Home > Articles > Programming > C/C++

Smart Pointers in C++

This chapter is from the book

7.3 Storage of Smart Pointers

To start, let's ask a fundamental question about smart pointers. Is pointee_'s type necessarily T*? If not, what else could it be? In generic programming, you should always ask yourself questions like these. Each type that's hardcoded in a piece of generic code decreases the genericity of the code. Hardcoded types are to generic code what magic constants are to regular code.

In several situations, it is worthwhile to allow customizing the pointee type. One situation is when you deal with nonstandard pointer modifiers. In the 16-bit Intel 80x86 days, you could qualify pointers with modifiers like __near, __far, and __huge. Other segmented memory architectures use similar modifiers.

Another situation is when you want to layer smart pointers. What if you have a LegacySmartPtr<T> smart pointer implemented by someone else, and you want to enhance it? Would you derive from it? That's a risky decision. It's better to wrap the legacy smart pointer into a smart pointer of your own. This is possible because the inner smart pointer supports pointer syntax. From the outer smart pointer's viewpoint, the pointee type is not T* but LegacySmartPtr<T>.

There are interesting applications of smart pointer layering, mainly because of the mechanics of operator->. When you apply operator-> to a type that's not a built-in pointer, the compiler does an interesting thing. After looking up and applying the user-defined operator-> to that type, it applies operator-> again to the result. The compiler keeps doing this recursively until it reaches a pointer to a built-in type, and only then proceeds with member access. It follows that a smart pointer's operator-> does not have to return a pointer. It can return an object that in turn implements operator->, without changing the use syntax.

This leads to a very interesting idiom: pre-and postfunction calls (Stroustrup 2000). If you return an object of type PointerType by value from operator->, the sequence of execution is as follows:

  • Constructor of PointerType

  • PointerType::operator-> called; likely returns a pointer to an object of type PointeeType

  • Member access for PointeeType—likely a function call

  • Destructor of PointerType

  • In a nutshell, you have a nifty way of implementing locked function calls. This idiom has broad uses with multithreading and locked resource access. You can have PointerType's constructor lock the resource, and then you can access the resource; finally, Pointer Type's destructor unlocks the resource.

    The generalization doesn't stop here. The syntax-oriented “pointer” part might sometimes pale in comparison with the powerful resource management techniques that are included in smart pointers. It follows that, in rare cases, smart pointers could drop the pointer syntax. An object that does not define operator-> and operator* violates the definition of a smart pointer, but there are objects that do deserve smart pointer–like treatment, although they are not, strictly speaking, smart pointers.

    Look at real-world APIs and applications. Many operating systems foster handles as accessors to certain internal resources, such as windows, mutexes, or devices. Handles are intentionally obfuscated pointers; one of their purposes is to prevent their users from manipulating critical operating system resources directly. Most of the time, handles are integral values that are indices in a hidden table of pointers. The table provides the additional level of indirection that protects the inner system from the application programmers. Although they don't provide an operator->, handles resemble pointers in semantics and in the way they are managed.

    For such a smart resource, it does not make sense to provide operator-> or operator*. However, you do take advantage of all the resource management techniques that are specific to smart pointers.

    To generalize the type universe of smart pointers, we distinguish three potentially distinct types in a smart pointer:

    • The storage type. This is the type of pointee_. By “default”—in regular smart pointers—it is a raw pointer.

    • The pointer type. This is the type returned by operator->. It can be different from the storage type if you want to return a proxy object instead of just a pointer. (You will find an example of using proxy objects later in this chapter.)

    • The reference type. This is the type returned by operator*.

    It would be useful if SmartPtr supported this generalization in a flexible way. Thus, the three types mentioned here ought to be abstracted in a policy called Storage.

    In conclusion, smart pointers can, and should, generalize their pointee type. To do this, SmartPtr abstracts three types in a Storage policy: the stored type, the pointer type, and the reference type. Not all types necessarily make sense for a given SmartPtr instantiation. Therefore, in rare cases (handles), a policy might disable access to operator-> or operator* or both.

    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