// Operator overloads
double   operator [] (const size_t p_i) const { return (*const_cast<Vector3d*>(this))[p_i]; }
double&  operator [] (const size_t p_i) {

    THROW_ERROR_IF(p_i < 0 || p_i > 2, ERROR::INDEX_OUT_OF_RANGE);                                  // this is a code defect

         if (p_i == 1) return m_x;
    else if (p_i == 2) return m_y;
    else               return m_z;
}

double&  operator [] (const size_t p_i) {

    // check that supplied index is in range.
    // if index is not in range, issue a warning and use index modulo 3
    size_t index = p_i;
    if (index > 2) {
        SHOW_WARN(ERROR::INDEX_OUT_OF_RANGE, "Using index modulo 3");                               // index out of range
        index %= 3;                                                                                 // use index modulo 3
    }
    
         if (index == 1) return m_x;
    else if (index == 2) return m_y;
    else                 return m_z;
}

         if (index == 1) return m_x;
    else if (index == 2) return m_y;