As everyone knows, setTitle automatically retains the string passed as a parameter. When the button caption needs to be changed, I guess that it is necesary to release the current (old) string before setting the new one. I wonder what is the most elegant way to dot it.
See my code sample (here, getPlayerHandFromGame method produces autoreleased strings which are retained when setTitle is invoked):
colourString = [pGame getPlayerHandFromGame:1 withColour:COLOUR_HEARTS];
// Split colourString into array of strings if not null.
if ([colourString length] != 0) {
listCards = [colourString componentsSeparatedByString:#" "];
for (cardCounterSameColour = 1; cardCounterSameColour <= [listCards count]; cardCounterSameColour ++) {
currentCardButton = [self buttonCardNumber:cardCounter];
// Objects are numbered from 0 in the array
[currentCardButton setTitle:[listCards objectAtIndex:cardCounterSameColour-1] forState:UIControlStateNormal];
cardCounter ++;
}
}
This portion of code will be called several times since the button caption will be updated several times. I guess that before setting the title, I should do something like this:
[currentCardButton titleForState:UIControlStateNormal release]
in order to release the string which will not be used anymore (titleForState returns a pointer to the NSString).
Is that the right way to avoid that the device memory gets loaded with unused strings ?
Many thanks,
Apple92
Any class that retains a value set on one of its properties is also responsible for releasing the old value when that property's value gets changed again. Don't worry about it.
As everyone knows, setTitle automatically retains the string passed as a parameter.
Really? I don't know it. In fact, I'll bet a pint of beer that it doesn't retain the string, but copies it.
Granted, for an NSString, -copy is probably implemented as doing a retain and returning self, but if you pass it an NSMutableString, a genuine copy will occur.
I guess that it is necesary to release the current (old) string before setting the new one.
Guess again, sucker!
Or less facetiously: any object is responsible for managing the ownership of other objects it wantsa to keep hold of. Once you have passed the title in setTitle: you do not need to worry about how the object disposes of it once it gets a new one.
Consider the code:
[currentCardButton titleForState:UIControlStateNormal]
and apply the memory management rules to the return result.
Did you obtain it with alloc, new or copy? No. Did you retain it? No (remember we are talking about the object passed back by the method,not the object you originally gave it). Therefore, you must not release it.
Take a look at the object ownership convention:
http://developer.apple.com/mac/library/documentation/Cocoa/Conceptual/MemoryMgmt/Articles/mmObjectOwnership.html#//apple_ref/doc/uid/20000043-BEHDEDDB
Every class should follow these rules (Apple's does it), so in order to them you dont have to worry because you haven't done a alloc/retain in your class, and the button will retain it for it's internal use.
Related
What if I have an object, say NSIndexPath in which before I do a copy I always release it first?
Is it possible to have a memory count under 0?
I am doing this to prevent memory leaks.. is this a good way?
//global already has some value before in here or it doesn't have a value.. I want to update
//this with a new value (I no longer care to the old pointer)
[global release]
global = [indexPath copy];
Don't. When the retain count reaches 0, your object will be dealloc'ed and its pointer will become invalid, so using it again will cause unpredictable results (namely crashing).
You should read through Apple's Memory Management Guide.
This is the fundamental rule:
You only release or autorelease objects you own. You take ownership of an object if you create it using a method whose name begins with “alloc”, “new”, “copy”, or “mutableCopy” (for example, alloc, newObject, or mutableCopy), or if you send it a retain message.
You use release or autorelease to relinquish ownership of an object. autorelease just means “send a release message in the future” (specifically: when the used autorelease pool receives a drain message).
Update:
As Josh pointed out, the one case you need to consider is when global and indexPath are the same. In that case, you would still "need" the pointer (to perform the copy), so you either autorelease (instead of releasing) or you use a temporary variable to handle that.
What you are doing is essentially correct, so long as global either has an old value that you no longer need or is nil. If it is one of a class's ivars, it will be nil when an instance of the class is created. The one catch is that if the new indexPath happens to be the same object as the one that's already in global, you will have over-released and you will crash.
// global points to object at 0x8BADFOOD
// indexPath also happens to be 0x8BADFOOD; for some reason, it
// hasn't changed since last time. This _can_ happen.
[global release]; // object at 0x8BADFOOD is deallocated
global = [indexPath copy]; // indexPath no longer valid!
// Crash! Bang! Boom!
The way to avoid this is to use a temp variable.
When you declare a property as copy and synthesize that property, the setter method that is created looks essentially like this, and you can do the same thing:
- (void)setMyFloozit:(Floozit *)newFloozit {
// Copy first in case newFloozit and myFloozit are for
// some reason the same object
// Take ownership of a copy of newFloozit
Floozit * tmp = [newFloozit copy];
// We no longer need old value of myFloozit, so we release it.
[myFloozit release];
// tmp contains a value that we own, we just need to assign
// it to the correct name.
myFloozit = tmp;
}
This could be made slightly better by checking first to see whether newFloozit and myFloozit are the same, and doing nothing if they are.
When a method returns an object that is taken from and NSMutableArray does the object must be autoreleased? Check the following method. Also should I autorelease the nil value?
-(NSObject*)getElementByID:(NSString*)ID{
for(int i=0;i<[elements count];i++){
NSObject *element = (NSObject*) [elements objectAtIndex:i];
if([element.key isEqualToString:ID]){
return [element autorelease];
}
}
return nil;
}
You must not autorelease element because you are not an owner of it (you have not put a retain on it). You would have become an owner of it if you acquired it using alloc, new or retain. Since you acquired this object calling objectAtIndex:, you do not own it. See Three Magic Words. Calling autorelease here will cause a crash later do to over-release.
Your method name is incorrect and breaks KVC. A method that begins with get must take a pointer that will be updated with the result. This should be elementForID:. As noted above with the three magic words, naming in ObjC is very important to writing stable code
As a side note, it is traditional to use id is most cases rather than NSObject*. They mean slightly different things, but typically id is correct.
You never need to do any memory management related things to nil. So, no, you should not send autorelease to nil.
You also should not need to send autorelease to the element object that you are returning from your elements array. That object you are returning will remain in memory by virtue of elements itself having retained it. If the calling method would like to retain the value that you return, it may. But if that calling method only uses the returned value within its own scope, it is safe for it to do so without retaining it.
I have a question about a retain and a NSString, if I have a method who a return a NSString, and I put the return NSString in a nsstring variable, I must do a retain or not?
NSString *myString = #"";
myString = [self methodWhoReturnString]; // I must do this?
myString = [[self methodWhoReturnString]retain]; // Or I must do this?
The Apple Developer Documentation on Memory Management explains the scenarios where you retain/release objects.
Simply put, if you want the string to stick around, you need to retain it until you're finished with it. If that is just the scope of the current function, you can get away without retaining it as if the string is already autorelease'd (likely) it won't get released until your function finishes and the current AutoReleasePool is purged.
Bear in mind that an NSString * could actually be pointing to an NSMutableString *. If it matters to you if the string is changed by some other function without you realizing, be sure to copy it: NSString * myCopyOfString = [mystring copy];
If the string is set to autorelease, which it most likely is, then yes you will need to retain it somehow. I would suggest doing this though:
myString = [[self methodWhoReturnString] copy];
this ensures you have retained the data in the string not just a reference to a string that might still be controlled elsewhere. Be sure you release your copy later!
Usually, methodWhoReturnString would return an autoreleased string, which means you should retain it if you want to keep it around.
So, if methodWhoReturnString is your method, I believe that to keep with convention you should return [stringToReturn autorelease]; from that method, and then retain it if you want to keep it.
You use retain if you're going to be using myString at a later point in time (i.e. after the current method has returned) to prevent it being autoreleased.
You don't need to use retain if it's just a temporary variable used within the current method, since in that case you do want it to be autoreleased.
One special case is properties. If you call self.blah = foo, you don't need to retain foo, since the setBlah: method should do it for you.
(there's a whole load of other complexities and edge cases, but this is a good rule of thumb to get you started on understanding what to do)
Given the code you provided, you shouldn't call -retain. In your example, the return value of a method that returns an instance of NSString is assigned to myString, an automatic local variable. If the assignment had been made to an instance variable or a static variable, you would want to call either retain or copy to guarantee that the reference remains valid beyond the end of the local scope.
In this case though, the reference to the NSString instance is stored in a variable that will be destroyed automatically at the end of the local scope, so your code needn't concern itself with the object's lifetime.
Any method that has alloc, new or copy in it automatically retains and infers that you have ownership of the object. All others shouldn't. It would be helpful if you had more context though. If we are in a contained method where this string is used briefly, then you probably don't need to retain. If it is going to be used for a while, you might want to use the #synthesize syntax to make it a property of the class you are in. When you use #property and #synthesize and call something like self.myProperty = something it will automatically retain.
1) What is the reason for the use of retain?
For example, in a setter method:
- (void) setCount: (int) input {
[intCount autorelease];
intCount = [input retain];
}
2) the autorelease-Method: Is it deleting an old object or preparing the new one?
3) Why the retain-method is called at the input-object?
Would
intCount = input;
be wrong?
And why?
Retain is used to increment the retainCount of an object. NSObjects have a property called retainCount which maintains a count on the number of references that are currently held on an object. When the retainCount of an object reaches 0, the object can be released from memory. Effectively this prevents an object from being released from memory if it's still in use elsewhere.
The autorelease method does not delete an old object and does not prepare the new object. It is effectively a pre-emptive call to release the object (autorelease is much more complicated than that and you should read up on it in the Memory Management Guide.)
In your case intCount = input wouldn't be wrong because you're working with a primative. However if input were an object then you'd need to be calling retain on it. In fact you don't even need to be writing your own getters/setters for primatives (or objects), but instead should be using Declared Properties. In fact you're almost always better off using Declared Properties and if you do want to roll your own, get to know the pitfalls of doing so first.
I recommend you read this. http://developer.apple.com/library/mac/#documentation/Cocoa/Conceptual/MemoryMgmt/MemoryMgmt.html
The answers to your questions have been answered fairly well so let me just add that if you can use garbage collection you should. It makes everything so much easier. It isn't a panacea and you still should learn to use the retain/release mechanism but unless you are dealing in some high volume memory calls, creating and deleting lots of objects, then just use garbage collection.
It can be found under Project | Edit Project Settings | Build
Then just search for "garbage" and you'll see it.
If you are doing iOS development and cannot use garbage collection I apologize for giving unhelpful information but it still stands for non-iOS development.
To answer ur question specifically:
1). the use of retain is to declare the ownership of an object. In this case, intCount retains the ownership of input, in case the input got released somewhere else, u can still use the intCount.
2). the autorelease of intCount is to relinquish the ownership of the old value. That avoid the memory leak of the old value. If u don't release the old value, and u assign a new value to this pointer, the old object will always be there and never got released, which will cause the memory leak.
3). if u don't retain the input, and the parameter of input got released somewhere else. then if nowhere else retain this object, it will get freed. So u can't use intCount as well. That's why u need to retain it or copy it.
But i think if u do intCount = input; it should be fine. Because int is not an object, it's just a type. So I think the whole method is okay to be written like this:
- (void) setCount: (int) input {
intCount = input;
}
But if its a pointer, u should not assign the new value to the old one directly.
In the code below, will tapsLable.text be deallocated when tapsMessage is released or does the assignment operator somehow increment the retain count so that tapsLabel.text continues to be available?
NSString *tapsMessage = [[NSString alloc] initWithFormat:#"%d taps detected", numTaps];
tapsLabel.text = tapsMessage; // tapsLabel is a UILabel object
[tapsMessage release];
Here's a tip
You can write the retainCounter for the object then you see what it is before and after the assignment.
e.g. NSLog( #"%d", [tapsMessage retainCount] );
That way you can answer such questions in the future by just writing out the retainCount as it always depends on how the property is declared.
tabsLabel.text is a property on tapsLabel. I think it's a string property that does [target copy] on assignment. Nevermind the details, yes, the assignment operator either increments the retain count or copies the value, so you can release tapsMessage and it is still available in tapsLabel.text.
Read more about properties here.
EDIT: looked up UILabel in the header, yes, it does a copy for the text property.
#property(nonatomic,copy) NSString *text; // default is nil
EDIT: to expand on the important question in the comment
How does anyone know when to release and when not to if you have to look at the implementation details of every object you assign something to?
You just follow the memory management rules. The point of the refcounted environment is exactly that there is some "loose coupling" going on between the objects in terms of memory management. As long as you retain and release properly, it is not your concern whether someone else also retains and releases these same objects, as long as all involved parties have their retains/releases matched.
In the first line, you have allocated and initialised an NSString. You own this object according to the memory management rules, which means you are responsible for releasing it.
In the second line, you are assigning the tapsMessage string the text property. This property (assuming tapsLabel is a UILabel) is declared with the copy attribute. For immutable strings (NSStrings), asking for a copy simply increments the retain count, since there is no need to make an actual duplicate of the data (the data can never change). Since UILabel has made a copy of the string, it has claimed ownership as well (objects can have more than one owner).
In the third line, you relinquish your ownership but the string still has one owner (the label), so the object is not deallocated.
It will not be deallocated.