Re: ublas::vector in std::map followup

martux wrote:

Hi, I tried using a ublas vector as key type in a std::map along the lines of

boost::ublas::vector<double> x(2);
std::map<boost::ublas::vector<double>, int> vm;
vm[x] = 1;

which wont compile (gcc 4.0.3) due to

/usr/lib/gcc/x86_64-linux-gnu/4.0.3/../../../../include/c++/4.0.3/bits/stl_function.h:227: error: no match
for ‘operator<’ in ‘__x < __y’.

Adding an implementation of

bool operator<(boost::ublas::vector<double> const& A, boost::ublas::vector<double> const& B) {...}

unfortunately doesnt help. Could anyone please point out what I am missing?

I am working on a fieldmap object based on std::map of ublas::bounded_vectors so that I can take advantage of matrix multiplication of ublas as well as the storage/iterators of std::map. However, ublas::vector (and ublas::bounded_vector) do not have the necessary properties to comply with the requirements of the Key value of a std::map.

I have a workaround, though I am sure it could be written in a much more general way, I just don't know how to do it. Since I am working with bounded_vector<int, dimension> where dimensions are small, I created a operator< and a functor bouded_vector_less<T> and pass this to the third template parameter of std::map. Thus:


template<typename _Tp, size_t _Dim>
inline bool
operator<(
        const bounded_vector<_Tp, _Dim>& __x,
        const bounded_vector<_Tp, _Dim>& __y
) {
        return std::lexicographical_compare(
                __x.begin(),
                __x.end(),
                __y.begin(),
                __y.end()
        );
}
template <typename _Tp>
struct my_less {
        bool
        operator()(
                const _Tp& __x,
                const _Tp& __y
        ) const {
                return __x < __y;
        }
};

This is more or less a copy of what is in the standard template library. Therefore, when I create the map I can do this:

std::map<
    ublas::bounded_vector<int,    3>,
    ublas::bounded_vector<float, 3>,
    my_less<
        ublas::bounded_vector<int,    3>
    >
> fieldmap;

Any suggestions on making this more general and/or more efficient would be greatly appreciated!