonathan Wakely:
On Thu, 29 Aug 2019 at 10:15, Christian Schneider
<> wrote:

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

class foo:
      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. 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
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

No, that would not conform to the standard.


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