Home > Articles > Programming > C/C++

Smart Pointers in C++

This chapter is from the book

7.13 Smart Pointers and Multithreading

Most often, smart pointers help with sharing objects. Multithreading issues affect object sharing. Therefore, multithreading issues affect smart pointers.

The interaction between smart pointers and multithreading takes place at two levels. One is the pointee object level, and the other is the bookkeeping data level.

7.13.1 Multithreading at the Pointee Object Level

If multiple threads access the same object and if you access that object through a smart pointer, it can be desirable to lock the object during a function call made through operator->. This is possible by having the smart pointer return a proxy object instead of a raw pointer. The proxy object's constructor locks the pointee object, and its destructor unlocks it. The technique is illustrated in Stroustrup (2000). Some code that illustrates this approach is provided here.

First, let's consider a class Widget that has two locking primitives: Lock and Unlock. After a call to Lock, you can access the object safely. Any other threads calling Lock will block. When you call Unlock, you let other threads lock the object.

class Widget
{
   ...
   void Lock();
   void Unlock();
};

Next, we define a class template LockingProxy. Its role is to lock an object (using the Lock/Unlock convention) for the duration of LockingProxy's lifetime.

template <class T>
class LockingProxy
{
public:
   LockingProxy(T* pObj) : pointee_ (pObj)
   { pointee_->Lock(); }
   ~LockingProxy()
   { pointee_->Unlock(); }
   T* operator->() const
   { return pointee_; }
private:
   LockingProxy& operator=(const LockingProxy&);
   T* pointee_;
};

In addition to the constructor and destructor, LockingProxy defines an operator-> that returns a pointer to the pointee object.

Although LockingProxy looks somewhat like a smart pointer, there is one more layer to it—the SmartPtr class template itself.

template <class T>
class SmartPtr
{
   ...
   LockingProxy<T> operator->() const
   { return LockingProxy<T>(pointee_); }
private:
   T* pointee_;
};

Recall from Section 7.3, which explains the mechanics of operator->, that the compiler can apply operator-> multiple times to one -> expression, until it reaches a native pointer. Now imagine you issue the following call (assuming Widget defines a function DoSomething):

SmartPtr<Widget> sp = ...;
sp->DoSomething();

Here's the trick: SmartPtr's operator-> returns a temporary LockingProxy<T> object. The compiler keeps applying operator->. LockingProxy<T>'s operator-> returns a Widget*. The compiler uses this pointer to Widget to issue the call to DoSomething. During the call, the temporary object LockingProxy<T> is alive and locks the object, which means that the object is safely locked. As soon as the call to DoSomething returns, the temporary LockingProxy<T> object is destroyed, so the Widget object is unlocked.

Automatic locking is a good application of smart pointer layering. You can layer smart pointers this way by changing the Storage policy.

7.13.2 Multithreading at the Bookkeeping Data Level

Sometimes smart pointers manipulate data in addition to the pointee object. As you read in Section 7.5, reference-counted smart pointers share some data—namely the reference count—under the covers. If you copy a reference-counted smart pointer from one thread to another, you end up having two smart pointers pointing to the same reference counter. Of course, they also point to the same pointee object, but that's accessible to the user, who can lock it. In contrast, the reference count is not accessible to the user, so managing it is entirely the responsibility of the smart pointer.

Not only reference-counted pointers are exposed to multithreading-related dangers. Reference-tracked smart pointers (Section 7.5.4) internally hold pointers to each other, which are shared data as well. Reference linking leads to communities of smart pointers, not all of which necessarily belong to the same thread. Therefore, every time you copy, assign, and destroy a reference-tracked smart pointer, you must issue appropriate locking; otherwise, the doubly linked list might get corrupted.

In conclusion, multithreading issues ultimately affect smart pointers' implementation. Let's see how to address the multithreading issue in reference counting and reference linking.

7.13.2.1 Multithreaded Reference Counting

If you copy a smart pointer between threads, you end up incrementing the reference count from different threads at unpredictable times.

As the appendix explains, incrementing a value is not an atomic operation. For incrementing and decrementing integral values in a multithreaded environment, you must use the type ThreadingModel<T>::IntType and the AtomicIncrement and AtomicDecrement functions.

Here things become a bit tricky. Better said, they become tricky if you want to separate reference counting from threading.

Policy-based class design prescribes that you decompose a class into elementary behavioral elements and confine each of them to a separate template parameter. In an ideal world, SmartPtr would specify an Ownership policy and a ThreadingModel policy and would use them both for a correct implementation.

In the case of multithreaded reference counting, however, things are much too tied together. For example, the counter must be of type ThreadingModel<T>::IntType. Then, instead of using operator++ and operator——, you must use AtomicIncrement and AtomicDecrement. Threading and reference counting melt together; it is unjustifiably hard to separate them.

The best thing to do is to incorporate multithreading in the Ownership policy. Then you can have two implementations: RefCounting and RefCountingMT.

7.13.2.2 Multithreaded Reference Linking

Consider the destructor of a reference-linked smart pointer. It likely looks like this:

template <class T>
class SmartPtr
{
public:
   ~SmartPtr()
   {
      if (prev_ == next_)
      {
         delete pointee_;
      }
      else
      {
         prev_->next_ = next_;
         next_->prev_ = prev_;
      }
   }
   ...
private:
   T* pointee_;
   SmartPtr* prev_;
   SmartPtr* next_;
};

The code in the destructor performs a classic doubly linked list deletion. To make implementation simpler and faster, the list is circular—the last node points to the first node. This way we don't have to test prev_ and next_ against zero for any smart pointer. A circular list with only one element has prev_ and next_ equal to this.

If multiple threads destroy smart pointers that are linked to each other, clearly the destructor must be atomic (uninterruptible by other threads). Otherwise, another thread can interrupt the destructor of a SmartPtr, for instance, between updating prev_->next_ and updating next_->prev_. That thread will then operate on a corrupt list.

Similar reasoning applies to SmartPtr's copy constructor and the assignment operator. These functions must be atomic because they manipulate the ownership list.

Interestingly enough, we cannot apply object-level locking semantics here. The appendix divides locking strategies into class-level and object-level strategies. A class-level locking operation locks all objects in a given class during that operation. An object-level locking operation locks only the object that's subject to that operation. The former technique leads to less memory being occupied (only one mutex per class) but is exposed to performance bottlenecks. The latter is heavier (one mutex per object) but might be speedier.

We cannot apply object-level locking to smart pointers because an operation manipulates up to three objects: the current object that's being added or removed, the previous object, and the next object in the ownership list.

If we want to introduce object-level locking, the starting observation is that there must be one mutex per pointee object—because there's one list per pointee object. We can dynamically allocate a mutex for each object, but this nullifies the main advantage of reference linking over reference counting. Reference linking was more appealing exactly because it didn't use the free store.

Alternatively, we can use an intrusive approach: The pointee object holds the mutex, and the smart pointer manipulates that mutex. But the existence of a sound, effective alternative—reference-counted smart pointers—removes the incentive to provide this feature.

In summary, smart pointers that use reference counting or reference linking are affected by multithreading issues. Thread-safe reference counting needs integer atomic operations. Thread-safe reference linking needs mutexes. SmartPtr provides only thread-safe reference counting.

InformIT Promotional Mailings & Special Offers

I would like to receive exclusive offers and hear about products from InformIT and its family of brands. I can unsubscribe at any time.

Overview


Pearson Education, Inc., 221 River Street, Hoboken, New Jersey 07030, (Pearson) presents this site to provide information about products and services that can be purchased through this site.

This privacy notice provides an overview of our commitment to privacy and describes how we collect, protect, use and share personal information collected through this site. Please note that other Pearson websites and online products and services have their own separate privacy policies.

Collection and Use of Information


To conduct business and deliver products and services, Pearson collects and uses personal information in several ways in connection with this site, including:

Questions and Inquiries

For inquiries and questions, we collect the inquiry or question, together with name, contact details (email address, phone number and mailing address) and any other additional information voluntarily submitted to us through a Contact Us form or an email. We use this information to address the inquiry and respond to the question.

Online Store

For orders and purchases placed through our online store on this site, we collect order details, name, institution name and address (if applicable), email address, phone number, shipping and billing addresses, credit/debit card information, shipping options and any instructions. We use this information to complete transactions, fulfill orders, communicate with individuals placing orders or visiting the online store, and for related purposes.

Surveys

Pearson may offer opportunities to provide feedback or participate in surveys, including surveys evaluating Pearson products, services or sites. Participation is voluntary. Pearson collects information requested in the survey questions and uses the information to evaluate, support, maintain and improve products, services or sites, develop new products and services, conduct educational research and for other purposes specified in the survey.

Contests and Drawings

Occasionally, we may sponsor a contest or drawing. Participation is optional. Pearson collects name, contact information and other information specified on the entry form for the contest or drawing to conduct the contest or drawing. Pearson may collect additional personal information from the winners of a contest or drawing in order to award the prize and for tax reporting purposes, as required by law.

Newsletters

If you have elected to receive email newsletters or promotional mailings and special offers but want to unsubscribe, simply email information@informit.com.

Service Announcements

On rare occasions it is necessary to send out a strictly service related announcement. For instance, if our service is temporarily suspended for maintenance we might send users an email. Generally, users may not opt-out of these communications, though they can deactivate their account information. However, these communications are not promotional in nature.

Customer Service

We communicate with users on a regular basis to provide requested services and in regard to issues relating to their account we reply via email or phone in accordance with the users' wishes when a user submits their information through our Contact Us form.

Other Collection and Use of Information


Application and System Logs

Pearson automatically collects log data to help ensure the delivery, availability and security of this site. Log data may include technical information about how a user or visitor connected to this site, such as browser type, type of computer/device, operating system, internet service provider and IP address. We use this information for support purposes and to monitor the health of the site, identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents and appropriately scale computing resources.

Web Analytics

Pearson may use third party web trend analytical services, including Google Analytics, to collect visitor information, such as IP addresses, browser types, referring pages, pages visited and time spent on a particular site. While these analytical services collect and report information on an anonymous basis, they may use cookies to gather web trend information. The information gathered may enable Pearson (but not the third party web trend services) to link information with application and system log data. Pearson uses this information for system administration and to identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents, appropriately scale computing resources and otherwise support and deliver this site and its services.

Cookies and Related Technologies

This site uses cookies and similar technologies to personalize content, measure traffic patterns, control security, track use and access of information on this site, and provide interest-based messages and advertising. Users can manage and block the use of cookies through their browser. Disabling or blocking certain cookies may limit the functionality of this site.

Do Not Track

This site currently does not respond to Do Not Track signals.

Security


Pearson uses appropriate physical, administrative and technical security measures to protect personal information from unauthorized access, use and disclosure.

Children


This site is not directed to children under the age of 13.

Marketing


Pearson may send or direct marketing communications to users, provided that

  • Pearson will not use personal information collected or processed as a K-12 school service provider for the purpose of directed or targeted advertising.
  • Such marketing is consistent with applicable law and Pearson's legal obligations.
  • Pearson will not knowingly direct or send marketing communications to an individual who has expressed a preference not to receive marketing.
  • Where required by applicable law, express or implied consent to marketing exists and has not been withdrawn.

Pearson may provide personal information to a third party service provider on a restricted basis to provide marketing solely on behalf of Pearson or an affiliate or customer for whom Pearson is a service provider. Marketing preferences may be changed at any time.

Correcting/Updating Personal Information


If a user's personally identifiable information changes (such as your postal address or email address), we provide a way to correct or update that user's personal data provided to us. This can be done on the Account page. If a user no longer desires our service and desires to delete his or her account, please contact us at customer-service@informit.com and we will process the deletion of a user's account.

Choice/Opt-out


Users can always make an informed choice as to whether they should proceed with certain services offered by InformIT. If you choose to remove yourself from our mailing list(s) simply visit the following page and uncheck any communication you no longer want to receive: www.informit.com/u.aspx.

Sale of Personal Information


Pearson does not rent or sell personal information in exchange for any payment of money.

While Pearson does not sell personal information, as defined in Nevada law, Nevada residents may email a request for no sale of their personal information to NevadaDesignatedRequest@pearson.com.

Supplemental Privacy Statement for California Residents


California residents should read our Supplemental privacy statement for California residents in conjunction with this Privacy Notice. The Supplemental privacy statement for California residents explains Pearson's commitment to comply with California law and applies to personal information of California residents collected in connection with this site and the Services.

Sharing and Disclosure


Pearson may disclose personal information, as follows:

  • As required by law.
  • With the consent of the individual (or their parent, if the individual is a minor)
  • In response to a subpoena, court order or legal process, to the extent permitted or required by law
  • To protect the security and safety of individuals, data, assets and systems, consistent with applicable law
  • In connection the sale, joint venture or other transfer of some or all of its company or assets, subject to the provisions of this Privacy Notice
  • To investigate or address actual or suspected fraud or other illegal activities
  • To exercise its legal rights, including enforcement of the Terms of Use for this site or another contract
  • To affiliated Pearson companies and other companies and organizations who perform work for Pearson and are obligated to protect the privacy of personal information consistent with this Privacy Notice
  • To a school, organization, company or government agency, where Pearson collects or processes the personal information in a school setting or on behalf of such organization, company or government agency.

Links


This web site contains links to other sites. Please be aware that we are not responsible for the privacy practices of such other sites. We encourage our users to be aware when they leave our site and to read the privacy statements of each and every web site that collects Personal Information. This privacy statement applies solely to information collected by this web site.

Requests and Contact


Please contact us about this Privacy Notice or if you have any requests or questions relating to the privacy of your personal information.

Changes to this Privacy Notice


We may revise this Privacy Notice through an updated posting. We will identify the effective date of the revision in the posting. Often, updates are made to provide greater clarity or to comply with changes in regulatory requirements. If the updates involve material changes to the collection, protection, use or disclosure of Personal Information, Pearson will provide notice of the change through a conspicuous notice on this site or other appropriate way. Continued use of the site after the effective date of a posted revision evidences acceptance. Please contact us if you have questions or concerns about the Privacy Notice or any objection to any revisions.

Last Update: November 17, 2020