Re: [boost] [move][unique_ptr] c++14 unique_ptr comes to town

Subject: Re: [boost] [move][unique_ptr] c++14 unique_ptr comes to town
From: Ion Gaztañaga (igaztanaga_at_[hidden])
Date: 2014-08-28 16:02:37

El 28/08/2014 1:13, Peter Dimov escribió:
> Ion Gaztañaga wrote:
>> Tested in several GCC, MSVC and Clang versions, both in C++03 and
>> C++11 modes. I won't have time in the following days to continue
>> working in your proposed changes, but at least I wanted to to show
>> that we've progressed a bit.
>
> Thanks!
>
> There's one other thing that I omitted in my last post:
>
> 85    template <class U>
> 86    unique_ptr_data(P p, BOOST_FWD_REF(U) d) BOOST_NOEXCEPT
> 87       : m_p(::boost::forward<U>(p)), d(::boost::forward<U>(d))
> 88    {}
>
> It's not clear to me why you need this constructor (in addition to the
> two others), and it's wrong - forward<U>(p) isn't likely to work.

Apart from the useless "forward<>()" (which I think will work as 
pointers are not move-enabled), if I disable this overload some 
compilers complain about move-only deleters not being copyable in C++03. 
I haven't investigated the issue, it might be provoked from some SFINAE 
provoked instantiations or compiler bugs. I'll try to simplify this if I 
find a way.

> In my implementation, I've went with perfect forwarding instead of the
> two constructor signatures described in the standard. Combined with the
> static_assert inside, It works for all cases. Not sure how this would
> translate to C++03 though; what does BOOST_FWD_REF(U) expand into? C++03
> can't really do perfect forwarding.

BOOST_FWD_REF(U) expands to const U &, which means that non-const 
references can't be forwarded to constructors. That could be somewhat 
workarounded using an utility like boost::ref internally, but I think 
deleter_arg_type1/2 allow a more portable C++03/C++11 implementation.

Best,

Ionest,

Ion

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