//-----------------------------------------------------------------------------
// Element Access
//-----------------------------------------------------------------------------

template<class T>
T& matrix<T>::at( std::size_t row, std::size_t col ){
  // TODO throw if out of bounds
  return data_.at(row*cols_+col);
}

template<class T>
const T& matrix<T>::at( std::size_t row, std::size_t col ) const{
  // TODO throw if out of bounds
  return data_.at(row*cols_+col);
}

template<class T>
typename matrix<T>::matrix_row matrix<T>::operator[]( std::size_t row ){
  return matrix_row(this,row);
}

template<class T>
const typename matrix<T>::matrix_row matrix<T>::operator[](
    std::size_t row ) const{
  return matrix_row(this,row);
}

//-----------------------------------------------------------------------------
// Row class element Access
//-----------------------------------------------------------------------------

template<class T>
T& matrix<T>::matrix_row::operator[](std::size_t col){
  return matrix_->data_[row_*matrix_->cols_+col];
}

template<class T>
const T& matrix<T>::matrix_row::operator[](std::size_t col) const{
  return matrix_->data_[row_*matrix_->cols_+col];
}


//-----------------------------------------------------------------------------
// Element Access
//-----------------------------------------------------------------------------

template<class T>
T& matrix<T>::at( std::size_t row, std::size_t col ){
  // TODO throw if out of bounds
  return data_.at(row*cols_+col);
}

template<class T>
const T& matrix<T>::at( std::size_t row, std::size_t col ) const{
  // TODO throw if out of bounds
  return data_.at(row*cols_+col);
}

template<class T>
typename matrix<T>::matrix_row matrix<T>::operator[]( std::size_t row ){
  return 0;
}

template<class T>
const typename matrix<T>::matrix_row matrix<T>::operator[](
    std::size_t row ) const{
  return 0;
}


--- 
+++ 
@@ -16,12 +16,26 @@
 
 template<class T>
 typename matrix<T>::matrix_row matrix<T>::operator[]( std::size_t row ){
-  return 0;
+  return matrix_row(this,row);
 }
 
 template<class T>
 const typename matrix<T>::matrix_row matrix<T>::operator[](
     std::size_t row ) const{
-  return 0;
+  return matrix_row(this,row);
 }
 
+//-----------------------------------------------------------------------------
+// Row class element Access
+//-----------------------------------------------------------------------------
+
+template<class T>
+T& matrix<T>::matrix_row::operator[](std::size_t col){
+  return matrix_->data_[row_*matrix_->cols_+col];
+}
+
+template<class T>
+const T& matrix<T>::matrix_row::operator[](std::size_t col) const{
+  return matrix_->data_[row_*matrix_->cols_+col];
+}
+
Add square bracket opertator access.
