template <typename ValueType,
          typename MutexType = stdx::mutex,
          typename LockPolicy = versioned_value_detail::DefaultLockPolicy>
class VersionedValue {
public:
    ...
    void peekAndMaybeUpdate(std::function<std::shared_ptr<ValueType>(const std::shared_ptr<ValueType>&)> cb) {
        auto lk = _lockPolicy.makeExclusiveLock(_mutex);
        if (auto maybeNewValue = cb(_current)) {
            _update(lk, std::move(maybeNewValue));
        }
    }

    void update(std::shared_ptr<ValueType> newValue) {
        auto lk = _lockPolicy.makeExclusiveLock(_mutex);
        _update(lk, std::move(newValue));
    }
    ...
private:
    void _update(WithLock, std::shared_ptr<ValueType> newValue) {
        _version.fetchAndAdd(1);
        _current = std::move(newValue);
    }
    ...
};