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

Subject: [boost] [iterator] transform_iterator and move-only types
From: Nathan Ridge (zeratul976_at_[hidden])
Date: 2012-08-25 02:25:42

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?

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);
                                         ^