I need few clarification in iPhone memory management.
Here is an example for setters;
1).
-(void)setValue:(NSString*)input{
[value autorelease];
value = [input retain];
}
In this example, why we have to use autorelease?
Can we use like as follows?
if(value)
[value release];
value = [input retain];
In the first example, Why we should not release the memory for input
2).
If I use following statement; what is the retain count for value
NSString *value;
value = #"welcome";
After the above statement, just I am trying to set one more value. Then what will happen?
eg:
value = #"For testing";
3).
What is the different between 2) and 3)?
NSString *value;
value = [input1 retain];
...
...
value = [input2 retain];// Now what is the retain count for value
4).
If I use following statement, why the app is getting crash?
NSString *value = [[[NSString alloc] init] autorelease];
...
...
Thanks in advance..
If "input" is the exact same object as "value" then calling [value release] could dealloc the object. So you must retain the new input value, release the old value, then assign the new value to the ivar:
[input retain];
[value release];
value = input;
After each of 2) and 3), the NSString *value points to a literal NSString object, the retain count will be 1 in each case, and releasing it is probably not a good idea
After this code:
value = [input2 retain];
value is an alias to the input2 object. The thing to realize is that objects have retain counts, variables do not.
As for your last case,
NSString *value = [[[NSString alloc] init] autorelease];
It creates an autoreleased empty string. If you reference that object again once the autorelease actually happens, you may get a crash because you'll be referring to an object that doesn't exist any more.
If you release a value before you retain the new value then you can have problems if the same value is being set twice. This happens if the caller hasn't retained their own copy, such as when they get the value from the same object they try to set it on, like this:
object.value = object.value;
That statement will cause the object to be released before it's retained again which could lead to the memory being deallocated and result in a dangling pointer being retained. By doing the autorelease it ensures that copying the same pointer onto itself will work correctly.
I usually write my setters as
- (void)setValue:(NString *)input {
if (value != input) {
[value release];
value = [input retain];
}
}
which avoids the problem of input and value both being the same object. If you just release it without checking then you might completely free it and the next line will try to retain an object that doesn't exist anymore.
However, it's best practice to copy strings instead of retaining them :)
(assuming you're working in a non-gc env)
1) you can create a temporary variable and release the temp after the retain/assign. otherwise, you'd need to compare the pointers. deferred release may also mask threading errors (whether you consider that good or bad is...)
2) technically, the NSString literals are valid for the life of your program. that is: while (1) [#"why won't i die?" release]; yields an infinite loop.
3) with explicit retain, alloc+init, new, and copy, you must counterbalance the retain count using release or autorelease. since you did not release value1, the static analysis may (correctly) spot that as a leak. since the string constants never die, the two aren't comparable in this regard.
4) there's nothing wrong with that staement. the problem lies elsewhere in your program. either you are assigning it to an ivar without retaining, or releasing it later.
try using static analysis often, and try reducing how much you use autorelease (you can't avoid it entirely).
none of this is magic, but you can at least reduce the location of many problems to the callsites (or very close) by not using autorelease all over the place. just use manual retain/release where possible.
lastly, check for leaks and run with NSZombies enabled.
1)
You will have to do autorelease because of the following:
When input is the same object as value and you will release value it's retain count will reach zero and get deallocated before you can retain it again though the input.
You can do it with retain but you have to change your code:
-(void)setValue:(NSString*)input{
if (value != input) {
[value autorelease];
value = [input retain];
}
}
2)
I believe #"Text" will be treated as a constant. When you want a object which you do not want any memory management with use:
NSString *value = [NSString stringWithString:#"Text"];
This will return an autoreleased object.
3) In this example it is not about the retain count of value, but about the retain count of both objects where value one is referenced to.
When you dont release input1 before you leave that method, you will have a memory management problem.
4) This statement should work. No points to argue. You rather use [NSString string].
Note:
For memory management: when you use alloc new or copy, you also have to use release or autorelease on the same object in the same scope.
Related
I decided I need to go back over some basics recently, mostly to do with memory management and I'm beginning to doubt.
The reason I ask is because I'm a bit muddled with how to memory safe process items in array using temporary variables.
Could somebody wise in the ways of arc please tell me if this simple code will leak memory?
self.array=[NSMutableArray new];
// Retain +1
Test *obj0 = [[Test alloc] init];
// Retain +1
[self.array addObject:obj0];
Test *obj1 = nil;
//Retain +1
obj1=self.array[0];// does need to be __weak even though it has no owner?
[self.array removeAllObjects];
// is not null
NSLog(#"A: %#", obj1);
When you add obj0 to the array, its retain count will be incremented, see e.g. here. It is then +2.
When you assign it to obj1, it still has an retain count of +2.
When you remove all objects from the array, their retain count will be decremented, so obj0 has again anretain count of +1.
It thus won't leak.
This is my test code:
for (int i = 0; i < 1000000000; ++i) {
NSString *string = #"Abc";
string = [string lowercaseString];
string = [string stringByAppendingString:#"xyz"];
}
In ARC environment,the loop will not make the memory explode.In my case,it just cost 1.2MB RAM to run this loop.
But in MRC,the loop will make the memory explode unless use a #autoreleasepool code block.What makes me confused is there are many articles say that it is needed to put the codes in a #autoreleasepool when the codes in a for loop. but in this case,it doesn't matter without the #autoreleasepool.Please help me with this.thx.
update:
if I write the code like this:
for (int i = 0; i < 1000000000; ++i) {
NSString *string = [NSString stringWithFormat:#"aaaaaaaaaaaaaaaaaaa"];
}
the code will make the memory explode both in ARC and MRC. why？
stringWithFormat:
also return a autorelease object. I am puzzled by this...
Code compiled with ARC has to interoperate with code that wasn't compiled with ARC. Furthermore, you can't assume that Foundation and, in particular, the methods of NSString that you're calling, were compiled with ARC.
Those NSString methods have to be compiled in such a way that they can be called from both ARC and non-ARC code. That means that they must autorelease the objects they return, whether or not they themselves were compiled with ARC.
However, if those NSString methods are compiled with ARC, then they may use objc_autoreleaseReturnValue() to do the autoreleasing. If the caller was also compiled with ARC and it retains the object (because, for example, it is assigned to a strong local variable), then it will likely use objc_retainAutoreleasedReturnValue() on it. In that case, the use of the autorelease pool can be avoided. objc_autoreleaseReturnValue() can detect that the caller will use objc_retainAutoreleasedReturnValue() on the returned value, by examining the stack and the caller's instructions, and it won't autorelease the value and it will communicate that fact through a side channel to objc_retainAutoreleasedReturnValue() so that it doesn't retain the value.
So, in certain specific circumstances that you can't rely on determining yourself, code that would "normally" autorelease and then retain an object won't. It will simply transfer ownership.
ARC does not automatically drain the autorelease pool or introduce inner autorelease pools.
Because of the uncertainties, you should always use #autoreleasepool around any code that has a chance of spiking memory due to autoreleased objects. For example:
for (int i = 0; i < 1000000000; ++i) #autoreleasepool {
NSString *string = #"Abc";
string = [string lowercaseString];
string = [string stringByAppendingString:#"xyz"];
}
Yes, with ARC memory management is automated. But it is not always doing it the right way. Especially in a loop where many temp objects are created.
Temporary objects are objects that are created and used in the current iteration only. Like small strings that are appended to a bigger string.
These temp objects are freed after the loop completed, which might be too late. Thats why you have to put the body of your loop in an #autoreleasepool, to ensure the objects are freed immediately after each iteration of your loop.
for (int i = 0; i < 1000000000; ++i) {
#autoreleasepool {
NSString *string = [NSString stringWithFormat:#"aaaaaaaaaaaaaaaaaaa"];
}
}
Further reading: https://developer.apple.com/library/ios/documentation/Cocoa/Conceptual/MemoryMgmt/Articles/mmAutoreleasePools.html
With ARC, the compiler is getting smarter.
Before ARC, methods like lowercaseString would create a retained/autoreleased object that is put into an autorelease pool. With ARC, the compiler is smart: lowercaseString checks that the method calling it immediately retains the result. In that case you would have a sequence retain / autorelease / retain, and in that case lowercaseString won't do it's retain / autorelease at all, so only the retain by the caller will be performed. That's faster (it saves a retain, and an autorelease, and the final release when the autorelease pool is closed), and the autorelease pool doesn't grow excessively.
That said, code that only works because the compiler performs an optimisation, is broken.
I am doing some research on reference count increase. Please help on finding it.
Below is sample code and research i'm doing what would happen of reference counting for each line below.
.h file
NSArray *tempArray;
#property (nonatomic, retain) NSArray *tempArray;
.m file
#synthesize tempArray;
-(void) sampleFunction
{
NSArray *myArray = [[NSArray alloc] init]; // Thinking reference count increases to "1"
tempArray = myArray;// reference count increases and tempArray gets retain count "1" now.
tempArray = myArray;// reference count increases and tempArray gets retain count "2" now.
tempArray = [NSArray arrayWithObject:#"SomeString"]; // retain count = ?
}
I know this code may not be for functioning, but this is for only researching about what will happen on reference counting for such scenarios. I tried printing retainCount, but it doesn't show the correct result. Please advise me how does the reference count works on this each line?
In lines 2, 3 and 4 you are affecting the instance variable tempArray to the same object as myArray. But if you write it this way, you try to affect an instance variable. As a matter of fact, if you didn't write any #synthesize tempArray or #synthesize tempArray = tempArray in your code, by default the instance variable generated automatically to store the property value is the same name as the property name, but prefixed with an underscore. So as the property name is tempArray, the instance variable is named _tempArray. The instance variable tempArray itself does not exist and your line of code is invalid.
So if we suppose you wrote instead:
-(void) sampleFunction
{
NSArray *myArray = [[NSArray alloc] init]; // (1)
self.tempArray = myArray; // (2)
self.tempArray = myArray; // (3)
self.tempArray = [NSArray arrayWithObject:#"SomeString"]; // (4)
}
In (1) you are creating a brand new instance of NSArray. "alloc" always initialize new instance with a reference count of 1
In (2) you write self.tempArray = myArray (which is equivalent to [self setTempArray:myArray]; and thus call the property setter), so you set the property to point to the same array you created in (1). This array is thus retained by the property, and its retainCount increses by one, because it is retained by myArray and by the self.tempArray property.
In (3) you affect the property to the very same object as before. This the ref count does not change at all. You could understand that as if you replaced the value of the self.tempArray with another value, so the setter of the property release the old value (decrementing its ref count), then retain the new value (thus incrementing its ref count). As in your case the old and new values are the same object, you would decrement the ref count of your array then re-increment it again. In practice, the ref count does not even change at all (instead of decrementing+incrementing again) to avoid any potential dealloc of the object, because the default implementation of a property setter is as follow:
-(void)setTempArray:(NSArray*)newValue
{
// check if old and new value are different. Only do sthg if they are different
if (newValue != _tempArray)
{
[_tempArray release]; // release old value
[newValue retain]; // retain new value
_tempArray = newValue; // store new value in the backing variable associated with the property
}
}
In (4) you replace again the value of the property tempArray, but this time with a completely new object. So the property will release its old value and retain the new one. Thus the first array you created in (1) which had a refcount of 2 (retained by myArray and by self.tempArray) decrease its ref count to 1 (because the property won't retain it anymore), and the new instance you created [NSArray arrayWithObject:#"SomeString"] is retained by the property, so its ref count is +1.
If you replaced self.tempArray = ... (so the use of the property) with the direct use of the instance variable, using instance variables don't retain the objects they are affected to (except if you are using ARC but it seems you don't), so the ref count of the object wouldn't have changed at all in (2), (3) and (4).
Actually, retain count increase only in new, alloc, retain and copy condition but if we are providing ownership to an object through this for retain count will increase other than that there is no possibility to increase retain count.
First things first, don't even try to rely upon retainCount.
After that: you're wondering which scenario happens among the ones you enumerated. Well, neither one.
Why? Because, in first palce, you're assigning to an instance variable directly - that won't change retain count. At all. Except if you use ARC, but it seems you don't.
You probably wanted to assign stuff to the property of the object, that is, write
self.tempArray = myArray;
etc. Now because the property itself (and not its backing ivar!) is declared as retain, the corresponding accessor method will increase the reference count of the object being assigned to the property. However, in order not to leak memory, an accessor method is usually implemented by releasing the previously assigned object when assigning and thus retaining the new one, i. e.
- (void)setTempArray:(NSArray *)tmp
{
[tmp retain];
[tempArray release];
tempArray = tmp;
}
So basically, when you reassign myArray to the self.tempArray property, it looses and gains a reference, thus its reference count doesn't chnage at all.
When you assign another, new array to the property, then again myArray loses a refcount, dropping to 0 it is deallocated, then the new array, created using + [NSArray arrayWithObject:] is retained. Its exact reference count is supposed to be 1 after this, since it was created using alloc - init - autorelease (that's how the method is implemented), and it has been retained by the property. However, the value returned by - retainCount is still (and never) to be relied upon.
In your particular example, you are assigning to tempArray directly and not self.tempArray, so the retainCount will stay at 1 throughout. But let's go through what would happen if you did what I think you meant.
In objective-c, a synthesized retained property will have a setter functionally equivalent to this:
-(void) setTempArray:(NSArray *value)
{
if(tempArray != value) {
[tempArray release];
tempArray = [value retain];
}
}
This increases the retain count when a new object is assigned to it, essentially does nothing when it is set to the same object, and releases it when something else is assigned to it. So the retain counts in your example go something like this:
-(void) sampleFunction
{
NSArray *myArray = [[NSArray alloc] init]; // Retain count of 1
self.tempArray = myArray; // 2
self.tempArray = myArray; // still 2
self.tempArray = [NSArray arrayWithObject:#"SomeString"];
// myArray.retainCount is 1,
// tempArray.retainCount is 2 but with 1 autorelease
// myArray leaks
}
I want to know that difference between following two lines
name1 = [[NSString stringWithUTF8String:(char *)sqlite3_column_text(compiledStatement,1)] retain];
name1 = [NSString stringWithUTF8String:(char *)sqlite3_column_text(compiledStatement,1)];
What will be effect on name1 if I do use retain at the last,
I face once problem, and couldn't use name1 in a method that is being call by NSTimer, and when I use retain, they it worked fine for me.
If I do call value from database in viewDidLoad, and use in a method that is being called by NSTimer after each second, then it will give bad-exec, but when I do use retain then it will work properly,
I want to know the reason
Here is the difference
- (void)func1 {
name1 = [[NSString stringWithUTF8String:...] retain];
name2 = [NSString stringWithUTF8String:...];
}
- (void)func2 {
NSLog(#"%#", name1); //OK, name1 is still there
NSLog(#"%#", name2); //Would be crashed because name2 could be released anytime after func1 is finished.
}
I wrote this answer to another question, but it explains what you're asking:
Objects in objective c have a retain count. If this retain count is greater that 0 when the object goes out of scope (when you stop using it), it leaks.
The following things increase the retain count
[[alloc] init]
new
copy
[retain]
adding an object to an array
adding an object as a child (e.g. views)
There are likely more, but you don't appear to use any others in your code
The following decrease the retain count
[release]
removing an object from an array
if you dealloc an array, all of its objects are released
You should go through your code and ensure each of the retains or additions to an array are matched with a corresponding release. (You can release member variables in the dealloc method).
Another user made a valid point that my answer doesn't
Once you add an object to an array, it takes ownership and will release the object when it is done with it. All you need to do is make sure you release anything you own according to the memory management rules
There are also autorelease objects, have a look at this example;
-(init){
...
stagePickerArray = [[NSMutableArray alloc] init];
for (int i = 0; i < 3; i++)
{
//this string is autoreleased, you don't have call release on it.
//methods with the format [CLASS CLASSwithsomething] tend to be autorelease
NSString *s = [NSString stringWithFormat:#"%d", i);
[stagePickerArray addObject:s];
}
...
}
Your issue is that when you come to use your string later, it has a retain count of zero and has been released. By calling retain on it, you're saying 'I want to use this later'. Don't forget to match every retain with a release or you're objects will 'leak'
I bet your code wouldn't crash if your name1 was a property - either (nonatomic, retain) or just (copy) depending on your needs.
Second condition is to have name1 initialized to sth meaningful at the time your other function tries to do sth with it.
EDIT:
With a property you'd have to use synthesized setter in this case with: self.name1 = #"your string";.
Normally you don't have to manually retain/release a string created with stringWith... methods since there's nothing you created in memory yourself by using explicit alloc. Also please note that with code:
NSString *str = [NSString stringWithUTF8String:#"your string"];
your str (if not used to set a property) will stop being available when the function gets out of scope (iOS eventloop will autorelease it).
I have the following example class:
Test.h:
#interface Test : UIButton {
NSString *value;
}
- (id)initWithValue:(NSString *)newValue;
#property(copy) NSString *value;
Test.m:
#implementation Test
#synthesize value;
- (id)initWithValue:(NSString *)newValue {
[super init];
NSLog(#"before nil value has retain count of %d", [value retainCount]);
value = nil;
NSLog(#"on nil value has retain count of %d", [value retainCount]);
value = newValue;
NSLog(#"after init value has retain count of %d", [value retainCount]);
return self;
}
Which produces the following output:
2008-12-31 09:31:41.755 Concentration[18604:20b] before nil value has retain count of 0
2008-12-31 09:31:41.756 Concentration[18604:20b] on nil value has retain count of 0
2008-12-31 09:31:41.757 Concentration[18604:20b] after init value has retain count of 2147483647
I am calling it like:
Test *test = [[Test alloc] initWithValue:#"some text"];
Shouldn't value have a retain count of 1? What am I missing?
Thanks for your help.
Don't look at retain counts. They're not useful and will only mislead you — you can't be certain that nothing else is retaining an object, that an object you get from somewhere isn't shared.
Instead, concentrate on object ownership and follow the Cocoa memory management rules to the letter. That way your memory management will be correct no matter what optimizations Cocoa may be doing behind the scenes for you. (For example, implementing -copy as just -retain for immutable objects.)
Furthermore, it's critical to understand the difference between properties of your objects and instance variables within your objects. In your question's code, you are assigning a value to an instance variable. That instance variable is just that: a variable. Assigning to it will behave like any other variable assignment. To use the property, you must use either dot syntax or bracket syntax to actually invoke the property's setter method:
self.value = newValue; // this is exactly equivalent to the next line
[self setValue:newValue]; // this is exactly equivalent to the previous line
The code generated for the dot syntax and the bracket syntax is identical, and neither will access the instance variable directly.
You are passing in a literal string. The compiler probably allocates it in static memory and sets the retain count to the maximum possible value.
Try a dynamically allocated string instead and see what happens.
NSString* string = [[NSString alloc] initWithString: #"some text"];
Test* test = [[Test alloc] initWithValue: string];
You've got a reference to an immutable string. Assignment doesn't need to copy the value (the string data) since it's immutable. If you do a mutable operation, like value = [newValue uppercaseString] then it should copy the bits into value, and value's retain count incremented.
You're passing in a string constant, which can't really be deallocated. I think that 2147483647 is probably UINT_MAX, which basically means that the object can't be released.
I think you want to do this:
self.value = newValue;
which will invoke the property setter and cause the copy to occur. "value = newValue" simply assigns a pointer value to the instance variable.
You shouldn't be paying attention to the retain counts, just follow the Cocoa memory management rules. http://iamleeg.blogspot.com/2008/12/cocoa-memory-management.html
hmm.. we're getting closer.
it appears that newValue's retain count is also 2147483647.
I tried dynamically allocating the string instead with the same retain count results.
I found a helpful article here: http://www.cocoadev.com/index.pl?NSString
FTA:
Does the NSString returned by #"" need to be released, or is it autoreleased?
Neither. #""-strings are of class NSConstantString?, and thus act like atoms in lisp; they hang around. That is, if you use #"cow" in two separate places in your code, they will be referencing the very same object.
I don't think -release or -autorelease does anything to either of them.
If I have "copy" on the property though, shouldn't it copy the contents of the target memory into new memory with a retain count of 1? It would seem the copy attribute does nothing in this case?
#import <Foundation/Foundation.h>
int main (int argc, const char * argv[]) {
NSAutoreleasePool * pool = [[NSAutoreleasePool alloc] init];
char *cstr = "this is a c string";
NSString *str = [[NSString alloc] initWithUTF8String:cstr];
NSLog(#"rc1: %d", [str retainCount]);
[pool drain];
return 0;
}
If you run the above code, it will display a retain count of 1
In Cocoa, many immutable objects will simply retain themselves when you ask for a copy within the same zone. If the object is guaranteed not to change (i.e. its immutableness) then an exact duplicate is redundant.
In Objective-C, the constant string class is separate to Cocoa's NSString class, although it may be a subclass of NSString (I'm not too sure). This constant string class may override NSObject's methods like retain, release and dealloc so that they do nothing, and also override retainCount so that it always returns the same number, UINT_MAX or so. This is because an Objective-C constant string is created in static memory. It must have the overall general behaviour of a Cocoa object (when using Cocoa) so that it can be added to arrays, used as keys to a dictionary etc, except in regards to its memory management, since it was allocated differently.
Disclaimer: I don't actually know what I'm talking about.