Objective-c – ARC and bridged cast

automatic-ref-countingclangobjective ctype conversion

With ARC, I can no longer cast CGColorRef to id. I learned that I need to do a bridged cast. According clang docs:

A bridged cast is a C-style cast annotated with one of three keywords:

(__bridge T) op casts the operand to the destination type T. If T
is a retainable object pointer type, then op must have a
non-retainable pointer type. If T is a non-retainable pointer type,
then op must have a retainable object pointer type. Otherwise the cast
is ill-formed. There is no transfer of ownership, and ARC inserts no
retain operations.

(__bridge_retained T) op casts the operand, which must have
retainable object pointer type, to the destination type, which must be
a non-retainable pointer type. ARC retains the value, subject to the
usual optimizations on local values, and the recipient is responsible
for balancing that +1.

(__bridge_transfer T) op casts the operand, which must have
non-retainable pointer type, to the destination type, which must be a
retainable object pointer type. ARC will release the value at the end
of the enclosing full-expression, subject to the usual optimizations
on local values.

These casts are required in order to transfer objects in and out of
ARC control; see the rationale in the section on conversion of
retainable object pointers.

Using a __bridge_retained or __bridge_transfer cast purely to convince
ARC to emit an unbalanced retain or release, respectively, is poor
form.

In what kind of situations would I use each?

For example, CAGradientLayer has a colors property which accepts an array of CGColorRefs. My guess is that I should use __brige here, but exactly why I should (or should not) is unclear.

Best Answer

I agree that the description is confusing. Since I just grasped them, I'll try to summarize:

(__bridge_transfer <NSType>) op or alternatively CFBridgingRelease(op) is used to consume a retain-count of a CFTypeRef while transferring it over to ARC. This could also be represented by id someObj = (__bridge <NSType>) op; CFRelease(op);
(__bridge_retained <CFType>) op or alternatively CFBridgingRetain(op) is used to hand an NSObject over to CF-land while giving it a +1 retain count. You should handle a CFTypeRef you create this way the same as you would handle a result of CFStringCreateCopy(). This could also be represented by CFRetain((__bridge CFType)op); CFTypeRef someTypeRef = (__bridge CFType)op;
__bridge just casts between pointer-land and Objective-C object-land. If you have no inclination to use the conversions above, use this one.

Maybe this is helpful. Myself, I prefer the CFBridging… macros quite a bit over the plain casts.

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Ios – What’s the difference between the atomic and nonatomic attributes

The last two are identical; "atomic" is the default behavior (~~note that it is not actually a keyword; it is specified only by the absence of nonatomic~~ -- atomic was added as a keyword in recent versions of llvm/clang).

Assuming that you are @synthesizing the method implementations, atomic vs. non-atomic changes the generated code. If you are writing your own setter/getters, atomic/nonatomic/retain/assign/copy are merely advisory. (Note: @synthesize is now the default behavior in recent versions of LLVM. There is also no need to declare instance variables; they will be synthesized automatically, too, and will have an _ prepended to their name to prevent accidental direct access).

With "atomic", the synthesized setter/getter will ensure that a whole value is always returned from the getter or set by the setter, regardless of setter activity on any other thread. That is, if thread A is in the middle of the getter while thread B calls the setter, an actual viable value -- an autoreleased object, most likely -- will be returned to the caller in A.

In nonatomic, no such guarantees are made. Thus, nonatomic is considerably faster than "atomic".

What "atomic" does not do is make any guarantees about thread safety. If thread A is calling the getter simultaneously with thread B and C calling the setter with different values, thread A may get any one of the three values returned -- the one prior to any setters being called or either of the values passed into the setters in B and C. Likewise, the object may end up with the value from B or C, no way to tell.

Ensuring data integrity -- one of the primary challenges of multi-threaded programming -- is achieved by other means.

Adding to this:

atomicity of a single property also cannot guarantee thread safety when multiple dependent properties are in play.

Consider:

 @property(atomic, copy) NSString *firstName;
 @property(atomic, copy) NSString *lastName;
 @property(readonly, atomic, copy) NSString *fullName;

In this case, thread A could be renaming the object by calling setFirstName: and then calling setLastName:. In the meantime, thread B may call fullName in between thread A's two calls and will receive the new first name coupled with the old last name.

To address this, you need a transactional model. I.e. some other kind of synchronization and/or exclusion that allows one to exclude access to fullName while the dependent properties are being updated.

Objective-c – How to disable ARC for a single file in a project

It is possible to disable ARC for individual files by adding the -fno-objc-arc compiler flag for those files.

You add compiler flags in Targets -> Build Phases -> Compile Sources. You have to double click on the right column of the row under Compiler Flags. You can also add it to multiple files by holding the cmd button to select the files and then pressing enter to bring up the flag edit box. (Note that editing multiple files will overwrite any flags that it may already have.)

I created a sample project that has an example: https://github.com/jaminguy/NoArc

xcode

See this answer for more info: Disable Automatic Reference Counting for Some Files

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Ios – What’s the difference between the atomic and nonatomic attributes

Objective-c – How to disable ARC for a single file in a project

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