Is there a non-atomic equivalent of std::shared_ptr? And why isn't there one in <memory>?

Question

This is a bit of a two part question, all about the atomicity of std::shared_ptr:

1. As far as I can tell, std::shared_ptr is the only smart pointer in <memory> that's atomic. I'm wondering if there is a non-atomic version of std::shared_ptr available (I can't see anything in <memory>, so I'm also open to suggestions outside of the standard, like those in Boost). I know boost::shared_ptr is also atomic (if BOOST_SP_DISABLE_THREADS isn't defined), but maybe there's another alternative? I'm looking for something that has the same semantics as std::shared_ptr, but without the atomicity.

2. I understand why std::shared_ptr is atomic; it's kinda nice. However, it's not nice for every situation, and C++ has historically had the mantra of "only pay for what you use." If I'm not using multiple threads, or if I am using multiple threads but am not sharing pointer ownership across threads, an atomic smart pointer is overkill. My second question is why wasn't a non-atomic version of std::shared_ptr provided in C++11? (assuming there is a why) (if the answer is simply "a non-atomic version was simply never considered" or "no one ever asked for a non-atomic version" that's fine!).

With question #2, I'm wondering if someone ever proposed a non-atomic version of shared_ptr (either to Boost or the standards committee) (not to replace the atomic version of shared_ptr, but to coexist with it) and it was shot down for a specific reason.

Answer 1

1. I'm wondering if there is a non-atomic version of std::shared_ptr available

Not provided by the standard. There may well be one provided by a "3rd party" library. Indeed, prior to C++11, and prior to Boost, it seemed like everyone wrote their own reference counted smart pointer (including myself).

2. My second question is why wasn't a non-atomic version of std::shared_ptr provided in C++11?

This question was discussed at the Rapperswil meeting in 2010. The subject was introduced by a National Body Comment #20 by Switzerland. There were strong arguments on both sides of the debate, including those you provide in your question. However, at the end of the discussion, the vote was overwhelmingly (but not unanimous) against adding an unsynchronized (non-atomic) version of shared_ptr.

Arguments against included:

• Code written with the unsynchronized shared_ptr may end up being used in threaded code down the road, ending up causing difficult to debug problems with no warning.

• Having one "universal" shared_ptr that is the "one way" to traffic in reference counting has benefits: From the original proposal:

  Has the same object type regardless of features used, greatly facilitating interoperability between libraries, including third-party libraries.

• The cost of the atomics, while not zero, is not overwhelming. The cost is mitigated by the use of move construction and move assignment which do not need to use atomic operations. Such operations are commonly used in vector<shared_ptr<T>> erase and insert.

• Nothing prohibits people from writing their own non-atomic reference-counted smart pointer if that's really what they want to do.

The final word from the LWG in Rapperswil that day was:

Reject CH 20. No consensus to make a change at this time.

Answer 2

Howard's answered the question well already, and Nicol made some good points about the benefits of having a single standard shared pointer type, rather than lots of incompatible ones.

While I completely agree with the committee's decision, I do think there is some benefit to using an unsynchronized shared_ptr-like type in special cases, so I've investigated the topic a few times.

If I'm not using multiple threads, or if I am using multiple threads but am not sharing pointer ownership across threads, an atomic smart pointer is overkill.

With GCC when your program doesn't use multiple threads shared_ptr doesn't use atomic ops for the refcount. This is done by updating the reference counts via wrapper functions that detect whether the program is multithreaded (on GNU/Linux this is done by checking a special variable in Glibc that says if the program is single-threaded^[1]) and dispatch to atomic or non-atomic operations accordingly.

I realised many years ago that because GCC's shared_ptr<T> is implemented in terms of a __shared_ptr<T, _LockPolicy> base class, it's possible to use the base class with the single-threaded locking policy even in multithreaded code, by explicitly using __shared_ptr<T, __gnu_cxx::_S_single>. You can use an alias template like this to define a shared pointer type that is not thread-safe, but is slightly faster^[2]:

template<typename T>
  using shared_ptr_unsynchronized = std::__shared_ptr<T, __gnu_cxx::_S_single>;

This type would not be interoperable with std::shared_ptr<T> and would only be safe to use when it is guaranteed that the shared_ptr_unsynchronized objects would never be shared between threads without additional user-provided synchronization.

This is of course completely non-portable, but sometimes that's OK. With the right preprocessor hacks your code would still work fine with other implementations if shared_ptr_unsynchronized<T> is an alias for shared_ptr<T>, it would just be a little faster with GCC.

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_{[1] Before Glibc 2.33 added that variable, the wrapper functions would detect whether the program links to libpthread.so as an imperfect method of checking for single-threaded vs multi-threaded.}

_{[2] Unfortunately because that wasn't an intended use case it didn't quite work optimally before GCC 4.9, and some operations still used the wrapper functions and so dispatched to atomic operations even though you've explicitly requested the `_S_single` policy. See point (2) at http://gcc.gnu.org/ml/libstdc++/2007-10/msg00180.html for more details and a patch to GCC to allow the non-atomic implementation to be used even in multithreaded apps. I sat on that patch for years but I finally committed it for GCC 4.9.}

Answer 3

My second question is why wasn't a non-atomic version of std::shared_ptr provided in C++11? (assuming there is a why).

One could just as easily ask why there isn't an intrusive pointer, or any number of other possible variations of shared pointers one could have.

The design of shared_ptr, handed down from Boost, has been to create a minimum standard lingua-franca of smart pointers. That, generally speaking, you can just pull this down off the wall and use it. It's something that would be used generally, across a wide variety of applications. You can put it in an interface, and odds are good people will be willing to use it.

Threading is only going to get more prevalent in the future. Indeed, as time passes, threading will generally be one of the primary means to achieve performance. Requiring the basic smart pointer to do the bare minimum needed to support threading facilitates this reality.

Dumping a half-dozen smart pointers with minor variations between them into the standard, or even worse a policy-based smart pointer, would have been terrible. Everyone would pick the pointer they like best and forswear all others. Nobody would be able to communicate with anyone else. It'd be like the current situations with C++ strings, where everyone has their own type. Only far worse, because interoperation with strings is a lot easier than interoperation between smart pointer classes.

Boost, and by extension the committee, picked a specific smart pointer to use. It provided a good balance of features and was widely and commonly used in practice.

std::vector has some inefficiencies compared to naked arrays in some corner cases too. It has some limitations; some uses really want to have a hard limit on the size of a vector, without using a throwing allocator. However, the committee didn't design vector to be everything for everyone. It was designed to be a good default for most applications. Those for whom it can't work can just write an alternative that suites their needs.

Just as you can for a smart pointer if shared_ptr's atomicity is a burden. Then again, one might also consider not copying them around so much.