I have a piece of code & I want to understand the memory leak possibility in this. I have gone through the Apple documentation here!
The code here process a array from parameter & stores the dictionary into a temporary array.
+ (void)setLinkedProfiles:(NSArray *)profileData {
NSMutableArray *returnArray = [[NSMutableArray alloc]init];
if([profileData count] > 0) {
for(NSDictionary *dict in profileData) {
NSDictionary *tempDict = #{
#"verifiedEmail" : ([[dict objectForKey:#"verifiedEmail"] isKindOfClass:[NSNull class]]) ? #"": [dict objectForKey:#"verifiedEmail"],
#"identifier" : [dict objectForKey:#"identifier"],
};
[returnArray addObject:tempDict];
}
}
[SharedApp sharedUserData].linkedProfiles = returnArray;
}
I suspect that the line of code SharedApp sharedUserData].linkedProfiles = returnArray might create a memory leak.
I want to understand why & in which scenario?
Can some one help me in understanding this ?
Thank you for reading & understanding my problem.
To clear things up:
Your line
NSMutableArray *returnArray = [[NSMutableArray alloc]init];
gives you ownership of that array. You are responsible to release it when you're done with it.
So adding
[returnArray release];
as the last line would work. You could use autorelease instead, or even work with an autoreleased array from the beginning, i.e.
NSMutableArray *returnArray = [NSMutableArray array];
linkedProfiles should be a strong reference (i.e. strong, or copy).
Although I strongly suggest to switch to ARC, understanding of the underlying memory management might come handy.
Related
I am stuck in the middle of memory management stuff. Please help me out in solving my question.
NSMutableArray *array = [[NSMutableArray alloc]init];
Object *obj = [[Object alloc]init];
[array addObject: obj];
[obj release];
Is it necessary to release obj in above code?
The answer to your question is: yes, if you don't use ARC. If you are writing a new app, you should seriously consider using ARC.
Container objects in Objective-C always balance their retain/release count. In other words, you should always manage memory as if you did not add the object and make sure your own code balances its retain count. Note that this is a convention and is not enforced, but you could always trust the built-in classes to follow this convention. Also, you can perform a static analysis (Cmd+Shift+B in XCode) to detect these problems. It would have pinpointed this in your code above.
The correct code in the case above would be:
NSMutableArray *array = [[NSMutableArray alloc]init];
Object *obj = [[[Object alloc]init]autorelease];
[array addObject: obj];
or
NSMutableArray *array = [[NSMutableArray alloc]init];
Object *obj = [[Object alloc]init];
[array addObject: obj];
[obj release];
since NSMutableArray (and its cousins) will retain the object as long as it is in the collection.
I was running Leaks tool and discovered a massive leak in my Dictionary mutableDeepCopy but I can't figure out what's wrong with the code. Any suggestions?
#interface RootViewController : UIViewController{
NSDictionary *immutableDictionary;
NSMutableDictionary *mutableDictionary;
}
Here is the line of code that's highlighted in Instruments
self.mutableDictionary = [self.immutableDictionary mutableDeepCopy];
Here is the method for creating a mutable copy of a Dictionary
#interface NSDictionary(MutableDeepCopy)
-(NSMutableDictionary *)mutableDeepCopy;
#end
Here is method implementation, I've highlighted the code that Leaks saids is leaking 100%
- (NSMutableDictionary *) mutableDeepCopy {
NSMutableDictionary *dictionaryToReturn = [NSMutableDictionary dictionaryWithCapacity:[self count]];
NSArray *keys = [self allKeys];
for(id key in keys) {
id value = [self valueForKey:key];
id copy = nil;
if ([value respondsToSelector:#selector(mutableDeepCopy)]) {
copy = [value mutableDeepCopy];
} else if ([value respondsToSelector:#selector(mutableCopy)]) {
copy = [value mutableCopy]; //This is the Leak
}
if (copy == nil) {
copy = [value copy];
}
[dictionaryToReturn setValue:copy forKey:key];
}
return dictionaryToReturn;
}
You need to analyse this in light of Apple's Memory Management Rules.
Starting with this line:
self.mutableDictionary = [self.immutableDictionary mutableDeepCopy];
I would expect mutableDeepCopy to return an object I own, so at some point I need to release or autorelease it. e.g.
NSMutableDeepCopy* temp = [self.immutableDictionary mutableDeepCopy];
self.mutableDictionary = temp;
[temp release];
or
self.mutableDictionary = [[self.immutableDictionary mutableDeepCopy] autorelease];
So now we need to look at mutableDeepCopy. Because it has 'copy' in the name it needs to returned an "owned" object which, in practice means "forgetting" to release the returned object. You have already failed to do that when you create the returned object in the first line, since dictionaryWithCapacity: gives you an object you do not own. Replace it with
NSMutableDictionary *dictionaryToReturn = [[NSMutableDictionary alloc] initWithCapacity:[self count]];
Now you own it.
It is important that you make your mutableDeepCopy obey the rules because it means you can treat the objects returned from mutableDeepCopy, mutableCopy and copy in exactly the same way. In all three cases you own the object copy that you insert into the array. Because you own it, you must release it or it'll leak as you found out. So, at the end of the loop, you need
[copy release];
That'll stop the leak.
How is your property declared? If is is retain or copy, then this doesn't leak.
Your problem is that the name mutableDeepCopy suggests that it returns a retained object, and not an autoreleased one as it actually does.
Edit:
And at the mutableDeepCopy itself, you need to release the copy variable after adding to the dictionary.
mutableCopy increments the retain count of the object, as does setValue:forKey:. This means that when dictionaryToReturn is dealloc'ed, the object that had mutableCopy called still has a retain count of one.
Try doing this instead:
copy = [[value mutableCopy] autorelease];
I have a memory leak when i call a method that return me a string----
the method definition is as follows
-(NSMutableArray *)read
{
NSMutableArray *dataArray = [[NSMutableArray alloc] init];
//picking data from database here
return dataArray;
}
this show a big memory leak
i also tried--- NSMutableArray *dataArray = [[[NSMutableArray alloc] init]autorelease];
but this time leack checking process gets hanged
i also cannot release that array before return
please help
-(NSMutableArray *)read
{
NSMutableArray *dataArray = [[NSMutableArray alloc] init];
//picking data from database here
return dataArray;
}
Anything that uses the method read will expect to get back an object it does not own. However, as written here, dataArray is still owned at the point of return. You can't release it because that might make it go away altogether. You must, in this instance autorelease the array. You can either do this:
-(NSMutableArray *)read
{
NSMutableArray *dataArray = [[[NSMutableArray alloc] init] autorelease];
//picking data from database here
return dataArray;
}
or this
-(NSMutableArray *)read
{
NSMutableArray *dataArray = [[NSMutableArray alloc] init];
//picking data from database here
return [dataArray autorelease];
}
You say "leak checking process get hanged" but I'm really not sure what you mean by that. Whether it hangs, crashes or plays the Botswana National Anthem, you definitely need to autorelease the returned array and any other problem is actually a different problem. Possibly, you are forgetting to retain the data elsewhere.
Another answer more...
There are many conventions in cocoa/cocoa-touch, there is one of them that says that if a method has the prefix init then you will have the ownership of that object (hence you have to release it)
This is NOT your case, hence if you do:
DatabaseReader *dbReader = [[DatabaseReader alloc] init];
NSMutableArray *mutArray = [dbReader read];
[dbReader release];
you are NOT supposed to release mutArray. BUT, the object created HAS to be released by someone. So you can do as JeremyP wrote. alloc/init and put it into a autorelease pool inside read method implementation. Or, you can do:
-(NSMutableArray *)read
{
NSMutableArray *dataArray = [NSMutableArray array];
//IMPORTANT:
//Did you noticed that I am not using any method
//with init prefix for the creation of dataArray ?
//so I don't need to release by my self ;)
//picking data from database here
return dataArray;
}
Which is basically the same. ;)
Ownership of the returned object may be returned to the object that receives from this function. You may do some debugging with the object's retain count using something like this...
NSLog(#"Retain count: %i", [dataArray retainCount]);
Turn on the debugging console (Command + R in Xcode) to see the NSLog output.
So I am trying to save arrays into an NSDictionary on the fly. Let me show you the code and explain what is going on.
for (int x= 0; x <[appDelegate.people count]; x++) {
Person *aPerson = [[Person alloc] init];
aPerson = [appDelegate.people objectAtIndex:x];
if ([appDelegate.groupedBusiness objectForKey:aPerson.business_name] == nil) {
NSMutableArray *newBusiness = [[NSMutableArray alloc] init];
//if the business does not exist in the dict, add the person to the business and add it to dict.
[newBusiness addObject:aPerson];
[appDelegate.groupedBusiness setObject:newBusiness forKey:aPerson.business_name];
[newBusiness release];
newBusiness = nil;
//NSLog(#"%#", appDelegate.groupedBusiness);
} else {
NSMutableArray *existingBusiness= [appDelegate.groupedBusiness objectForKey:aPerson.business_name];
[existingBusiness addObject:aPerson];
//THIS IS THE LINE I AM NOT SURE ABOUT!!!
[appDelegate.groupedBusiness setObject:existingBusiness forKey:aPerson.business_name];
[existingBusiness release];
existingBusiness = nil;
//NSLog(#"%#", appDelegate.groupedBusiness);
}
}
Alright, so the appDelegate has an array of "People" that has a whole bunch of attributes about a person. I am trying to set up a dictionary to sort them by their business names. I am doing this by creating an array and saving it in the dictionary with the business_name as the key. Each iteration of the loop I check to see if the key exists, if it does, pull out the existing array, add the person you are checking, and resave it to the dictionary. However, this does not appear to be happening. Is there some exotic behavior in the NSDictionary class that would prevent that? I have poured over the class web page and can't find anything. Sorry if this is a noobie question, I am still trying to understand the objective-c classes. Thanks!
Why do you release existingBusiness? You are not creating an object, just taking the pointer from an array. When you invoke release, retainCount became 0 and object deallocs.
Just remove the following two lines:
[existingBusiness release];
existingBusiness = nil;
and everything should work fine.
You're way overcomplicating this, not to mention leaking a couple things.
for (Person *aPerson in appDelegate.people) {
NSMutableArray *business = [appDelegate.groupedBusiness objectForKey:aPerson.business_name];
if (!business) {
business = [NSMutableArray array];
[appDelegate.groupedBusiness setObject:business forKey:aPerson.business_name];
}
[business addObject:aPerson];
}
Not an answer, but some coding style issues.
Use fast iteration if you don't need the index:
for (Person *aPerson in appDelegate.people) {
Use convenience constructors; it makes your code more readable (remember to remove the "release" at the end):
NSMutableArray *newBusiness = [NSMutableArray arrayWithObject:aPerson];
Avoid duplicate logic where possible:
NSMutableArray * business = [appDelegate.groupedBusiness objectForKey:aPerson.business_name;
if (!business) {
business = [NSMutableArray array];
}
[business addObject:aPerson];
[appDelegate.groupedBusiness setObject:business forKey:aPerson.business_name];
The "setObject:existingBusiness" call does changes nothing apart from wasting CPU cycles, but in the case above, it makes the code somewhat more readable.
[appDelegate.groupedBusiness setObject:existingBusiness forKey:aPerson.business_name];
[existingBusiness release];
existingBusiness = nil;
This should all be removed. existingBusiness is already in the dict, and it's a mutable object - when you're adding a person to it, this will be reflected in the dictionary as well as it's the same object you're dealing with. Apart from that you have a couple of memory leaks as Daniel points out.
I have a list of shops in a ListController file.
I've setted up a sqlite db, in which i've stored 60 shops.
On the top of the list i have a search bar.
I've made a class called DataController, that is responsible to load and store db datas.
#interface DataController : NSObject {
sqlite3 *database;
NSArray *shops;
NSDictionary* dictionaryOfShops;
}
#property (nonatomic, retain) NSDictionary *dictionaryOfShops;
#property (nonatomic, retain) NSArray* shops;
-(void)initializeShops;
initializeShops method loads data from the db, and stores results into the 2 props in this way:
-(void)initializeShops{
[dictionaryOfShops release];
[shops release];
NSMutableDictionary *dictionary = [[[NSMutableDictionary alloc] init] autorelease];
if (sqlite3_open(....))
NSString *query = ....
if (sqlite3_prepare_v2(database, [query UTF8String],-1, &statement, nil) == SQLITE_OK)
{
while (sqlite3_step(statement) == SQLITE_ROW) {
int rId = sqlite3_column_int(statement, 0);
char *rName = (char *)sqlite3_column_text(statement, 1);
Shop* s = [[Shop alloc] init];
s.ID = rId;
if(sName != nil) s.Name = [NSString stringWithUTF8String:rName];
NSString *shopID = [[NSString alloc] initWithFormat:#"%d",s.ID];
[dictionary setObject:s forKey:shopID];
[shopID release];
[s release];
}
sqlite3_finalize(statement);
}
[query release];
dictionaryOfShops = [[NSDictionary alloc] initWithDictionary:dictionary];
shops = [[NSArray alloc] initWithArray:[dictionary allValues]];
dictionary = nil;
[dictionary release];
//Sorting
NSSortDescriptor *sort = [[NSSortDescriptor alloc] initWithKey:#"Name" ascending:YES];
NSArray *sortedList =[self.shops sortedArrayUsingDescriptors:[NSArray arrayWithObject:sort]];
self.shops = sortedList;
[sort release];
}
The problem is that when user enters some text into the search
bar, I change the value of the query (adding LIKE....) and then call the initializeShops method again. This second time makes
so many leaks, (related to the Shop class properties) and
leaks also a NSDictionary and a NSArray.
Before posting this to you I've tried different solutions, but
at least this doesn't leaks anything the first time I call
initilizeShops.
I accept any suggestion, since I'm really stuck
on it.
MORE:
The really strange thing is memory management of my var dictionary and the 2 props shops and dictionaryOfShops. With this code
NSMutableDictionary *dictionary = [[NSMutableDictionary alloc] init];
//add data to dictionary
dictionaryOfShops = [[NSDictionary alloc] initWithDictionary:dictionary];
shops = [[NSArray alloc] initWithArray:[dictionary allValues]];
[dictionary release]
Considering that dictionaryOfShops and shops are two properties (nonatomic,retain) synthesized, how can I change value to them without leaks?
The very first time I pass through this method nothing gets leaked, from the second time it starts to leak so many objects (the contents of the collections).
The first question is Why not just use Core Data? It is very likely going to be faster, will require less code, and will be significantly easier to maintain over time. To be blunt; SQLite is deceptively hard. Easy to get started, exceptionally difficult to get right.
In any case, the memory management of dictionary is wrong. It only isn't crashing because you swapped the order of the nil assignment and release as kennyTM suggested. I would suggest not creating an autoreleased dictionary.
Otherwise, the code as written seems pretty leakless at first glance. So:
Can you provide some more code?
Anything interesting memory wise
going on elsewhere?
Are you using threading at all (or
NSOperationQueue)?
Have you run under the Leaks
instrument and retrieved the
backtraces of allocation of the
specific objects being leaked?
dictionary = nil;
[dictionary release];
Please swap these 2 statements. In this form it means [nil release] which is a no-op.
Ok, I've found the error.
In my class Shop, i realize i didn't implement the method
-(void)dealloc
So when I release the old dictionary (to prepare for a new assignment), all the fields inside of it didn't get released.