caffe2/utils/IdWrapper.h

#pragma once

#include <functional>

namespace c10 { namespace guts {

/**
 * This template simplifies generation of simple classes that wrap an id
 * in a typesafe way. Namely, you can use it to create a very lightweight
 * type that only offers equality comparators and hashing. Example:
 *
 *   struct MyIdType final : IdWrapper<MyIdType, uint32_t> {
 *     constexpr explicit MyIdType(uint32_t id): IdWrapper(id) {}
 *   };
 *
 * Then in the global top level namespace:
 *
 *   C10_DEFINE_IDWRAPPER(MyIdType);
 *
 * That's it - equality operators and hash functions are automatically defined
 * for you, given the underlying type supports it.
 */
template <class ConcreteType, class UnderlyingType>
class IdWrapper {
public:
    using underlying_type = UnderlyingType;
    using concrete_type = ConcreteType;

protected:
    constexpr explicit IdWrapper(underlying_type id) noexcept(noexcept(underlying_type(std::declval<underlying_type>())))
        : id_(id) {}

    constexpr underlying_type underlyingId() const noexcept(noexcept(underlying_type(std::declval<underlying_type>()))) {
        return id_;
    }

private:
    friend struct std::hash<ConcreteType>;

    // TODO Making operator== noexcept if underlying type is noexcept equality comparable doesn't work with GCC 4.8.
    //      Fix this once we don't need GCC 4.8 anymore.
    friend constexpr bool operator==(const concrete_type& lhs, const concrete_type& rhs) {
        return lhs.id_ == rhs.id_;
    }

    // TODO Making operator!= noexcept if operator== is noexcept doesn't work with GCC 4.8.
    //      Fix this once we don't need GCC 4.8 anymore.
    friend constexpr bool operator!=(const concrete_type& lhs, const concrete_type& rhs) {
        return !(lhs == rhs);
    }

    underlying_type id_;
};

}}

#define C10_DEFINE_HASH_FOR_IDWRAPPER(ClassName)             \
  namespace std {                                            \
  template <>                                                \
  struct hash<ClassName> {                                   \
    size_t operator()(ClassName x) const {                   \
      return std::hash<ClassName::underlying_type>()(x.id_); \
    }                                                        \
  };                                                         \
  }