flat_trait_2.h

// The minimum atomic size we believe to generate lock free code, i.e. all
// trivially copyable types not bigger this size generate lock free code.
static constexpr int kMinLockFreeAtomicSize = 8;

template <typename T>
struct IsAtomicFlagTypeTrait {
  static constexpr bool value = (sizeof(T) <= kMinLockFreeAtomicSize
                                 && std::is_trivially_copyable<T>::value);
}

// If not atomic fittable type, use mutex implementation.
template <typename T,
          typename std::enable_if<
              !flags_internal::IsAtomicFlagTypeTrait<T>::value, int>::type = 0>
ABSL_MUST_USE_RESULT T GetFlag(const absl::Flag<T>& flag) {
  return flag.Get();
}

// If it is a trivially copyable type, use atomic hack.
template <typename T,
          typename std::enable_if<
              flags_internal::IsAtomicFlagTypeTrait<T>::value, int>::type = 0>
ABSL_MUST_USE_RESULT T GetFlag(const absl::Flag<T>& flag) {
  // T might not be default constructible.
  union U {
    T value;
    U() {}
  };
  U result;
  if (flag.AtomicGet(&result.value)) {
    return result.value;
  }
  return flag.Get();
}