Thread: Functor to allow access by callback

Hi,

I want to make the two functions to acces the matrix elements in a valarray user-modifyable by using a callback to a functor, which is passed as class-template parameter and instanciated as the data member "accessor" in the class. These are the functions 'at', two versions for const-correctness:

Code:

    template<typename T, typename AccessorT>
	inline T Matrix<T,AccessorT> :: at(size_t const& i, size_t const& j) const
	{
	    if(i>=this->nb_rows() || j>=this->nb_cols()) {
		throw MatrixException("index out of bounds");
	    }
	    return this->_elms[this->accessor(i,j)];          //   <<-----------------  use of the accessor
	}

    template<typename T, typename AccessorT>
	inline T& Matrix<T,AccessorT> :: at(size_t const& i, size_t const& j)
	{
	    if(i>=this->nb_rows() || j>=this->nb_cols()) {
		throw MatrixException("index out of bounds");
	    }
	    return this->_elms[this->accessor(i,j)];          //   <<-----------------  use of the accessor
	}

The functor "accessor" simply translates the two indices 'i,j' in one index 'k' . I have a default accessor defined as:

Code:

    class MatrixAccessor
    {
    public:
	virtual size_t operator()(size_t const& i, size_t const& j) = 0;
	virtual ~MatrixAccessor(){}
    };

    class C_Accessor : public MatrixAccessor // for row-oriented memory access
    {
    public:
    C_Accessor() : _nbrows(0), _nbcols(0) {}
    C_Accessor(size_t const& nb_rows) : _nbrows(nb_rows), _nbcols(nb_rows) {}
    C_Accessor(size_t const& nb_rows, size_t const& nb_cols) : _nbrows(nb_rows), _nbcols(nb_cols) {}
	size_t operator()(size_t const& i, size_t const& j)
	{
	    return(i+j*_nbrows);
	}

    private:
	size_t _nbrows;
	size_t _nbcols;
    };

My problem is const-correctness :

error C3848: expression having type 'const LinearAlgebra::C_Accessor' would lose some const-volatile qualifiers in order to call 'size_t LinearAlgebra::C_Accessor:: operator ()(const size_t &,const size_t &)'

Is this due to the operator[] of the valarray class? Should the operator() of the functor return a 'const size_t' ?

I haven't found the operator[] of the valarray class in the documentation, by the way...
http://publib.boulder.ibm.com/infoce...r_valarray.htm
http://msdn.microsoft.com/en-us/libr...y8(VS.80).aspx

Thanks a lot for your help!

Make operator() const?

Another thing is that there doesn't seem to be much point in inheriting (and even polymorphically) the accessors: they are used as a template argument after all.