I have the following controller class which will do different tasks based on combination of the flag and param property. The value of these two properties will be set by many other classes having a reference to this controller. The question is how does each of the calling class assign value and when should they release it so that there will be no memory leak ?
#interface SampleController {
NSMutableArray *param;
NSString *flag;
}
#property (nonatomic, retain) NSMutableArray *param;
#property (nonatomic, retain) NSString *flag;
#end
#implementation SampleController
#synthesize param;
#synthesize flag;
- (id)init
{
param = [[NSMutableArray alloc] initWithCapacity:0];
flag = #"nothing";
}
#end
Well it depends on how you call your controller :
in an instance variable of an other object : you have to release it in this object's deallocate methode
in a function : you should release it when you do not need it anymore (retained by another object for example or it finished the job in this function), if you want to return it, just sent the message "autorelease" to it and the NSAutoReleasePool will do the job for you.
To assign value, you can
set the mutable array with the setParam:(*NSMutableArray)theArrayYouWantToReplaceYourArrayWith
access it directly with [[yourSampleController param]addObject:(id)objectYouWantToAdd]...
or more convenient : [yourSampleController.param addObject:(id)objectYouWantToAdd]
The addObject: message here is an example, you can see the methods for modifying an array (remove, sort,...) in the NSMutableArray class reference.
You will not be able to modify your string since it is a NSString and not a NSMutableString, but you can accessit by
[yourSampleController getParam]
[yourSampleController param]
yourSampleController.param
If you want to avoid leaks in general, build your project with the Instrument tool in leak mode and look at the objects that are leaked if you found some that are declared in your functions.
You can also check the Clang Static Analyzer (free static debugger) which is quite good if you have a lot of files.
I hope i helped you
Julien
Related
I am studying Objective-C. I asked a question about this code earlier but I came up with further questions. The below code is trying to make NSArray externally but really makes NSMutableArray internally so I can add pointers or remove in NSMutableArray
I face two questions.
1) What is the purpose of doing like this? Is there a specific reason you make NSArray externally? Why can't I just declare a property of NSMutableArray?
2)I learn that instance variable (_assets) is made when I declare a property of NSArray *assets. And I also declared NSMutableArray *_assets under the interface. I think those two _assets conflict each other even though they have different types. Am I thinking this in a wrong way?
#interface BNREmployee : BNRPerson
{
NSMutableArray *_assets;
}
#property (nonatomic) unsigned int employeeID;
#property (nonatomic) unsigned int officeAlarmCode;
#property (nonatomic) NSDate *hireDate;
#property (nonatomic, copy) NSArray *assets;
I'll try put your answers the way you have asked them. Let hope they clear your doubts. By now I guess you would be knowing that NSArray once initialised with data you wont be able to add or delete the data inside it which is different from NSMutableArray.
The benefit here no one else can change your externally visible data. Also when you try to sort or iterate the array you are sure that no other data would be removed or added. Also if you use NSMutableArray for such cases the application would crash if you add data while you iterate the array.
Like #KirkSpaziani Explained
#synthesize assets = _assets;
would create an instance variable for your property. However you are actually supposed to use this _assets only in getter and setter. Else places you should be using self.assets.
You can also synthesize your other array NSMutableArray *_assets as follows
#synthesize _assets = __assets;
Which would have double underscore, but frankly we shouldn't be using the underscore for a starting variable name. Plus would be great if you have different names altogether.
Also with advances in Objective C you dont require to synthesize these variables at all. Just use the self.variableName and you can access it.
Hope it clears some of your queries.
Put
{
NSMutableArray *_assets;
}
in the #implementation block
#implementation {
NSMutableArray *_assets;
}
Putting the NSMutableArray in the implementation block hides the fact that it is mutable from consumers (it is no longer in the header file).
Follow it with:
#synthesize assets = _assets;
This might not be necessary actually, but makes things clearer. When you declare a property an ivar will be automatically created (unless you #dynamic the property). However an explicitly declared ivar of the same name will override the automatically created one - so long as the type is the same or a subclass.
The reason to make it an NSArray publicly visible is so that no one else can mutate your data structure. You will have control of it. If it is an NSMutableArray internally then you can add and remove items without exposing that functionality to consumers.
You can declare your property to be readonly or readwrite - a readwrite NSArray means you can replace the whole array with a property set, but you can't add or remove items. If internally you are adding and removing items, this can make things messy. Try to stick with readonly when having a mutable internal version.
Here's something you can do if you want _assets to be a mutable array, but you don't want other classes to modify it, implement the setter and getter of the assets property so they look like this (implementing the getter and the setter will cause the property to not be synthesised, which means the NSArray *_assets will not be created automatically):
-(NSArray *)assets{
return [_assets copy]; // Copy creates an immutable copy
}
-(void)setAssets:(NSArray *)assets{
_assets = [NSMutableArray arrayWithArray:assets];
}
Keep in mind that if you access the assets array a LOT, it might be slow since you're creating an immutable copy every time, so you can create an NSArray whenever your _assets array is modified and return that in the -(NSArray *)assets method
The reason you'd internally keep an NSMutableArray, but expose an NSArray externally is so that users of your API won't abuse it and mutate its data. Keeping it visible as immutable makes people less prone to mess with it.
Another approach you could take to this is to not use a property at all, but simply have a getter and a mutable property in a class extension. For example, in your .h:
#interface BNREmployee : BNRPerson
- (NSArray *)assets;
#end
In your .m
#interface BNREmployee ()
// Inside of the class manipulate this property
#property (nonatomic, strong) NSMutableArray *mutableAssets;
#end
#implementation BNREmployee
// Clients of your class rely on this
- (NSArray *)assets
{
// copy makes the result immutable
return [self.mutableAssets copy];
}
#end
Another approach might be to make the property only writable to the implementation of you class.
To do that you declare your property as readonly in the header:
//BNREmployee.h
#property (nonatomic, readonly) NSMutableArray *assets;
Than declare it as readwrite inside an inner interface in your implementation:
//BNREmployee.m
#interface BNREmployee()
#property (nonatomic, readwrite) NSMutableArray *assets;
#end
#implementation
...
Suppose I have a property called myPropertyName defined in my class MyClassName. Manual memory management is used throughout this post.
MyClassName.h
#import <UIKit/UIKit.h>
#interface MyClassName : NSObject {
#private
NSObject* myPropertyName;
#public
}
#property (nonatomic, retain) NSObject* myPropertyName;
// Some methods prototypes are here
#end
MyClassName.m
#import "MyClassName.h"
#implementation MyClassName
#synthesize myPropertyName;
// Some methods are here
#end
I'm confused with usages such as the place of myPropertyName declaration, its difference between instance variable. For example, what is the difference among these three statement of initialization code, for example, in the customized -(void)init method for my class myClassName.
self.myPropertyName = [[[NSObject alloc] init] autorelease];
This one is calling myPropertyName setter, but I'm not sure what is the name of the instance variable being used in the setter, myPropertyName (since I've declared a #private field named myPropertyName) or _myPropertyName (people say that this one with underbar is the default)?
myPropertyName = [[NSObject alloc] init];
Does this initialize the instance variable of the myPropertyName property? If I don't have #synthesize myPropertyName = _myPropertyName;, would it be wrong since the default instance variable for the property is said to be _myPropertyName.
_myPropertyName = [[NSObject alloc] init];
Is _myPropertyName still declared as the instance variable for my property myPropertyName even if I use #synthesize myPropertyName; and #private NSObject* myPropertyName;?
In my understanding, a property is just a name (such as myPropertyName), there should be some instance variable encapsulated to be used in actual operations in the code, such as assigning values.
First off, I highly recommend reading Apple's documentation on properties, also linked by nhgrif. However, I understand docs can be a bit dense reading material (though Apple's, I find, are not so bad), so I'll give a brief overview of properties here.
I like examples, so I'm going to rewrite your two classes in a bit more current form.
MyClassName.h
#import <UIKit/UIKit.h>
#interface MyClassName : NSObject
#property (nonatomic, strong) NSObject *myPropertyName;
// method prototypes here
#end
MyClassName.m
#import "MyClassName.h"
#implementation MyClassName
// some methods here
#end
The class MyClassName now has a property called myPropertyName of type NSObject *. The compiler will do a lot of work for you for "free" in this instance. Specifically, it will generate a backing variable, and also generate a setter and getter for myPropertyName. If I were to rewrite the two files, and pretend I'm the compiler, including that stuff, they would look like this:
MyClassName.h
#import <UIKit/UIKit.h>
#interface MyClassName : NSObject {
NSObject *_myPropertyName;
}
#property (nonatomic, strong) NSObject *myPropertyName;
- (void)setMyPropertyName:(NSObject *)obj;
- (NSObject *)myPropertyName;
#end
MyClassName.m
#import "MyClassName.h"
#implementation MyClassName
- (void)setMyPropertyName:(NSObject *)obj
{
_myPropertyName = obj;
}
- (NSObject *)myPropertyName
{
return _myPropertyName;
}
#end
Again, all of this is happening for "free": I'm just showing you what's happening under the hood. Now for your numbered questions.
self.myPropertyName = [[[NSObject alloc] init] autorelease];
First of all, you should probably be using Automatic Reference Counting, or ARC. If you are, you won't be allowed to call autorelease. Ignoring that part, this works fine. Excluding the autorelease, this is exactly equivalent to:
[self setMyPropertyName:[[NSObject alloc] init]];
Which, if you look at the second .m file I wrote out, above, will basically translate to:
`_myPropertyName = [[NSObject alloc] init];
myPropertyName = [[NSObject alloc] init];
As written, this code will give a compiler error, since there is no variable called myPropertyName in this class. If you really want to access the instance variable underlying (or, "backing") the myPropertyName property, you can, by using its real name:
_myPropertyName = [[NSObject alloc] init]; // note the underscore
But most of the time, it's better to use the setter, as in point 1., since that allows for side effects, and for Key-Value Coding, and other good stuff.
_myPropertyName = [[NSObject alloc] init];
Oh. Well you got it. See point 2.
You mentioned that:
I'm confused with usages such as the place of myPropertyName declaration, its difference between instance variable. For example, what is the difference among these three statement of initialization code, for example, in the customized -(void)init method for my class myClassName.
In case it hasn't been made clear, a property is something of an abstract concept; its data is stored in a normal instance variable, typically assigned by the compiler. Its access should usually be restricted to the setter and getter, with important exceptions. To keep this answer short, I won't go into more detail than that.
One more thing: as nhgrif mentioned, you don't need to use the #synthesize keyword anymore. That is implicitly understood by the compiler now.
If you're not sure about any of this, post a comment or, better yet, read the docs.
Let's take this example:
#property NSString *fullName;
If in the implementation, we override the setters and getters, and in these setters and getters, we don't use an instance variable fullName, it is never created. For example:
- (NSString *)fullName
{
return [NSString stringWithFormat:#"%# %#", self.firstName, self.lastName];
}
- (void)setFullName:(NSString *)fullName
{
//logic to split fullName into two strings
//self.firstName = etc
//self.lastName = etc.
}
In this example, there is no instance variable for fullName created.
This is according to Apple's Official Documentation
If, however, you don't override both the setter and getter, an instance variable is created.
As a sidenote, you can declare a property readonly, and then simply overriding the getter (without using the variable) will prevent an ivar being created. Likewise, you can declare a property writeonly and just override the setter.
I have a -dealloc() method which I am assuming I can use to dealloc instance variables, I have another variable that is not in the instance, rather a class level variable, wondering when & how I dealloc this? I can't do it in the instance method dealloc() right? Code below for reference (on varaible: levelHash):
#interface Level : CCNode
{
//Instance variables
PlayBackgroundLayer* playBGLayer;
PlayTilemapLayer* playTilemapLayer;
PlayUILayer* playUILayer;
PlayElementLayer* playElementLayer;
}
//Property declarations for instance variables
#property (nonatomic, retain) PlayBackgroundLayer* playBGLayer;
#property (nonatomic, retain) PlayTilemapLayer* playTilemapLayer;
#property (nonatomic, retain) PlayUILayer* playUILayer;
#property (nonatomic, retain) PlayElementLayer* playElementLayer;
//Static methods
+(void) Initialize: (NSString*) levelReference;
+(void) InitLevel: (NSString*) levelReference;
+(Level*) GetCurrentLevel;
#end
//static variables
NSMutableDictionary *levelHash;
and my implementation:
+(void) Initialize: (NSString*) levelReference
{
levelHash = [[NSMutableDictionary alloc] init];
[levelHash setObject:NSStringFromClass([LevelOne class]) forKey:#"1"];
//EG CALL IT [levelHash objectForKey:#"foo"];
//WHEN DO I CALL THIS??? [levelHash release];
}
Classes aren't deallocated for the life of your program, so there doesn't seem to be much point in releasing that dictionary. All the memory your app uses is reclaimed when it terminates. You can create a "tear down" method for the class where you release the dictionary if you want to, just as you've created a custom initialization method.
(By the way, it is neither a class nor a static variable; ObjC doesn't have class variables and, lacking the static keyword, it's actually global. This is why there's no need to worry about a leak -- global variables also exist for the entire lifetime of your program.
Also, you shouldn't be putting it in your header file, as I mentioned earlier. Every file that imports this header is going to be re-defining it, which will cause a linker error -- you can only define things once.)
I'm not sure I understood how alloc and retain work.
Recently I discovered that the NSString properties were not retained and I had to add [myString copy] when I set them. Which makes me wonder if I misunderstood the whole way of using retain/alloc
Please, may someone tell me if I'm doing it correctly? I read a lot and had a look on open source projects, this let me thing that I may have been wrong since the beginning.
Here is my way of doing it:
/**** VIEW.h *****/
#import "MyClass.h"
#interface MyViewController : UIViewController {
//Is the following line really necessary?
MyClass *myObject;
}
#property (nonatomic, retain) MyClass *myObject;
- (void)defineObject;
#end
.
/**** VIEW.m *****/
#import "VIEW.h"
#implementation MyViewController
#dynamic myObject;
- (void)viewDidLoad
{
[super viewDidLoad];
[self defineObject];
NSLog(#"My object's name is: %#", myObject.name);
}
- (void)defineObject
{
//Here particularly, Why doesn't it work without both alloc and init
//shouldn't "#property (nonatomic, retain) MyClass *myObject;" have done that already?
myObject = [[MyClass alloc] initPersonalised];
[myObject setName:#"my name"];
}
.
/**** MyClass.h *****/
#interface MyClass : NSObject {
//not sure if this line is still necessary
NSString *name;
}
#property (nonatomic, retain) NSString *name;
- (id)initPersonalised;
- (void)setName:(NSString *)name;
- (NSString *)name;
#end
.
/**** MyClass.m *****/
#import "MyClass.h"
#implementation MyClass
#dynamic name;
(id)initPersonalised{
self = [super init];
name = #"Undefined";
}
- (void)setName:(NSString *)name{
self.name = [name copy];
}
- (NSString *)name{
return [self.name copy];
}
#end
I hope you can bring a bit of light, after months of programming this way, I'm less and less sure of doing it well.
This is indeed a topic that every Objective C programmer stumbles upon. There are a few things one needs to know:
Instance variable vs. property access
Within MyViewController,
myObject = xxx;
and
self.myObject = xxx;
are two different things. The first directly assigns to the instance variable and does neither release to old referenced insance nor retain the newly assigned instance. The latter one uses the property setter and thus releases the old and retains the new value.
Deallocation
Even when you have declared an implemented a property that takes care of retaining and releases the values, it won't take care of deallocation when your object (MyViewController in your case) is released. So you must explicitly release it in dealloc:
-(void) dealloc {
[myObject release];
[super dealloc];
}
Now to your code:
The snippet:
myObject = [[MyClass alloc] initPersonalised];
is perfectly okay. When you create an object, you use the pair of alloc and initXXX. The always create an instance with the reference count set to 1. So by directly assigning it to the instance variable, you create a clean constellation. I don't see no other way of creating the instance.
In MyClass you could use #synthesize name instead of #dynamic. Then the compiler would implement name and setName: automatically and you wouldn't need to do it yourself.
Finally, your missing dealloc.
Update:
If you use:
self.myObject = [[MyClass alloc] initPersonalised];
then you have a memory leak because initPesonalised sets the reference count to 1 and the setter of myObject increases it to two. If you want to use the setter, then I has to be:
MyClass* mo = [[MyClass alloc] initPersonalised];
self.myObject = [[MyClass alloc] initPersonalised];
[mo release];
It would be different if you weren't using initXXX to create a new instance. The class NSString for example has many methods called stringXXX, which create a new instance (or return a shared one) that has (conceptually) a reference count of 1 that will later automatically decreased by one. Then you better use the setter:
self.name = [NSString stringWithFormat: #"instance %d", cnt];
If you want to use copy instead of retain for your string property (which is good practice), then you can simply declare your property like this:
#property (nonatomic, copy) NSString *name;
When you then use #synthesize to implement the getter and setter, the compiler will generate them using copy instead of retain.
And NSString *name; is necessary even if you use #property and/or #synthesize to implement the property.
Alloc and init are methods that always go hand-in-hand. alloc allocates space for your object, and init initializes your object to some value. When you call alloc, you are responsible for freeing that object later. If you call copy, you are also responsible for releasing that object later. It's considered good practice to always initialize your objects right after you allocate them.
Now, to answer the questions I found in your code.
#interface MyViewController : UIViewController {
//Is the following line really necessary?
MyClass *myObject;
}
So is that line necessary? That depends. Does it make sense that your object has a MyClass as a property? This is a question only you can answer based on your design. I recommend you to study Object-Oriented Programming in more depth.
- (void)defineObject
{
//Here particularly, Why doesn't it work without both alloc and init
//shouldn't "#property (nonatomic, retain) MyClass *myObject;" have done that already?
myObject = [[MyClass alloc] initPersonalised];
[myObject setName:#"my name"];
}
Not necessarily. You are just providing a pointer to an object of the specified kind. The moment you set your property, depending on the property modifiers, your class will know what to do with MyObject.
In that way, there's no need to call [yourObject copy]. In this way your properties will be copied instead of being retained. Just don't forget to release it later in your -dealloc method, like you would with retain properties.
All in all, this is what I recommend you to study a bit more:
Object-Oriented Programming (not related to your issue, but I can tell you are not comfortable using it. Objective-C is heavily object oriented, so you want to understand OOP).
iOS Memory Management.
You can have a look at the Memory Management Guide. It will help you to better understand the alloc & retain concepts; hope this helps you.
Is there a difference between an "instance variable" and a "property" in Objective-c?
I'm not very sure about this. I think that an "property" is an instance variable that has accessor methods, but I might think wrong.
A property is a more abstract concept. An instance variable is literally just a storage slot, like a slot in a struct. Normally other objects are never supposed to access them directly. A property, on the other hand, is an attribute of your object that can be accessed (it sounds vague and it's supposed to). Usually a property will return or set an instance variable, but it could use data from several or none at all. For example:
#interface Person : NSObject {
NSString *name;
}
#property(copy) NSString *name;
#property(copy) NSString *firstName;
#property(copy) NSString *lastName;
#end
#implementation Person
#synthesize name;
- (NSString *)firstName {
[[name componentsSeparatedByString:#" "] objectAtIndex:0];
}
- (NSString *)lastName {
[[name componentsSeparatedByString:#" "] lastObject];
}
- (NSString *)setFirstName:(NSString *)newName {
NSArray *nameArray = [name componentsSeparatedByString:#" "];
NSArray *newNameArray [[NSArray arrayWithObjects:newName, nil] arrayByAddingObjectsFromArray:[nameArray subarrayWithRange:NSMakeRange(1, [nameArray size]-1)]];
self.name = [newNameArray componentsJoinedByString:#" "];
}
- (NSString *)setLastName:(NSString *)newName {
NSArray *nameArray = [name componentsSeparatedByString:#" "];
NSArray *newNameArray [[nameArray subarrayWithRange:NSMakeRange(0, [nameArray size]-2)] arrayByAddingObjectsFromArray:[NSArray arrayWithObjects:newName, nil]];
self.name = [newNameArray componentsJoinedByString:#" "];
}
#end
(Note: The above code is buggy in that it assumes the name already exists and has at least two components (e.g. "Bill Gates" rather than just "Gates"). I felt that fixing those assumptions would make the actual point of the code less clear, so I'm just pointing it out here so nobody innocently repeats those mistakes.)
A property is a friendly way of implementing a getter/setter for some value, with additional useful features and syntax. A property can be backed by an instance variable, but you can also define the getter/setter to do something a bit more dynamic, e.g. you might define a lowerCase property on a string which dynamically creates the result rather than returning the value of some member variable.
Here's an example:
// === In your .h ===
#interface MyObject {
NSString *propertyName;
}
// ...
#property (nonatomic, retain) NSString *propertyName;
// === In your .m #implementation ===
#synthesize propertyName /* = otherVarName */;
The #property line defines a property called propertyName of type NSString *. This can be get/set using the following syntax:
myObject.propertyName = #"Hello World!";
NSLog("Value: %#", myObject.propertyName);
When you assign to or read from myObject.propertyName you are really calling setter/getter methods on the object.
The #synthesize line tells the compiler to generate these getter/setters for you, using the member variable with the same name of the property to store the value (or otherVarName if you use the syntax in comments).
Along with #synthesize you can still override one of the getter/setters by defining your own. The naming convention for these methods is setPropertyName: for the setter and propertyName (or getPropertyName, not standard) for the getter. The other will still be generated for you.
In your #property line you can define a number of attributes in parens for the property that can automate things like thread-safety and memory management. By default a property is atomic meaning the compiler will wrap #synthesized get/set calls with appropriate locks to prevent concurrency issues. You can specify the nonatomic attribute to disable this (for example on the iPhone you want to default most properties to nonatomic).
There are 3 attribute values that control memory management for any #synthesized setters. The first is retain which will automatically send release to old values of the property, and retain to the new values. This is very useful.
The second is copy which will make a copy of any values passed in rather than retaining them. It is good practice to use copy for NSString because a caller could pass in an NSMutableString and change it out from under you. copy will make a new copy of the input which only you have access to.
The third is assign which does a straight pointer assign without calling retain/release on the old or new object.
Lastly you can also use the readonly attribute to disable the setter for the property.
I use properties for the interface part - where the object interfaces with other objects
and instance variables are stuff that you need inside your class - nobody but you is supposed to see and manipulate those.
By default, a readwrite property will be backed by an instance variable, which will again be synthesized automatically by the compiler.
An instance variable is a variable that exists and holds its value for the life of the object. The memory used for instance variables is allocated when the object is first created (through alloc), and freed when the object is deallocated.
Unless you specify otherwise, the synthesized instance variable has the same name as the property, but with an underscore prefix. For a property called firstName, for example, the synthesized instance variable will be called _firstName.
Previously people use properties publicly and ivars for private usage, but since several years ago, you can also define properties in #implementation to use them privately. But I'd still use ivars when possible, since there are less letters to type, and it runs faster according to this article. It makes sense since properties are mean to be "heavy": they are supposed to be accessed from either generated getters/setters or the ones manually written.
However, in recent codes from Apple, ivars are not used anymore. I guess because it's more like objc rather than C/C++, plus it's easier to use properties with assign, nullable, etc.
Objective-C Property vs Instance variable (iVar)
[Swift variable, property...]
Instance variable
#interface SomeClass: NSObject
NSString *someVariable;
#end
Property
#interface SomeClass: NSObject
#property (nonatomic, strong) NSString *someVariable;
#end
Property uses Instance variable inside. property = variable + bounded getter/setter. It is a method call with variable syntax and access
#property generates getter and setter methods(accessor methods) which uses backing ivar(aka backing field) which you can use via underscore _<var_name> (_someVariable).
Since it calls a method - method dispatch mechanism is used that is why KVO[About] can be applied
When you override accessor methods backing iVar is not generated that is why you can declare a new property explicitly or use #synthesize[About] to generate a new one or link with existing
#import "SomeClass.h"
#interface SomeClass()
#property (nonatomic, strong) NSString *someVariable;
#end
#implementation SomeClass
- (void) foo {
//property getter method
NSString *a1 = self.someVariable; //NSString *a1 = [self someVariable];
//property setter method
self.someVariable = #"set someVariable"; //[self setSomeVariable:#"set someVariable"];
//iVar read
NSString *a2 = _someVariable;
//iVar write
_someVariable = #"set iVar";
}
//if you overriding someVariable getter and setter the iVar(_someVariable) is not generated, that is why you can:
//1. create some variable explicitly
NSString *_someVariable;
//or
//2. use #synthesize
#synthesize someVariable = _someVariable;
//overriding
- (NSString*) someVariable {
return _someVariable;
}
- (void)setSomeVariable: (NSString*) updatedSomeVariable {
_someVariable = updatedSomeVariable;
}
#end
[property attributes]