Re: Specs for shared_ptr; compatibility between classic pointers and shared_ptr

From:

Peter Dimov <pdimov@gmail.com>

Newsgroups:

comp.lang.c++.moderated

Date:

Sun, 31 Aug 2008 02:58:51 CST

Message-ID:

<7b596790-1e7a-4fec-aa4d-1446ba33e131@34g2000hsh.googlegroups.com>

Fokko Beekhof wrote:
....

When we replace classical pointers with shared_ptrs, it does not work
anymore:

....

struct S
{
    S(std::tr1::shared_ptr<BaseA> pa_) : pa(pa_) {}
    S(std::tr1::shared_ptr<BaseB> pb_) : pb(pb_) {}

    std::tr1::shared_ptr<BaseA> pa;
    std::tr1::shared_ptr<BaseB> pb;
};

int main()
{
// S s(std::tr1::shared_ptr<BaseA>(new DerivA()) ); // works
// S s( new DerivA() ); // Doesn't work, SP constructor is explicit
    S s(std::tr1::shared_ptr<DerivA>(new DerivA()) ); // breaks

    return 0;
}

....

Your code works with the latest version of boost::shared_ptr; it uses
essentially the same technique that Kai-Uwe Bux outlined.

Rumor has is that part of the discussion on standadization is now taking
place here, so at least I would like to bring the matter to attention -
although I have no idea whose attention :-)

The specification of std::shared_ptr in the latest draft standard has
already been changed to account for this conversion ambiguity. Your
post has prompted me to take a second look, though, and I think that
your code is actually not covered. I'll make sure it's fixed. Thanks
for bringing it up.

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