I am reading Apple Doc for understanding property instance variable but bit confused
From Apple Doc:
Most Properties Are Backed by Instance Variables 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.
Although it’s best practice for an object to access its own properties
using accessor methods or dot syntax, it’s possible to access the
instance variable directly from any of the instance methods in a class
implementation. The underscore prefix makes it clear that you’re
accessing an instance variable rather than, for example, a local
variable:
If using accessor methods or dot syntax is best practice then why user _ivarPropertyName?
Why use ivar for presenting properties? what are its benefits? when apple says "using accessor methods or dot syntax is best practice"
#property declares the existence of a property (describing its interface), but doesn't specify the implementation of that property. But properties need to store their contents somewhere. By default, the compiler synthesizes an ivar for that (and matching setters and getters). So normally you can ignore the existence of the ivar and just use dot syntax.
I follow Apple's advice and try to avoid using ivars directly. But somtimes you want to access a property without invoking its getter. The most common exception in my code is lazily-initialized read-only properties:
#interface MyObject : NSObject
#property ( nonatomic, readonly ) id someProperty ;
#end
#implementation MyObject
#synthesize someProperty = _someProperty ; // required; compiler will not auto-synthesize ivars for readonly properties
-(id)someProperty
{
if ( !_someProperty )
{
_someProperty = ... create property here
}
return _someProperty ;
}
#end
Also, you may not want to invoke the getter for a property in your -dealloc method... for example, a timer property. To avoid creating a timer in -dealloc, access the ivar directly:
-(void)dealloc
{
[ _myTimer invalidate ] ; // don't use self.myTimer here, that would create a timer even though we're going away...
}
There are probably more use cases. For most properties you don't even need to use the ivar, just use <value> = self.property and self.property = <new value>.
edit:
Also, there will be some additional overhead for accessing the property via message dispatch (using dot-accessor syntax or the getter) vs directly accessing the ivar, but it will make no difference in almost all cases.
Related
I am so confused between self and underscore to access the property in Objective c, whenever we create property, its getter-setter automatically generated. So we can access the same property with self.property and same as _property. In my opinion, there shoulb be some difference which i am not getting. PLease tell me with examples.
The underbar (underscore) version is the actual instance variable, and should not be referenced directly. You should always go via the property name, which will ensure that any getter/setter actions are honoured.
So if you code _property = 4, you have directly set the variable. If you code self.property = 4, you are effectively making the method call [self setProperty:4], which will go via the setter (which might do something such as enforce property having a max value of 3, or updating the UI to reflect the new value, for example).
Edit: I though it was worth mentioning that the setter (setProperty) will issue a _property = 4 internally to actually set the instance variable.
when you are using the self.XX, you access the property via the setter or getter.
when you are using the _XX, you access the property directly skip the setter or getter.
Let's say you have a property defined as follows:
#property (nonatomic,strong) NSString* name;
The getters and setters of the name property are automatically generated for you.Now, the difference between using underscore and self is that:
self.name =#"someName"; // this uses a setter method generated for you.
_name = #"someName"; // this accesses the name property directly.
The same applies for getting the name property;
I recently mess up with property and ivar.
One very simple question but could not find answer.
Can I say instance var is strong and atomic ?
Thanks~
For instance variables containing object references, you can use the __weak or the __strong keyword. Default is __strong. If you also declare a property using the instance variable, this needs to agree with the property.
Instance variables cannot be atomic. Being atomic is not a property of an instance variable, it is a property of the code that is used to access it. Therefore properties can be atomic or nonatomic, but instance variables can't. Note that being atomic isn't particularly useful; that's discussed elsewhere.
Instance variables also cannot be readonly. If you have an instance variable, anyone who can access it can both read it and write it. On the other hand, instance variables can be #private, #protected or #public which properties cannot.
To give a broader answer:
A. ownership qualification (strong, weak, unsafe unretained)
As mentioned, (explicitly) declared ivars are strong by default as every other local var. You can change that behavior in the usual way with __weak or __unsafe_unretained.
If you have an (implicitly or explicitly) synthesized property and no (explicitly) declared ivar, the ivar is synthesized and inherits its ownership qualification from the property, which has an default strong ownership.
If you have an (explicitly) declared ivar that is bound to an (implicitly or explicitly) synthesized property, the ownership qualification of the property and the ivar shall be the same.
If a property is synthesized, then the associated instance variable is the instance variable which is named, possibly implicitly, by the #synthesize declaration. If the associated instance variable already exists, then its ownership qualification must equal the ownership of the property; otherwise, the instance variable is created with that ownership qualification.
http://clang.llvm.org/docs/AutomaticReferenceCounting.html#property-declarations
B. Atomicity
Atomicity has two effects:
The accessors are synchronized with an object level lock.
The lifetime of the return value exceeds the lifetime of the receiver.
The first subject is obviously related to control flow, not to ownership, so it has no effect on the ivar and you cannot qualify an ivar with atomicity.
The second subject means that you can imagine that the return value gets two additional messages in MRR:
return [[ivar retain] autorelease]
ARC respects that convention. However, using ARC the meaning of that is academical in usual code.
An instance variable is strong by default for objects, unless you use the __weak keyword, in which case it's weak.
It's never atomic unless you arrange to use a thread-locking mechanism when assigning it or accessing it.
Generally it's more convenient to use properties over instance variables.
Properties can be strong and atomic, and because strong and atomic are the default you can specify them like this:
#property id name;
Instance variables are strong by default but as they do not have accessors you need to do any locking yourself. One of the reasons it is better to use properties rather than access instance variables directly.
This question already has answers here:
Is there a difference between an "instance variable" and a "property" in Objective-c?
(6 answers)
Closed 9 years ago.
What is the difference between these declarations, lets call them red and orange:
pros, cons?
The red set is properties, and the orange set is instance variables.
A property declaration tells the compiler to define a getter method, and possibly a setter method. (No setter method if the property is readonly.)
In newer versions of Objective C, declaring a property also creates an instance variable that is used to save values for the property. By convention the instance variable has the same name as the property, but with an "_" prefix. There is a way to change the name of the instance variable, but let's ignore that for now.
The property foo:
#property (nonatomic, strong) NSString *foo;
Would have a getter method:
- (NSString *) foo;
and a setter method
- (void) setFoo: (NSString *) foo;
That enables you to use code like this:
NSString *aString = self.foo;
or
NSString *aString = [self foo];
(2 different, equally valid ways of invoking the getter)
And invoking the setter
self.foo = #"a string";
or
[self setFoo: #"a string"];
(2 different, equally valid ways of invoking the setter)
Properties are really useful when you want to create a public interface to get and set values in your class from outside. If you declare a property as "atomic" the compiler adds additional code to the getter and the setter so reads and writes to the property are "thread safe", and can be accessed from background threads.
Before ARC, properties also were a very clean way to manage retains and releases. You declared a property as "retain" and the setter was written to retain the object that was passed in. That's less of an issue in ARC, because the system takes care of retains and releases for you.
It is also possible to write a custom getter or setter method that invokes your own code instead of the compiler-written code. You can use that to do things like log information, send notifications about changes, update labels, etc, etc. You simply add a method body to your .m file that has the same method signature as the getter or setter and the compiler uses that method instead of the automatically generated one.
As I said before, the code:
self.foo = #"a string";
is the same as
[self setFoo: #"a string"];
and invokes the setter method. The setter method sets the internal instance variable _foo.
However, the code
_foo = #"a string";
changes the instance variable directly, without invoking the setter. If you do define a property, you should use it instead of the instance variable.
Objective-c use to be simple...and tedious. You would declare instance variables for a class (the orange) and then you would define (usually) 2 methods for each, one so that an external class could set each instance variable to a new value, and one that returned the ivars value so an external object could read it. Aka, you had to write a getter and setter for each ivar. This was two lines of code in the interface and sometimes around 10 lines of code for the implementation file.
Then came properties, declared with #property. There was much rejoicing and drinking in the streets. These single #property lines told the compiler to write those methods for you, including the correct memory management code and even mutex locking code (depending on what you specified when declaring the #property)
Theres a whole lot of history, but nowadays, with automatic reference counting, it really only makes sense to use #properties in your interface, when you want to make an ivar public, and declare your private ivars in your implementation file.
Lastly, not that #property not only tells the compiler to implement your getter and setter, but it also automatically provides an instance variable with the same name, but prefixed with an underscore (this is only if you have implicit synthesizing of properies enabled...more history)
So, thats the difference. #property tells the compiler to write code for you (essentially). What code it actually writes is modified by all the different ways you can declare a #property.
I see someone writes codes like this
#interface SomeClass:<NSObject>{
NSString * _iVar;
}
#property(nonatomic,retain)NSString *iVar;
and in implement file
#synthesize iVar = _iVar;
But I always like write codes like this:
#interface SomeClass:<NSObject>
#property(nonatomic,retain)NSString *iVar;
and in implement file
#synthesize iVar = _iVar;
They all works fine, and I can use an instance someClass to get iVar "someClass.ivar".I just know why?
You now don't even have to synthesize the iVars anymore. The latest Xcode will automatically create a backing instance variable for each property, with the property name prefixed with an underscore.
In Objective-C when you are using anything via "." (dot syntax), it is accessing property. You have declared and synthesize a property with name iVar, so you can access it with someClass.iVar.
When you are declaring a property, a variable is automatically created with it. So, even if you do not create a variable, it will be there.
To avoid ambiguity, people are creating properties and iVar with different names. So, they can identify if the variable is being used or a property is being used. In that case people let the compiler know that a particular variable is associated with a property by low way:
#synthesize iVar = _iVar;
If you want access variable directly outside the class instead of property, you should use "->" (arrow syntax).
This is a two part question in hopes that I can understand more about the topic.
1) It seems to me that you have two popular options for declaring a property for a class in objective c. One is to add the property to the header's class body eg.
#interface MyClass : NSObject {
NSArray *myArray;
}
Or you can add it after the #interface body and before the #end statement like so.
#interface MyClass : NSObject {
//
}
#property (nonatomic, retain) NSArray *myArray;
What is the difference between these two "styles" and when do you choose one over the other?
2) after the #property you find options such as (nonatomic, retain). What are those for and why/when do you use different options?
Here are the only property modifiers that Xcode recognizes:
nonatomic (does not enforce thread safety on the property, mainly for use when only one thread shall be used throughout a program)
atomic (enforces thread safety on the property, mainly for use when multiple threads shall be used throughout a program) (default)
retain / strong (automatically retains / releases values on set, makes sure values do not deallocate unexpectedly) (default if ARC and object type)
readonly (cannot set property)
readwrite (can both set and get property) (default)
assign / unsafe_unretained (no memory management shall be done with this property, it is handled manually by the person assigning the value) (default if not ARC or object type)
copy (copies the object before setting it, in cases where the value set must not change due to external factors (strings, arrays, etc).
weak (automatically zeroes the reference should the object be deallocated, and does not retain the value passed in)
getter=method (sets the selector used for getting the value of this property)
setter= method (set the selector used for setting the value of this property)
1) #property is a special way to define getter- and setter-methods, or as we call them accessors in Objective-C. Your first snippet just declares an array for which you have to declare and write accessors yourself. For example setMyArray: and myArray.
Using #property will declare your accessors for you and is equivalent to declaring setMyArray: and myArray yourself. It is the preferred way to declare accessors since Objective-C 2.0. Note that you still have to declare the property (in your case myArray) yourself.
2) You first need to know about #synthesize. Remember that #property DECLARES the accessors for your property, #synthesize will IMPLEMENT them. When you use an #property in your #interface you mostly likely write an #synthesize in #implementation. Using #synthesize is equivalent to implementing setMyArray: and myArray.
The attributes (nonatomic, retain) tell the compiler, among others, how the memory management should work and therefore how the methods will be implemented. Note that you never actually see these accessors, but be assured that they are there and ready for you to be used.
To read more on that topic I recommend reading Section 9 on Properties from the following Tutorial or buy a Book that covers an Introduction to Objective-C.
Also you should familiarize yourself with at least the following attributes:
Access
Choose readwrite (default) or readonly. If readonly is set, ONLY the getter methods will be available.
Setter Memory Management
assign (default), simply assigns the new value. You mostly likely only use this with primitive data types.
retain, releases the old value and retains the new. If you use the garbage collector, retain is equivalent to assign. Why? The manual release of the old value will be done by the garbage collector.
copy will copy the new value and release the old value. This is often used with strings.
Threading
atomic (default) will ensure that the setter method is atomic. This means only one thread can access the setter at once.
nonatomic, use this when you don't work with threads.
This post gives you a good introduction to memory management and assign, retain and copy.
Properties are basically the accessor methods. They define the scope of the variable.
First case as given above,the variable is not accessible in other classes whereas by declaring a property as in the second case,variable is accessible in other classes also.
Also, they are useful for memory management.
First one will be private declaration and will not be accessible by other classes if you do not define the second one. Second is used together with #synthesize in .m module , and setter/getter's are created for you by the compiler. You can still define your own getter or setter with this. In this case all iVars defined in #property can be accessed by other classes.Retain/release operations are done automatically.
You should read Apple documentation for more details.
please check:
What's the difference between the atomic and nonatomic attributes?
Properties are basically the accessor methods. They define the scope of the variable. by default access specifior of variable is protected and properties set its Specifier from protected to Public