Re: [boost] [iterator] transform_iterator and move-only types

Subject: Re: [boost] [iterator] transform_iterator and move-only types
From: Jeffrey Lee Hellrung, Jr. (jeffrey.hellrung_at_[hidden])
Date: 2012-08-25 12:20:55

On Fri, Aug 24, 2012 at 11:25 PM, Nathan Ridge <zeratul976_at_[hidden]>wrote:

>
> Hi,
>
> The following code fails to compile for me:
>
>
> #include <memory>
> #include <boost/iterator/transform_iterator.hpp>
>
> std::unique_ptr<int> foo(int i)
> {
>     return std::unique_ptr<int>(new int(i));
> }
>
> int main()
> {
>     int a[5] = {1, 2, 3, 4, 5};
>     auto it = boost::make_transform_iterator(a, foo);
> }
>
>
> The error message I get with GCC 4.8 is pasted below.
>
> I think this should work because returning a move-only type
> from a function by value is a perfectly valid thing to do.
> Do you agree? Should I file a bug?
>

Seems reasonable, but upon an initial inspection, this looks like a hiccup
in boost::is_convertible...I'm not saying that once that gets sorted out,
there won't be something wrong in boost::transform_iterator, but
is_convertible should probably be addressed first.

In any case, this looks like this same problem that John Bytheway
identified in the thread titled "[boost] [type_traits][parameter]
Inconsistent boost::is_convertible between gcc and clang".

- Jeff

> Thanks,
> Nate
>
>
> In file included from boost/iterator/iterator_categories.hpp:20:0,
>                  from boost/iterator/iterator_adaptor.hpp:14,
>                  from boost/iterator/transform_iterator.hpp:12,
>                  from test.cpp:2:
> boost/type_traits/is_convertible.hpp: In instantiation of 'const bool
> boost::detail::is_convertible_basic_impl<std::unique_ptr<int>&,
> std::unique_ptr<int> >::value':
> boost/type_traits/is_convertible.hpp:296:5:   required from 'const bool
> boost::detail::is_convertible_impl<std::unique_ptr<int>,
> std::unique_ptr<int> >::value'
> boost/type_traits/is_convertible.hpp:419:1:   required from 'struct
> boost::is_convertible<std::unique_ptr<int>, std::unique_ptr<int> >'
> boost/mpl/aux_/nested_type_wknd.hpp:26:31:   required from 'struct
> boost::mpl::aux::nested_type_wknd<boost::is_convertible<std::unique_ptr<int>,
> std::unique_ptr<int> > >'
> boost/mpl/aux_/preprocessed/gcc/and.hpp:23:8:   required from 'struct
> boost::mpl::aux::and_impl<true, boost::is_convertible<std::unique_ptr<int>,
> std::unique_ptr<int> >, mpl_::bool_<true>, mpl_::bool_<true>,
> mpl_::bool_<true> >'
> boost/mpl/aux_/preprocessed/gcc/and.hpp:48:8:   required from 'struct
> boost::mpl::and_<boost::is_convertible<boost::random_access_traversal_tag,
> boost::single_pass_traversal_tag>,
> boost::is_convertible<std::unique_ptr<int>, std::unique_ptr<int> >,
> mpl_::bool_<true>, mpl_::bool_<true>, mpl_::bool_<true> >'
> boost/mpl/if.hpp:67:11:   [ skipping 6 instantiation contexts, use
> -ftemplate-backtrace-limit=0 to disable ]
> boost/iterator/detail/facade_iterator_category.hpp:187:8:   required from
> 'struct
> boost::detail::facade_iterator_category<boost::random_access_traversal_tag,
> std::unique_ptr<int>, std::unique_ptr<int> >'
> boost/iterator/iterator_facade.hpp:104:17:   required from 'struct
> boost::detail::iterator_facade_types<std::unique_ptr<int>,
> boost::random_access_traversal_tag, std::unique_ptr<int>, long int>'
> boost/iterator/iterator_facade.hpp:621:9:   required from 'class
> boost::iterator_facade<boost::transform_iterator<std::unique_ptr<int>
> (*)(int), int*, boost::use_default, boost::use_default>,
> std::unique_ptr<int>, boost::random_access_traversal_tag,
> std::unique_ptr<int>, long int>'
> boost/iterator/iterator_adaptor.hpp:261:9:   required from 'class
> boost::iterator_adaptor<boost::transform_iterator<std::unique_ptr<int>
> (*)(int), int*, boost::use_default, boost::use_default>, int*,
> std::unique_ptr<int>, boost::use_default, std::unique_ptr<int>,
> boost::use_default>'
> boost/iterator/transform_iterator.hpp:74:9:   required from 'class
> boost::transform_iterator<std::unique_ptr<int> (*)(int), int*,
> boost::use_default, boost::use_default>'
> test.cpp:12:52:   required from here
> boost/type_traits/is_convertible.hpp:136:23: error: use of deleted
> function 'std::unique_ptr<_Tp, _Dp>::unique_ptr(const std::unique_ptr<_Tp,
> _Dp>&) [with _Tp = int; _Dp = std::default_delete<int>]'
>      static bool const value = sizeof(
> boost::detail::checker<To>::_m_check(_m_from, 0) )
>                        ^
> In file included from c++/4.8.0/memory:81:0,
>                  from test.cpp:1:
> c++/4.8.0/bits/unique_ptr.h:259:7: error: declared here
>        unique_ptr(const unique_ptr&) = delete;
>        ^
> In file included from boost/iterator/iterator_categories.hpp:20:0,
>                  from boost/iterator/iterator_adaptor.hpp:14,
>                  from boost/iterator/transform_iterator.hpp:12,
>                  from test.cpp:2:
> boost/type_traits/is_convertible.hpp:129:41: error:   initializing
> argument 1 of 'static boost::type_traits::yes_type
> boost::detail::checker<T>::_m_check(T, int) [with T = std::unique_ptr<int>;
> boost::type_traits::yes_type = char]'
>      static boost::type_traits::yes_type _m_check(T, int);
>                                          ^
>