null_iterator.h

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#ifndef MYSQL_ITERATORS_NULL_ITERATOR_H
#define MYSQL_ITERATORS_NULL_ITERATOR_H
 
/// @file
/// Experimental API header
 
#include <ranges>  // range
 
/// @addtogroup GroupLibsMysqlIterators
/// @{
 
namespace mysql::iterators {
 
/// Returns an iterator object for a given range type, created without passing a
/// range object, while guaranteeing that two such iterators for the same type
/// compare as equal.
///
/// The use case is a view which does not have a range: then the view can return
/// null iterators from its begin and end member functions, making it behave as
/// a view over an empty range.
///
/// Note that default-constructed iterators do not generally work in this case,
/// because comparison for default-constructed standard library iterators is
/// undefined behavior.
///
/// This is implemented by returning the end iterator for a (singleton)
/// default-constructed range object.
template <std::ranges::range Range_t>
[[nodiscard]] auto null_iterator() {
  static const Range_t default_instance{};
  return default_instance.end();
}
 
}  // namespace mysql::iterators
 
// addtogroup GroupLibsMysqlIterators
/// @}
 
#endif  // ifndef MYSQL_ITERATORS_NULL_ITERATOR_H