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enable_shared_from_this fails at runtime when inherited privately


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.

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>')

I'm think, it would be helpful, if the std implemantions also would fail at compile time already, and wanted to ask if this would be possible/feasible.

BR, Christian

compilers:
gcc-Version 9.2.0 (Gentoo 9.2.0 p1)
clang version 8.0.1 (tags/RELEASE_801/final) (used with both libstdc++.so.6 and libc++.so.1 (v8.0.1))


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