podio/detail/OrderKey.h

0001 #ifndef PODIO_DETAIL_ORDERKEY_H
0002 #define PODIO_DETAIL_ORDERKEY_H
0003 #include <functional>
0004 namespace podio::detail {
0005 /// Internal class allowing datatype objects to be placed in data structures like maps and sets by providing a way to
0006 /// compare them. The comparison is based on addresses of their internal data objects.
0007 ///
0008 /// This class is intended to be used as the return value of internal `podio::detail::getOrderKey` free functions. These
0009 /// functions are friends of each datatype, allowing them to access the internal data objects and define less-than
0010 /// comparison operators for both datatypes and interface types.
0011 ///
0012 /// The friend free function design is used in order to reduce the coupling between interfaces and datatypes. Interfaces
0013 /// do not need to be friends of datatypes to define the less-than comparison operator, which allows using datatypes
0014 /// from different datamodels in an interface type.
0015 class OrderKey {
0016 public:
0017   OrderKey(const void* orderKey) noexcept : m_orderKey(orderKey) {
0018   }
0019   friend bool operator<(const OrderKey& lhs, const OrderKey& rhs) noexcept {
0020     return std::less<const void*>{}(lhs.m_orderKey, rhs.m_orderKey);
0021   }
0022
0023 private:
0024   const void* m_orderKey;
0025 };
0026 } // namespace podio::detail
0027
0028 #endif // PODIO_DETAIL_ORDERKEY_H