visible_fields.h

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   Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301  USA */
 
#ifndef SQL_VISIBLE_FIELDS_H
#define SQL_VISIBLE_FIELDS_H
 
/**
  @file
  An adapter class to support iteration over an iterator of Item *
  (typically mem_root_deque<Item *>), while skipping over items that
  are hidden (item->hidden == true). This is such a common operation
  that it warrants having its own adapter. You can either do
 
    for (Item *item : VisibleFields(fields))
 
  or use select->visible_fields().
 
  Behavior is undefined if you modify the hidden flag of an item during
  iteration.
 
  TODO(sgunders): When we get C++20, replace with an std::views::filter.
 */
 
#include <iterator>
 
#include "mem_root_deque.h"
#include "sql/item.h"
 
template <class Iterator>
class VisibleFieldsAdapter {
 public:
  VisibleFieldsAdapter(Iterator base, Iterator end) : m_it(base), m_end(end) {
    while (m_it != m_end && (*m_it)->hidden) {
      ++m_it;
    }
  }
 
  // Pre-increment.
  VisibleFieldsAdapter &operator++() {
    do {
      ++m_it;
    } while (m_it != m_end && (*m_it)->hidden);
    return *this;
  }
 
  // Post-increment.
  VisibleFieldsAdapter operator++(int) {
    VisibleFieldsAdapter ret = *this;
    do {
      m_it++;
    } while (m_it != m_end && (*m_it)->hidden);
    return ret;
  }
 
  auto &operator*() const { return *m_it; }
 
  bool operator==(const VisibleFieldsAdapter &other) const {
    return m_it == other.m_it;
  }
  bool operator!=(const VisibleFieldsAdapter &other) const {
    return m_it != other.m_it;
  }
 
  // Define aliases needed by std::iterator_traits, to allow using this iterator
  // type with standard library templates.
  using difference_type = typename Iterator::difference_type;
  using value_type = typename Iterator::value_type;
  using pointer = typename Iterator::pointer;
  using reference = typename Iterator::reference;
  using iterator_category = std::forward_iterator_tag;
 
 private:
  Iterator m_it, m_end;
};
 
template <class Container, class Iterator>
class VisibleFieldsContainer {
 public:
  explicit VisibleFieldsContainer(Container &fields) : m_fields(fields) {}
  VisibleFieldsAdapter<Iterator> begin() {
    return VisibleFieldsAdapter<Iterator>(m_fields.begin(), m_fields.end());
  }
  VisibleFieldsAdapter<Iterator> end() {
    return VisibleFieldsAdapter<Iterator>(m_fields.end(), m_fields.end());
  }
 
 private:
  Container &m_fields;
};
 
inline auto VisibleFields(mem_root_deque<Item *> &fields) {
  return VisibleFieldsContainer<mem_root_deque<Item *>,
                                mem_root_deque<Item *>::iterator>(fields);
}
 
inline auto VisibleFields(const mem_root_deque<Item *> &fields) {
  return VisibleFieldsContainer<const mem_root_deque<Item *>,
                                mem_root_deque<Item *>::const_iterator>(fields);
}
 
#endif  // SQL_VISIBLE_FIELDS_H