I have written code that I believe addresses this issue, however, due to some underlying issues specifically with Rcpp::Vector::const_iterator it got a little bit more complicated (and intrusive) than expected.

To solve the initial issue of no const versions of row() and column() being available in Matrix I did the following:

Create ConstMatrixColumn and ConstMatrixRow classes
- These are just like their non-const brothers, except they only provide const_iterators to traverse them.
- They still inherit from VectorBase. After careful analysis of this class, it is const safe so there needn't be any change here. This is good because VectorBase is used extensively throughout the codebase.
Add ConstMatrixColumn column(int i) const and ConstMatrixRow row(int i) const functions to the Matrix class.
Add ConstMatrixColumn operator()(internal::NamedPlaceHolder, int i) const and ConstMatrixRow operator()(int i, internal::NamedPlaceHolder) const functions to the Matrix class.

Now this all works wonderfully except for the fact that the Vector::const_iterator is so very different from its counterpart Vector::iterator.

Vector::iterator has specializations for well-known types that are instantiations of internal::Proxy_Iterator as shown below (from Rcpp/vector/proxy.h).

        template <int RTYPE> struct proxy_based_iterator{
                typedef ::Rcpp::internal::Proxy_Iterator< typename r_vector_proxy<RTYPE>::type > type ;
        } ;
        template<> struct r_vector_iterator<VECSXP> : proxy_based_iterator<VECSXP>{} ;
        template<> struct r_vector_iterator<EXPRSXP> : proxy_based_iterator<EXPRSXP>{} ;
        template<> struct r_vector_iterator<STRSXP> : proxy_based_iterator<STRSXP>{} ;

Vector::const_iterator was merely defined by (from Rcpp/vector/proxy.h):

        template <int RTYPE>
        struct r_vector_const_iterator {
                typedef const typename storage_type<RTYPE>::type* type ;
        };

It had no specializations and was not a full-featured iterator like Vector::iterator was. This led to some problems with the unit tests passing.

My solution to the problem (which I believe only adds functionality) was to create a new class called internal::Const_Proxy_Iterator which is identical to internal::Proxy_Iterator in every way except that operator*() and operator->() return const references and pointers respectively. Then simply adding the following to Rcpp/vector/proxy.h gives us a full-featured and appropriately specialized Vector::const_iterator.

        template <int RTYPE> struct proxy_based_const_iterator{
                typedef ::Rcpp::internal::Const_Proxy_Iterator< typename r_vector_const_proxy<RTYPE>::type > type ;
        } ;
        template<> struct r_vector_const_iterator<VECSXP> : proxy_based_const_iterator<VECSXP>{} ;
        template<> struct r_vector_const_iterator<EXPRSXP> : proxy_based_const_iterator<EXPRSXP>{} ;
        template<> struct r_vector_const_iterator<STRSXP> : proxy_based_const_iterator<STRSXP>{} ;

All of these changes (with the addition of unit tests for the const variants of Matrix::row() and Matrix::column()) yield functional const access to Matrix rows and columns as well as a passing R CMD check.

While this is a rather large change (i.e. the definition of const_iterator is significantly different) I believe that it only adds functionality and shouldn't (famous last words) break any user's implementations. That is to say, because the old implementation of const_iterator was so bare bones, replacing it with a more full-featured implementation should be both desirable and non-intrusive.

Anyhow, that's where my code is at. Please let me know if you are interested in a PR for this.

Uh oh!

const versions of row and column for Matrix #200

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