text: This is the mail archive of the gcc@gcc.gnu.org mailing list for the GCC project.
  Am 29.08.19 um 13:44 schrieb Jonathan Wakely:
  On Thu, 29 Aug 2019 at 12:43, Jonathan Wakely wrote:On Thu, 29 Aug 2019 at 11:50, Christian Schneider <cschneider@radiodata.biz> wrote: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 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.No. It only required it to be publicly accessible if you called shared_from_this().Do you know (or someone else), why it was removed?Yes (look at the author of the paper :-). As I wrote in that paper: "The proposed wording removes the preconditions on shared_from_this so that it is now well-defined to call it on an object which is not owned by any shared_ptr, in which case shared_from_this would throw an exception." Previously it was undefined behaviour to call shared_from_this() if the base class hadn't been initialized to share ownership with a shared_ptr. That meant the following was undefined: #include <memory> struct X : std::enable_shared_from_this<X> { }; int main() { X x; // not owned by a shared_ptr x.shared_from_this(); } Now this program is perfectly well-defined, but it throws an exception. There is no good reason to say that program has undefined behaviour (which means potentially unbounded types of errors) when we can just make it valid code that throws an exception when misused.And in order to make it well-defined, we tightened up the specification to say exactly how and when the weak_ptr in a enable_shared_from_this base class gets initialized. If it's not possible to initialize it (e.g. because it's private) then it doesn't initialize it.
  OK, thx for clarification and insights.Since it is a requirement from the standard, I will add a note on cppreference.com, so that it is clear that it needs to be public inherited, and it silently fails if you don't inherit public.
  I find it a little, umm..., inconvenient, that the compiler happily accepts it when it is clear that it never ever can work...The code compiles and runs. It just doesn't do what you thought it would do. Welcome to C++.