Home > Articles > Programming > C/C++

New Objective-C Runtime Features in OS X 10.7 and iOS 5

The latest versions of the Objective-C runtimes shipped with iOS, Mac OS X, and even GNUstep provide several new features. David Chisnall looks at a few of these features.
Like this article? We recommend

Like this article? We recommend

Apple's Objective-C runtime acquired several new features in the version shipped with OS X 10.7 and iOS 5. Most of these features are undocumented, although they're present in the headers.

The most obvious change is the addition of Automatic Reference Counting (ARC) support. On OS X 10.6, Apple provides a partial implementation of ARC, but without support for weak references. OS X 10.7 incorporates a much more complex version into the runtime.

Three functions also look quite interesting, with signatures like this:

NS_RETURNS_RETAINED id objc_retainedObject(objc_objectptr_t CF_CONSUMED pointer);

objc_objectptr_t is a typedef for void*, and the implementation of this function does nothing. The ARC optimizer in LLVM will remove any calls that you make to these functions. They're hints to the compiler about pointer ownership. You use this feature to indicate that you've taken ownership of an object pointer. In general, you should use bridging casts instead of calling these functions—they're really useful only if you have some inline functions in headers that need to support ARC and non-ARC modes.

Blocks as Methods

One of my favorite additions—and the one that required the most effort for me to implement in the GNUstep Objective-C runtime—is imp_implementationWithBlock(). This function takes a block and returns an instance method pointer (IMP). The Apple implementation has a hard limit on the number of these pointers that can be allocated.

The block that you pass into this function must take an object as the first argument. The function will then return an IMP for a method that takes the same explicit arguments as the block takes arguments after the object pointer. For example, if you pass a block like this:

^(id, NSUInteger, NSRect)

you'll get back an IMP like this:

*(id, SEL, NSUInteger, NSRect)

This technique works because the block function, like an Objective-C or C++ method, has a hidden argument: a pointer to the block structure. The imp_implementationWithBlock() function returns a trampoline that modifies the first two arguments. When you call the method, the trampoline moves the receiver from the first argument to the second (overwriting the selector) and loads the block pointer into the first argument. It doesn't touch any of the subsequent arguments.

I don't know how Apple is using this functionality, but I strongly suspect that it's related to key-value observing. Being able to swizzle a method with a block can make attaching the observers quite fast.

Memory management is a little bit tricky if you use this function. Blocks are reference counted, but IMPs are not. You can attach an IMP as a method to multiple classes, so the runtime can't automatically release the block when the method is removed. You must do this yourself, tracking the uses of the IMP and calling imp_removeBlock() when the block is removed.

The most irritating thing about this API is that the compiler automatically generates a type encoding for every block and stores it in the block descriptor. When you register the method, you must provide its type encoding. If you use arguments like NSUInteger or NSRect, this encoding can change, depending on whether you're targeting a 32-bit or 64-bit system. Transforming the block encoding into an IMP encoding is trivial, but there's no public API for doing it—or even for accessing the block's type encoding—on OS X. In the GNUstep runtime, I've added a block_copyIMPTypeEncoding_np() function, which returns the type encoding that should be used for the IMP generated from a block.

Using this API is quite simple. This example adds a +count: method that uses a block:

__block int b = 0;
void* blk = ^(id self, int a) {
      b += a;
      return b;
};
blk = Block_copy(blk);
IMP imp = imp_implementationWithBlock(blk);
char *type = block_copyIMPTypeEncoding_np(blk);
assert(NULL != type);
class_addMethod((objc_getMetaClass("Foo")), @selector(count:), imp, type);
free(type);
assert(2 == [Foo count: 2]);
assert(6 == [Foo count: 4]);

In the Apple implementation, you would need to use a hard-coded type-encoding string, "i@:i", instead of the block_copyIMPTypeEncoding_np() function. Not too much extra effort, but you can't write generic functions that do this with a user-provided block (unless they also provide the type encoding). Also, if you change the types of the block, you must remember to change the type encoding.

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