c++ - Get submatrix of matrix with smaller size -

i representing map ( matrix rows x columns ) bits using bitset stl or dynamic_bitset<> boost ( can use whatever want ). need submatrix of matrix smaller size( example a(2,2) a(2,3) a(3,2) a(3,3) there size 2 ). there efficient structure representing matrix bits , getting submatrix startindex , length without iteration ?

it possible efficiently share data between matrices , submatrices. trick track 3 variables in class.

• row_stride
• start
• data

the data needs shared_ptr structure underlying data can destroyed once done it. start pointer data referenced data, row_stride tells how far move next row.

additional things might track

• column stride (this can allow take other interesting views matrix, , support transpose efficiently).
• row , column length - these can handy debugging or if want make loops, , multiplies easier work with.

here's how might non-bit based approach (i've omitted .. gist).

template<typename t> struct matrixdata {    t * data;    explicit matrixdata( size_t n ) { new t[n]; }    ~matrixdata() { delete [] data; } private:     matrixdata( const matrixdata & );     matrixdata& operator=( const matrixdata & ); };  template<typename t> class matrix {     matrix(size_t nni, size_t nnj) :       data( new matrixdata( nni*nnj ) ),       ni(nni),       nj(nnj),       row_stride(ni),       col_stride(1)     {     }      t operator()( size_t i, size_t j)     {        assert( < ni );        assert( j < nj );        return start + * col_stride + j * row_stride;     }      matrix submatrix( size_t i_start, size_t j_start, size_t new_ni, size_t new_nj )     {        assert( i_start + new_ni < ni );        assert( j_start + new_nj < nj );         matrix retval(*this);        retval.start += i_start * col_stride + j_start * row_stride;        retval.ni = new_ni;        retval.nj = new_nj;        return retval;     }      matrix transpose()     {        matrix retval(*this);        std::swap(retval.ni,retval.nj);        std::swap(retval.row_stride,retval.col_stride);     }    private:     shared_ptr<matrixdata> data;     t* start;     size_t ni;     size_t nj;     size_t row_stride;     size_t col_stride;  }; 

making work bit based version mean changing matrixdata hold 1 of bot based structures, changing start index structure , changing operator() access data correctly.