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I'm just beginning to have a look at Objective-C and Cocoa with a view to playing with the iPhone SDK. I'm reasonably comfortable with C's malloc and free concept, but Cocoa's references counting scheme has me rather confused. I'm told it's very elegant once you understand it, but I'm just not over the hump yet.
How do release, retain and autorelease work and what are the conventions about their use?
(Or failing that, what did you read which helped you get it?)
Let's start with retain and release; autorelease is really just a special case once you understand the basic concepts.
In Cocoa, each object keeps track of how many times it is being referenced (specifically, the NSObject base class implements this). By calling retain on an object, you are telling it that you want to up its reference count by one. By calling release, you tell the object you are letting go of it, and its reference count is decremented. If, after calling release, the reference count is now zero, then that object's memory is freed by the system.
The basic way this differs from malloc and free is that any given object doesn't need to worry about other parts of the system crashing because you've freed memory they were using. Assuming everyone is playing along and retaining/releasing according to the rules, when one piece of code retains and then releases the object, any other piece of code also referencing the object will be unaffected.
What can sometimes be confusing is knowing the circumstances under which you should call retain and release. My general rule of thumb is that if I want to hang on to an object for some length of time (if it's a member variable in a class, for instance), then I need to make sure the object's reference count knows about me. As described above, an object's reference count is incremented by calling retain. By convention, it is also incremented (set to 1, really) when the object is created with an "init" method. In either of these cases, it is my responsibility to call release on the object when I'm done with it. If I don't, there will be a memory leak.
Example of object creation:
NSString* s = [[NSString alloc] init]; // Ref count is 1
[s retain]; // Ref count is 2 - silly
// to do this after init
[s release]; // Ref count is back to 1
[s release]; // Ref count is 0, object is freed
Now for autorelease. Autorelease is used as a convenient (and sometimes necessary) way to tell the system to free this object up after a little while. From a plumbing perspective, when autorelease is called, the current thread's NSAutoreleasePool is alerted of the call. The NSAutoreleasePool now knows that once it gets an opportunity (after the current iteration of the event loop), it can call release on the object. From our perspective as programmers, it takes care of calling release for us, so we don't have to (and in fact, we shouldn't).
What's important to note is that (again, by convention) all object creation class methods return an autoreleased object. For example, in the following example, the variable "s" has a reference count of 1, but after the event loop completes, it will be destroyed.
NSString* s = [NSString stringWithString:#"Hello World"];
If you want to hang onto that string, you'd need to call retain explicitly, and then explicitly release it when you're done.
Consider the following (very contrived) bit of code, and you'll see a situation where autorelease is required:
- (NSString*)createHelloWorldString
{
NSString* s = [[NSString alloc] initWithString:#"Hello World"];
// Now what? We want to return s, but we've upped its reference count.
// The caller shouldn't be responsible for releasing it, since we're the
// ones that created it. If we call release, however, the reference
// count will hit zero and bad memory will be returned to the caller.
// The answer is to call autorelease before returning the string. By
// explicitly calling autorelease, we pass the responsibility for
// releasing the string on to the thread's NSAutoreleasePool, which will
// happen at some later time. The consequence is that the returned string
// will still be valid for the caller of this function.
return [s autorelease];
}
I realize all of this is a bit confusing - at some point, though, it will click. Here are a few references to get you going:
Apple's introduction to memory management.
Cocoa Programming for Mac OS X (4th Edition), by Aaron Hillegas - a very well written book with lots of great examples. It reads like a tutorial.
If you're truly diving in, you could head to Big Nerd Ranch. This is a training facility run by Aaron Hillegas - the author of the book mentioned above. I attended the Intro to Cocoa course there several years ago, and it was a great way to learn.
If you understand the process of retain/release then there are two golden rules that are "duh" obvious to established Cocoa programmers, but unfortunately are rarely spelled out this clearly for newcomers.
If a function which returns an object has alloc, create or copy in its name then the object is yours. You must call [object release] when you are finished with it. Or CFRelease(object), if it's a Core-Foundation object.
If it does NOT have one of these words in its name then the object belongs to someone else. You must call [object retain] if you wish to keep the object after the end of your function.
You would be well served to also follow this convention in functions you create yourself.
(Nitpickers: Yes, there are unfortunately a few API calls that are exceptions to these rules but they are rare).
If you're writing code for the desktop and you can target Mac OS X 10.5, you should at least look into using Objective-C garbage collection. It really will simplify most of your development — that's why Apple put all the effort into creating it in the first place, and making it perform well.
As for the memory management rules when not using GC:
If you create a new object using +alloc/+allocWithZone:, +new, -copy or -mutableCopy or if you -retain an object, you are taking ownership of it and must ensure it is sent -release.
If you receive an object in any other way, you are not the owner of it and should not ensure it is sent -release.
If you want to make sure an object is sent -release you can either send that yourself, or you can send the object -autorelease and the current autorelease pool will send it -release (once per received -autorelease) when the pool is drained.
Typically -autorelease is used as a way of ensuring that objects live for the length of the current event, but are cleaned up afterwards, as there is an autorelease pool that surrounds Cocoa's event processing. In Cocoa, it is far more common to return objects to a caller that are autoreleased than it is to return objets that the caller itself needs to release.
Objective-C uses Reference Counting, which means each Object has a reference count. When an object is created, it has a reference count of "1". Simply speaking, when an object is referred to (ie, stored somewhere), it gets "retained" which means its reference count is increased by one. When an object is no longer needed, it is "released" which means its reference count is decreased by one.
When an object's reference count is 0, the object is freed. This is basic reference counting.
For some languages, references are automatically increased and decreased, but objective-c is not one of those languages. Thus the programmer is responsible for retaining and releasing.
A typical way to write a method is:
id myVar = [someObject someMessage];
.... do something ....;
[myVar release];
return someValue;
The problem of needing to remember to release any acquired resources inside of code is both tedious and error-prone. Objective-C introduces another concept aimed at making this much easier: Autorelease Pools. Autorelease pools are special objects that are installed on each thread. They are a fairly simple class, if you look up NSAutoreleasePool.
When an object gets an "autorelease" message sent to it, the object will look for any autorelease pools sitting on the stack for this current thread. It will add the object to the list as an object to send a "release" message to at some point in the future, which is generally when the pool itself is released.
Taking the code above, you can rewrite it to be shorter and easier to read by saying:
id myVar = [[someObject someMessage] autorelease];
... do something ...;
return someValue;
Because the object is autoreleased, we no longer need to explicitly call "release" on it. This is because we know some autorelease pool will do it for us later.
Hopefully this helps. The Wikipedia article is pretty good about reference counting. More information about autorelease pools can be found here. Also note that if you are building for Mac OS X 10.5 and later, you can tell Xcode to build with garbage collection enabled, allowing you to completely ignore retain/release/autorelease.
Joshua (#6591) - The Garbage collection stuff in Mac OS X 10.5 seems pretty cool, but isn't available for the iPhone (or if you want your app to run on pre-10.5 versions of Mac OS X).
Also, if you're writing a library or something that might be reused, using the GC mode locks anyone using the code into also using the GC mode, so as I understand it, anyone trying to write widely reusable code tends to go for managing memory manually.
As ever, when people start trying to re-word the reference material they almost invariably get something wrong or provide an incomplete description.
Apple provides a complete description of Cocoa's memory management system in Memory Management Programming Guide for Cocoa, at the end of which there is a brief but accurate summary of the Memory Management Rules.
I'll not add to the specific of retain/release other than you might want to think about dropping $50 and getting the Hillegass book, but I would strongly suggest getting into using the Instruments tools very early in the development of your application (even your first one!). To do so, Run->Start with performance tools. I'd start with Leaks which is just one of many of the instruments available but will help to show you when you've forgot to release. It's quit daunting how much information you'll be presented with. But check out this tutorial to get up and going fast:
COCOA TUTORIAL: FIXING MEMORY LEAKS WITH INSTRUMENTS
Actually trying to force leaks might be a better way of, in turn, learning how to prevent them! Good luck ;)
Matt Dillard wrote:
return [[s autorelease] release];
Autorelease does not retain the object. Autorelease simply puts it in queue to be released later. You do not want to have a release statement there.
My usual collection of Cocoa memory management articles:
cocoa memory management
There's a free screencast available from the iDeveloperTV Network
Memory Management in Objective-C
NilObject's answer is a good start. Here's some supplemental info pertaining to manual memory management (required on the iPhone).
If you personally alloc/init an object, it comes with a reference count of 1. You are responsible for cleaning up after it when it's no longer needed, either by calling [foo release] or [foo autorelease]. release cleans it up right away, whereas autorelease adds the object to the autorelease pool, which will automatically release it at a later time.
autorelease is primarily for when you have a method that needs to return the object in question (so you can't manually release it, else you'll be returning a nil object) but you don't want to hold on to it, either.
If you acquire an object where you did not call alloc/init to get it -- for example:
foo = [NSString stringWithString:#"hello"];
but you want to hang on to this object, you need to call [foo retain]. Otherwise, it's possible it will get autoreleased and you'll be holding on to a nil reference (as it would in the above stringWithString example). When you no longer need it, call [foo release].
The answers above give clear restatements of what the documentation says; the problem most new people run into is the undocumented cases. For example:
Autorelease: docs say it will trigger a release "at some point in the future." WHEN?! Basically, you can count on the object being around until you exit your code back into the system event loop. The system MAY release the object any time after the current event cycle. (I think Matt said that, earlier.)
Static strings: NSString *foo = #"bar"; -- do you have to retain or release that? No. How about
-(void)getBar {
return #"bar";
}
...
NSString *foo = [self getBar]; // still no need to retain or release
The Creation Rule: If you created it, you own it, and are expected to release it.
In general, the way new Cocoa programmers get messed up is by not understanding which routines return an object with a retainCount > 0.
Here is a snippet from Very Simple Rules For Memory Management In Cocoa:
Retention Count rules
Within a given block, the use of -copy, -alloc and -retain should equal the use of -release and -autorelease.
Objects created using convenience constructors (e.g. NSString's stringWithString) are considered autoreleased.
Implement a -dealloc method to release the instancevariables you own
The 1st bullet says: if you called alloc (or new fooCopy), you need to call release on that object.
The 2nd bullet says: if you use a convenience constructor and you need the object to hang around (as with an image to be drawn later), you need to retain (and then later release) it.
The 3rd should be self-explanatory.
Lots of good information on cocoadev too:
MemoryManagement
RulesOfThumb
As several people mentioned already, Apple's Intro to Memory Management is by far the best place to start.
One useful link I haven't seen mentioned yet is Practical Memory Management. You'll find it in the middle of Apple's docs if you read through them, but it's worth direct linking. It's a brilliant executive summary of the memory management rules with examples and common mistakes (basically what other answers here are trying to explain, but not as well).
I'm writing a C++ application using Metal API (objective C) and MRC (Manual Reference Counting). I have a very limited experience with ObjC. Most of the Metal API objects are defined as protocols and created as an object returned from C-function or other object's method (ex. MTLDevice newBufferWithLength). To know how to release objects created this way I need to know if they have been set autoreleased or not (I can't call release an autoreleased object with retain count 1 for instance). The problem is I can't find any description in Metal API documentation which would answer this question. I've only read in user guide that all so called lightweight objects are created autoreleased. There are three examples of autoreleased objects but not sure if I can just assume that rest of the objects are not autoreleased. In cocoa many objects also may be created without alloc+init, being returned from a static method (ex NSString string) so the problem seems not to be only Metal related.
Thank you for your help.
The usual Objective-C rule is that creating scope is also responsible to release object. So, in virtually all cases, except well-documented exceptions, returned object is autoreleased (both returned through return value or out-arguments). More correct way to see it is that object is always returned with +0 scope-local retain count, so you are expected to retain it if needed. Reading the manual it seems that Metal API is one of Apple's frameworks, so it should follow this rule unless warned with bold statements. C functions in Objective-C are also expected to behave that way. (There is no difference between a method and a function in terms of resource management.)
For that "alloc-init vs. [NSString string]" part: MRC code either returns [NSString string], which is already +0, or [[[NSString alloc] init] autorelease]. Otherwise it breaks the convention. Exceptions are -[init] and +[new...] methods itself that return +1. Under ARC there is no difference between alloc-init/string, because ARC knows the convention and does the right thing, optimizing excessive retains/releases where possible.
Also note that -[retainCount] is meaningless and "considered harmful", because you never know how many retain-autorelease calls were performed and what objc-runtime accounting is in effect even with your own objects.
ARC is really a great option unless you're writing some sort of Objective-C -to- Whatever bridge, where retain counts and/or cycles must be managed explicitly due to lack of context. It doesn't take anything from you, giving in most cases a big advantage of not managing resources at all.
I am a experienced developer that is new to iOS developement (on iPhone in particular). A project I am working on is using Automatic Reference Counting (ARC).
I was doing some research on ARC and came across this document: http://clang.llvm.org/docs/AutomaticReferenceCounting.html and in a section titled: "Retained return values" it states that an attribute can be used to indicate that the caller expects to take ownership of a +1 retain count. It also goes on to state that ARC will automatically add the attribute to methods if it detects it is in particular method families (it specifically names: alloc, copy, init, mutableCopy, and new)
From further reading it seems that method families are simply method naming conventions. My understanding is that if the method name starts with a method family then it is in that method family. So for example
+(id) init
+(id) initWithName:(NSString*)name
are both part of the init method family.
My question is: Is there a formal list of defined Method Families for iOS development and if so, what are they / where might I find it?
below is the section llvm.org I mentioned above:
Section 3.2.2: Retained return values states:
A function or method which returns a retainable object pointer type
may be marked as returning a retained value, signifying that the
caller expects to take ownership of a +1 retain count. This is done by
adding the ns_returns_retained attribute to the function or method
declaration, like so:
id foo(void) __attribute((ns_returns_retained));
- (id) foo __attribute((ns_returns_retained)); This attribute is part of the type of the function or method.
When returning from such a function or method, ARC retains the value
at the point of evaluation of the return statement, before leaving all
local scopes.
When receiving a return result from such a function or method, ARC
releases the value at the end of the full-expression it is contained
within, subject to the usual optimizations for local values.
Rationale: this formalizes direct transfers of ownership from a callee
to a caller. The most common scenario this models is the retained
return from init, alloc, new, and copy methods, but there are other
cases in the frameworks. After optimization there are typically no
extra retains and releases required.
Methods in the alloc, copy, init, mutableCopy, and new families are
implicitly marked attribute((ns_returns_retained)). This may be
suppressed by explicitly marking the method
attribute((ns_returns_not_retained)).
It is undefined behavior if the method to which an Objective-C message
send statically resolves has different retain semantics on its result
from the method it dynamically resolves to. It is undefined behavior
if a block or function call is made through a static type with
different retain semantics on its result from the implementation of
the called block or function.
Rationale: Mismatches with returned results will cause over-retains or
over-releases, depending on the direction. Again, the rule about
function calls is really just an application of the existing C/C++
rule about calling functions through an incompatible function type.
Section 5 of the same document is titled "Method Families". It lists all the currently defined families, which are the ones you identified above; alloc, new, copy, mutableCopy, and init. It also lists the criteria for being part of that family. Mostly, it's just based on the method name, but it also depends on the return type. For example, a member of the copy family must return an object; a copy method that has a void return type will not be considered part of the copy method family.
I don't know that I've seen a formal list of method families, but your analysis is basically correct. I believe the only method naming conventions that have compiler-enforced semantics are those mentioned in your post, namely alloc, init, copy (and mutableCopy), and new. Even before/without ARC, the clang static analyzer included in Xcode uses those naming conventions to find memory management problems, and they're also the basis of the longstanding memory management rules for Objective-C programmers prior to ARC.
Namely, if you create an object using a method that begins with init, copy, mutableCopy or new, you "own" that object and are responsible for releasing it later. Objects created or returned by methods with any other name, are not owned by you, the caller, and if you want to keep a reference to them, you must retain them. Every retain you make must be balanced by a later release, when you're done with the object reference. Finally, calling autorelease on an object puts it in the innermost autorelease pool. The pool is responsible for releasing the object later, where later is sometime after the current scope. So, release and autorelease are both valid ways of relinquishing your retain on an object.
Now, the rules I've set forth shouldn't (and can't) be followed by you, the programmer, under ARC. However, ARC is essentially a system whereby the compiler with help from the static analyzer knows these rules and inserts the necessary memory management calls for you at compile time. So, if you're looking to develop an understanding of what ARC is doing behind-the-scenes, understanding these rules is a good start.
This turned out a bit longwinded, but the point is that the methods you've named are the only ones I know of that have this enforced naming convention in Objective-C, and it stems from the memory management rules set forth above. There are other naming conventions in Objective-C, to be sure, but they're standard style conventions, not something that's enforced anywhere.
Look at official Apple Developer documentation about Memory Management Rules. Seems, your list is complete: “alloc”, “new”, “copy”, or “mutableCopy” (only 4 keywords).
I would include the convenience constructors provided by some Cocoa classes:
- +stringWith* // Various ways to make NSString objects
- +arrayWith*, +dictionaryWith*, etc. // Make collections
- +dateWith* // Make NSDate objects
- etc. // Many, many others
They all return a new, autoreleased instance of the class they are sent to - except for some special cases, see for instance #Peter Hosey's comment.
I don't know if there are any formal rules, but the pattern is that the constructor name is composed of the class name without the NS part (and without the Mutable part, too, for mutable classes) followed by With<Parameter Type>: and one or more parameters. The name begins with a lower case letter (except the usual suspects, like URLWithString:.)
For instance, the following line provides an autoreleased NSArray instance with a single element:
[NSArray arrayWithObject: #"Test"]
I don't know if there are any special rules in the compiler to recognize this type of constructor.
I am trying to lower the footprint of memory my application uses at runtime....
In the standard usage to get a font based on system font size the api is used as font = [NSFont systemFontOfSize]
+ (NSFont *)systemFontOfSize:(CGFloat)fontSize; // Aqua System font
As per my understanding that the font will be freed by the autorelease pool, First I need to confirm that this understating is correct?
If this is correct, then if I have a drawWithFrame function where I am using this every time I draw a cell. Should I not release it there? and would increasing the memory footprint of the application, since the fonts would be freed when the applications autorelease pool is called?
Now is again this understanding is correct? There are 2 approaches
Find a way to release NSFont
Put a NSAutoReleasePool *pool. alloc / release there itself in draw with frame
What do you guys suggest?
The function name systemFontOfSize: doesn't begin with any of the magic keywords "alloc", "new", "copy", or "mutableCopy", so you aren't responsible for releasing what it returns. See the Cocoa basic memory management rules for details.
I'm not use to dealing with Font so I might be out of the way.
But to me if you frequently need and object, I would cache it in a property.
Creating and deletion is more expensive than reading a property.
And if this is a key feature of your class, I would event create it in the init of my class.
That would save the unnecessary test to see if it already exist.
You are correct on both items. By convention, all named initializers return autoreleased objects. Since you (scope of drawWithFrame) did not excplicitly allocate font you are not responsible for releasing it. Like you also suggested, this can be optimizated by putting an autorelease pool around it.
Wether or not the benefit is worth the extra typing work of the coder is debatable.
As with most non-"init" methods that return an object ("copy" is an exception that pops to mind) the object returned by systemFontOfSize is autoreleased and so it will be released at some point in the (very near) future.
If you want to use the object pointed to by your variable "font" outside of the local scope you should retain it and release it when it's no longer needed (or before being replaced by another font)
Edit: I just reread realized you're creating the font every time so the 2nd paragraph isn't so relevant to your question, but anyway, unless the font is something that has to change many times a second I'd consider creating the font object outside the drawrect method. I don't know how much of a practical effect this specifically would have on your current FPS/responsiveness but it's generally good to get used to not doing stuff such as object creation redundantly multiple times in loops etc.
I'm creating a library that will be used by multiple types of iOS apps. Part of my API allows a user to specify routines that will be used for the library's allocations. My library is implemented mostly in C++, so this has been straightforward so far.
However, I've recently been adding some user interface functionality to the library: displaying a UIWebView using a custom view controller. I'm not sure how to ensure that my allocators are used for these objects.
How can I ensure that all of the Cocoa UI objects created by my library are allocated with my own functions?
I've tried a few things including overriding -initWithZone and calling CFAllocatorSetDefault before my -init. None of them have worked yet; and honestly I'm still a beginner with Objective C and Cocoa, so I'd like to know what the "correct" way to do this is.
I'm unable to find evidence of it now, but it certainly was the case that CFAllocator, malloc_zone_t and NSZone were all toll-free bridged. So you could just cast your allocator to an NSZone and pass it along.
I think the problem you're going to face is that NSZone was added at NextStep so as to allow a program to maintain multiple heaps, with the feeling being that it would allow programmers to keep related objects close to one another in memory — which is good for caching — and in some cases to throw away entire object graphs without walking the graph, which is obviously fast. However the former was of little benefit in practice and the latter is more likely to create problems than to be of actual benefit. So Apple has back-peddled from NSZones, gradually turning the related runtime calls into no-ops and removing detailed documentation. Apple's feeling is that, at the Objective-C level, you should not only maintain only a single heap (which is a side issue from your point of view) but that they'll always know best how to maintain it.
EDIT: an alternative idea is to replace NSObject's alloc, that being the thing that creates memory. The Objective-C runtime is well-defined enough that we know exactly what behaviour alloc exhibits, so that a vanilla version might be:
+ (id)alloc
{
Class *newInstance;
// we'll use calloc to ensure we get correct initial behaviour of
// everything equal to 0, and use the runtime's class_getInstanceSize
// so that we're allocating the correct amount of memory irrespective
// of whether this call has fallen through from a subclass
newInstance = (Class *)calloc(1, class_getInstanceSize(self));
// the thing that defines a class is that the first thing stored in
// it is the isa pointer, which points to the metaclass. So we'll
// set the isa pointer appropriately
*newInstance = self;
return (id)newInstance;
}
To replace NSObject's normal init, you'd probably want to define your alternative as a category method on NSObject named, say, customInit, then use class_getClassMethod, class_getMethodImplementation and method_setImplementation directly on [NSObject class] to switch it into place. See the Object-C Runtime Reference for documentation on those.