[Boost-users] [boost.multi_array] Question about iterators on multi_array_views

24 May 2006

      Hello everyone

I'm trying to use the boost.multi_array's iterators and came across a
problem: calling end() on a certain multi_array_view only yields the
same iterator, if it is called on exactly the same instance of the
view. When, however, the view is created twice (using the same
expression, of course), end() produces an iterator that doesn't
compare to the first one.

In other words:
      m_a[indices[index_range()][1]].end()
            != m_a[indices[index_range()][1]].end()

I'm not sure whether this is just me, trying to use something in a way
it shouldn't be used, or if this is a limitation of the multi_array
library. I'd appreciate your opinion!

Thanks a lot!

Bernhard Maeder

Following is the code that produces an error due to using the
multi_array in the above manner:

-----------------------------------------------------------------

#include <iostream>
#include <boost/multi_array.hpp>

using namespace boost;
typedef boost::multi_array_types::index_range index_range;
using boost::indices;

template<typename IteratorType>
void output(IteratorType begin, IteratorType end)
{
    for (IteratorType it = begin; it != end; ++it)
    {
        std::cout << *it << std::endl;
    }
}

template<typename ArrayType>
void output(ArrayType const & ar)
{
   std::cout << "----------" << std::endl;
   std::cout << (ar.end() == ar.end()) << std::endl;
   std::cout << "----------" << std::endl;
   output(ar.begin(), ar.end());
}

int main()
{
    typedef boost::multi_array<double, 2> array_type;

    // Create a 3x3 array:
    array_type m_a(boost::extents[3][3]);

    for (std::size_t i=0; i<9; i++)
        m_a.data()[i] = static_cast<double>(i);

    // Works:
    std::cout << "Use a sub_array:" << std::endl;
    output(m_a[1]);

    // Works:
    std::cout << "Use a multi_array_view:" << std::endl;
    output(m_a[indices[index_range()][1]]);

    // Doesn't work:
    std::cout << "Use subsequent versions of a multi_array_view:" <<
std::endl;
    // This returns false:
    std::cout << "----------" << std::endl;
    std::cout << (
            m_a[indices[index_range()][1]].end()
         == m_a[indices[index_range()][1]].end()
          ) << std::endl;
    std::cout << "----------" << std::endl;

    output( m_a[indices[index_range()][1]].begin()
          , m_a[indices[index_range()][1]].end()
          );

    return 0;
}
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[Boost-users] [boost.multi_array] Question about iterators on multi_array_views

bernhard.maeder＠zkb.ch