[Boost-users] Problems with nesting boost::container::small_vector and C++98

[Daniel Trstenjak]

Hi,


first the used versions:
boost 1.63.0
gcc 5.4.0 20160609
Ubuntu 16.04.1


the example I've problems with is:

#include <map>
#include <boost/container/small_vector.hpp>

using boost::container::small_vector;

typedef std::pair<int, small_vector<float, 8> > mypair;

typedef std::map<int, mypair> mymap;
typedef std::map<int, small_vector<mypair, 8> > mymap2;
typedef std::map<int, small_vector<small_vector<float, 8>, 8> > mymap3;

int main()
{
mymap m;
mypair& sv = m[0];

mymap2 m2;
small_vector<mypair, 8>& sv2 = m2[0];

mymap3 m3;
small_vector<small_vector<float, 8>, 8>& sv3 = m3[0];
}


I can compile it without errors with 'g++ -std=c++11', but without
'-std=c++11' I'm getting an error for the 'operator[]' on mymap2:

***@octa ~> g++ -Iboost_1_63_0/ small_vector_test.cpp
In file included from boost_1_63_0/boost/container/small_vector.hpp:27:0,
from small_vector_test.cpp:2:
boost_1_63_0/boost/container/vector.hpp: In instantiation of ‘void boost::container::vector<T, Allocator>::assign(FwdIt, FwdIt, typename boost::move_detail::disable_if_or<void, boost::move_detail::is_same<typename boost::container::container_detail::version<Allocator>::type, boost::move_detail::integral_constant<unsigned int, 0u> >, boost::move_detail::is_convertible<InIt, typename boost::container::allocator_traits<Allocator>::size_type>, boost::container::container_detail::is_input_iterator<FwdIt> >::type*) [with FwdIt = boost::container::container_detail::vec_iterator<std::pair<int, boost::container::small_vector<float, 8ul> >*, true>; T = std::pair<int, boost::container::small_vector<float, 8ul> >; Allocator = boost::container::small_vector_allocator<boost::container::new_allocator<std::pair<int, boost::container::small_vector<float, 8ul> > > >; typename boost::move_detail::disable_if_or<void, boost::move_detail::is_same<typename boost::container::container_detail::version<Allocator>::type, boost::move_detail::integral_constant<unsigned int, 0u> >, boost::move_detail::is_convertible<InIt, typename boost::container::allocator_traits<Allocator>::size_type>, boost::container::container_detail::is_input_iterator<FwdIt> >::type = void]’:
boost_1_63_0/boost/container/small_vector.hpp:563:7: required from ‘boost::container::small_vector<T, N, Allocator>::small_vector(const boost::container::small_vector<T, N, Allocator>&) [with T = std::pair<int, boost::container::small_vector<float, 8ul> >; long unsigned int N = 8ul; Allocator = boost::container::new_allocator<std::pair<int, boost::container::small_vector<float, 8ul> > >]’
/usr/include/c++/5/bits/stl_pair.h:113:31: required from ‘std::pair<_T1, _T2>::pair(const _T1&, const _T2&) [with _T1 = const int; _T2 = boost::container::small_vector<std::pair<int, boost::container::small_vector<float, 8ul> >, 8ul>]’
/usr/include/c++/5/bits/stl_map.h:487:23: required from ‘std::map<_Key, _Tp, _Compare, _Alloc>::mapped_type& std::map<_Key, _Tp, _Compare, _Alloc>::operator[](const key_type&) [with _Key = int; _Tp = boost::container::small_vector<std::pair<int, boost::container::small_vector<float, 8ul> >, 8ul>; _Compare = std::less<int>; _Alloc = std::allocator<std::pair<const int, boost::container::small_vector<std::pair<int, boost::container::small_vector<float, 8ul> >, 8ul> > >; std::map<_Key, _Tp, _Compare, _Alloc>::mapped_type = boost::container::small_vector<std::pair<int, boost::container::small_vector<float, 8ul> >, 8ul>; std::map<_Key, _Tp, _Compare, _Alloc>::key_type = int]’
small_vector_test.cpp:18:39: required from here
boost_1_63_0/boost/container/vector.hpp:1261:15: error: binding ‘const std::pair<int, boost::container::small_vector<float, 8ul> >’ to reference of type ‘std::pair<int, boost::container::small_vector<float, 8ul> >&’ discards qualifiers
*cur = *first;
^
In file included from /usr/include/c++/5/bits/stl_algobase.h:64:0,
from /usr/include/c++/5/bits/stl_tree.h:63,
from /usr/include/c++/5/map:60,
from small_vector_test.cpp:1:
/usr/include/c++/5/bits/stl_pair.h:96:12: note: initializing argument 1 of ‘std::pair<int, boost::container::small_vector<float, 8ul> >& std::pair<int, boost::container::small_vector<float, 8ul> >::operator=(std::pair<int, boost::container::small_vector<float, 8ul> >&)’
struct pair
^

Any ideas?
Is 'boost:container::small_vector' supposed to work without C++11 support?

Thanks!


Greetings,
Daniel

[Daniel Trstenjak]

Hi,

Post by Daniel Trstenjak
typedef std::pair<int, small_vector<float, 8> > mypair;

If I'm replacing 'mypair' with 'mystruct':

struct mystruct {
mystruct& operator=(const mystruct& other) {
i = other.i;
sv = other.sv;

return *this;
}

int i;
small_vector<float, 8> sv;
};


Then I can compile it without C++11 support, but I've to
implement the 'operator=' by hand, otherwise I'm still getting
a compile error:

***@octa ~> g++ -Iboost_1_63_0/ small_vector_test_2.cpp
In file included from boost_1_63_0/boost/container/small_vector.hpp:27:0,
from small_vector_test_2.cpp:2:
boost_1_63_0/boost/container/vector.hpp: In instantiation of ‘void boost::container::vector<T, Allocator>::assign(FwdIt, FwdIt, typename boost::move_detail::disable_if_or<void, boost::move_detail::is_same<typename boost::container::container_detail::version<Allocator>::type, boost::move_detail::integral_constant<unsigned int, 0u> >, boost::move_detail::is_convertible<InIt, typename boost::container::allocator_traits<Allocator>::size_type>, boost::container::container_detail::is_input_iterator<FwdIt> >::type*) [with FwdIt = boost::container::container_detail::vec_iterator<mystruct*, true>; T = mystruct; Allocator = boost::container::small_vector_allocator<boost::container::new_allocator<mystruct> >; typename boost::move_detail::disable_if_or<void, boost::move_detail::is_same<typename boost::container::container_detail::version<Allocator>::type, boost::move_detail::integral_constant<unsigned int, 0u> >, boost::move_detail::is_convertible<InIt, typename boost::container::allocator_traits<Allocator>::size_type>, boost::container::container_detail::is_input_iterator<FwdIt> >::type = void]’:
boost_1_63_0/boost/container/small_vector.hpp:563:7: required from ‘boost::container::small_vector<T, N, Allocator>::small_vector(const boost::container::small_vector<T, N, Allocator>&) [with T = mystruct; long unsigned int N = 8ul; Allocator = boost::container::new_allocator<mystruct>]’
/usr/include/c++/5/bits/stl_pair.h:113:31: required from ‘std::pair<_T1, _T2>::pair(const _T1&, const _T2&) [with _T1 = const int; _T2 = boost::container::small_vector<mystruct, 8ul>]’
/usr/include/c++/5/bits/stl_map.h:487:23: required from ‘std::map<_Key, _Tp, _Compare, _Alloc>::mapped_type& std::map<_Key, _Tp, _Compare, _Alloc>::operator[](const key_type&) [with _Key = int; _Tp = boost::container::small_vector<mystruct, 8ul>; _Compare = std::less<int>; _Alloc = std::allocator<std::pair<const int, boost::container::small_vector<mystruct, 8ul> > >; std::map<_Key, _Tp, _Compare, _Alloc>::mapped_type = boost::container::small_vector<mystruct, 8ul>; std::map<_Key, _Tp, _Compare, _Alloc>::key_type = int]’
small_vector_test_2.cpp:21:41: required from here
boost_1_63_0/boost/container/vector.hpp:1261:15: error: binding ‘const mystruct’ to reference of type ‘mystruct&’ discards qualifiers
*cur = *first;
^
small_vector_test_2.cpp:6:8: note: initializing argument 1 of ‘mystruct& mystruct::operator=(mystruct&)’


For whatever reasons the operator 'mystruct& mystruct::operator=(mystruct&)' is used, with a non const
reference parameter, which can't work in this context.

Looks like a bug? boost? gcc?


Greetings,
Daniel

[Daniel Trstenjak]

Attached the source file with 'mypair' replaced by 'mystruct'.

[Ion Gaztañaga]

Post by Daniel Trstenjak
Hi,
boost 1.63.0
gcc 5.4.0 20160609
Ubuntu 16.04.1
#include <map>
#include <boost/container/small_vector.hpp>
using boost::container::small_vector;
typedef std::pair<int, small_vector<float, 8> > mypair;
typedef std::map<int, mypair> mymap;
typedef std::map<int, small_vector<mypair, 8> > mymap2;
typedef std::map<int, small_vector<small_vector<float, 8>, 8> > mymap3;
int main()
{
mymap m;
mypair& sv = m[0];
mymap2 m2;
small_vector<mypair, 8>& sv2 = m2[0];
mymap3 m3;
small_vector<small_vector<float, 8>, 8>& sv3 = m3[0];
}

In C++98 std::pair copy constructor is implicitly declared. Since
small_vector declares operator= taking a non-const reference (as that's
how move semantics are emulated), then std::pair's operator= takes a
non-const pair argument:

http://en.cppreference.com/w/cpp/language/copy_assignment#Implicitly-declared_copy_assignment_operator

So the problem you are facing it's a limitation of the move emulation
library. When you define your own class holding a small_vector, you have
the same problem. When you explicitly declare operator= taking a const
reference the problem is solved.

See:

http://www.boost.org/doc/libs/1_63_0/doc/html/move/emulation_limitations.html

and

http://www.boost.org/doc/libs/1_63_0/doc/html/container/known_issues.html#container.known_issues.move_emulation_limitations

Best,

Ion

[Daniel Trstenjak]

So the problem you are facing it's a limitation of the move emulation library.

Thank you for the info!

It seems a bit unfortunate that the move emulation library breaks that easily.


Greetings,
Daniel