This question already has answers here:
Do declared properties require a corresponding instance variable?
(6 answers)
Closed 9 years ago.
Why in many fragment code declare instance variable like and for what? what different about property and non property
#import <Foundation/Foundation.h>
#interface class1:NSObject
{
NSMutableString *currentData;
}
#property (nonatomic, retain) NSMutableString * currentData;
What you saw is an "old code"... but sometimes you still need to support old versions (e.g. 10.5).
A property is simply a couple of getter and setter (well.. it depends on the attributes you choose: e.g. readonly will generate only a getter).
But a property operates (and so it needs) an instance variable. Usually what you see in the implementation file is something like
#implementation class1
#synthesize currentData = currentData;
#end
This means create getter and setter which uses currentData as variable.
For newer version you don't need to create the instance variable and you can just type the property and synthesize statement. In the most recent language version you don't even need the synthesize statement. Automatically an instance variable named _propertyName (underscore + name of the property) is created.
BTW: sometimes you still need to make your own getter and/or setter. Classic naming convention applies (e.g. - (void)setCurrentData: (NSMutableString*)newData; for setter and - (NSMutableString*)currentData; for getter), but same rules as before for properties: if you support only the most recent OSes you can just write the #property statement and right your getter and setter by using the "underscored" variable...
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This question already has answers here:
Under what conditions is #synthesize automatic in Objective-c?
(5 answers)
Closed 7 years ago.
I've only been in the Objective-C & Cocoa world for a year, so I wasn't around when properties weren't automatically synthesized
Whenever I create new classes in our project, I can declare #property BOOL testFlag without declaring an instance variable. Then, in the implementation, I could either call self.testFlag, [self testFlag], [self setTestFlag:NO]. But, I could also call _testFlag. I always assumed because properties are automatically synthesized with instance variables, that's why I can just append a "_" underscore before the property name to directly access the instance variable.
However, I have been refactoring really old files, that clearly were created before auto-synthesizing was a thing
So now in the header, I'd see something like this:
#interface testClass
{
BOOL _testFlag
}
#property BOOL testFlag;
In the implementation, there could be a custom setter/getter:
#implementation testClass
#synthesize testFlag = _testFlag;
-(void)setTestFlag:(BOOL)testFlag
{
_testFlag = testFlag;
}
-(BOOL)testFlag
{
return _testFlag;
}
But I thought because of auto-synthesizing, I could remove the _testFlag declaration in the header, and I could remove the #synthesize in the implementation. But when I do this, there are just a truck ton of errors; wherever somebody was directly trying to access the ivar _testFlag. (This actually includes the custom getter/setter above ^^, too)
Is there perhaps something I am missing with the project settings that prevent this old file from generating an implied instance variable for properties?
Remove the getter and setter to have automatic synthesis of the backing variable. When you supply both for the property, the assumption is that you're going to look after storage yourself.
This question already has answers here:
How does an underscore in front of a variable in a cocoa objective-c class work?
(9 answers)
Closed 9 years ago.
In the tutorial i am following, it creates a property of an array like so
#property (nonatomic, retain) NSMutableArray *entries;
And in the implementation file and defines it defines it as...
entries = [[NSMutableArray alloc] init];
However in my program, defining like that gives me an error
"Use of undeclared identifier 'entries'; did you mean '_entries'?"
Does this affect my program?
In simple:
#property (nonatomic, retain) NSMutableArray *entries;
creates the following code for you:
An ivar called _entries and two methods
- (void)setEntries:(NSMutableArray *)entries;
- (NSMutableArray *)entries;
If you want to give the ivar a different name (e. g. entries without underscore) you have to synthesize them. But you hardly ever need the actual, just use your properties like self.entries.
The rare cases where you actually want the ivar is when you want to override the setter and getter method
- (void)setEntries:(NSMutableArray *)entries
{
_entries = entries;
// do more stuff
}
When you declare property compilator create private variable which you can access with underscore (_entries) and compilator also create setter (if you do not specify readonly) and getter. You can call getter in two ways:
[self entries] or self.entries
When you declare property you should access in it two ways:
_entries - You access your private variable directly (can be use just inside the class)
self.entries - You can access this property view setter/getter it's safe way because this method manage way how to access it (release, retain, copy, etc.)
#property (nonatomic, retain) NSMutableArray *entries; is nothing but an instance variable of name _entries. (or any other name, that you can specify while doing the synthesize)
When you do #synthesize entries, two accessor methods (setter and getter) are created for you
- (void)setEntries:(NSMutableArray *)entries;
- (NSMutableArray *)entries;
If you access it directly as _entries, then it is just changing the value of the variable.
But if you say self.entries, the accessor methods are called. The accessor methods are made according to what property type you specified, strong (or retain) copy assign atomic, say assign will just copy the value to the variable, atomic makes sure that if setter and getter are called together from different threads, getter always gets the value either prior to the start of setter or after the completion of setter.
This question already has answers here:
What does #synthesize window=_window do?
(3 answers)
Closed 9 years ago.
I'm just starting to use Objective-C and I need to clarify something
When I #synthesize a #property, it is common convention to do the following:
#interface Class : ParentClass
#property propertyName
#end
#implementation
#synthesize propertyName = _propertyName;
#end
I've seen plenty of questions and answers suggesting that "_propertyName" is widely accepted as the "correct" way to synthesize properties. However, does it serve ANY purpose? Or is it merely to increase readability and identify instance variables?
It makes it so that if you accidentally leave off "self." you get a nice compiler error instead of silently not having your methods called.
From http://developer.apple.com/library/ios/#documentation/cocoa/conceptual/ProgrammingWithObjectiveC/EncapsulatingData/EncapsulatingData.html
You Can Customize Synthesized Instance Variable Names
As mentioned earlier, the default behavior for a writeable property is
to use an instance variable called _propertyName.
If you wish to use a different name for the instance variable, you
need to direct the compiler to synthesize the variable using the
following syntax in your implementation:
#implementation YourClass #synthesize propertyName =
instanceVariableName; ... #end
Also:
Note: The compiler will automatically synthesize an instance variable
in all situations where it’s also synthesizing at least one accessor
method. If you implement both a getter and a setter for a readwrite
property, or a getter for a readonly property, the compiler will
assume that you are taking control over the property implementation
and won’t synthesize an instance variable automatically. If you still
need an instance variable, you’ll need to request that one be
synthesized: #synthesize property = _property;
By doing this the generated accessor actually got to know which variable(iVar) to use.
Yea, It increases the readability & also separates the private & public variables to understand & use. Private variable of Class generally written in "propertyName" format.You can say it is a coding convention where Private Variable Names use '' as prefix and Public Variables or Property Names are lowerCamelCase.
This question already has answers here:
Closed 10 years ago.
Possible Duplicate:
Is there a difference between an “instance variable” and a “property” in Objective-c?
Difference between self.ivar and ivar?
What is the difference between declaring variables in brackets immediately after the #interface line, and defining properties below?
For example...
#interface GCTurnBasedMatchHelper : NSObject {
BOOL gameCenterAvailable;
BOOL userAuthenticated;
}
#property (assign, readonly) BOOL gameCenterAvailable;
Defining the variables in the brackets simply declares them instance variables.
Declaring (and synthesizing) a property generates getters and setters for the instance variable, according to the criteria within the parenthesis. This is particularly important in Objective-C because it is often by way of getters and setters that memory is managed (e.g., when a value is assigned to an ivar, it is by way of the setter that the object assigned is retained and ultimately released). Beyond a memory management strategy, the practice also promotes encapsulation and reduces the amount of trivial code that would otherwise be required.
It is very common to declare an ivar in brackets and then an associated property (as in your example), but that isn't strictly necessary. Defining the property and synthesizing is all that's required, because synthesizing the property implicitly also creates an ivar.
The approach currently suggested by Apple (in templates) is:
Define property in header file, e.g.:
#property (assign, readonly) gameCenter;
Then synthesize & declare ivar in implementation:
#synthesize gameCenter = __gameCenter;
The last line synthesizes the gameCenter property and asserts that whatever value is assigned to the property will be stored in the __gameCenter ivar. Again, this isn't necessary, but by defining the ivar next to the synthesizer, you are reducing the locations where you have to type the name of the ivar while still explicitly naming it.
{
BOOL gameCenterAvailable;
BOOL userAuthenticated;
}
the above two are called member Variables
They can't be accessed outside the class.(Important point) (unless you provide custom getters and setters)
if you make a #property then the variable can be read inside the class as well as outside the class..so the setters and getters are generated for you..automatically
then declaring the same as a member variable isn't required..
It is just done to increase Readability .. you can read it easily than reading
#property (non..)
When you define a property a getter and setter is created for you. When you access them usingobject.member setters and getters are called automatically.
When you declare variable in interface setters and getters are not written for you. you can also specify some visibility modifiers to them like #private,#public etc.
This question already has an answer here:
Closed 11 years ago.
Possible Duplicate:
Properties declared as instance variables too?
Let's say I have an objective c class interface like:
#interface {
NSString * someString;
}
#property (nonatomic, retain) NSString * someString;
with an #implementation that #synthesizes the string and does some other stuff.
What I'd like to know is, whether it's necessary to have that NSString * someString object pointer declaration within the brackets at all. (As an aside, is there a specific term for the space between the brackets where variables are defined?) It seems like the code will work fine if I omit the NSString * someString declaration, and the #property line has all the same information about type. So, if it's not necessary to have the variable declaration, why does the option exist at all? What happens if you have conflicting types in the variable declaration and the #property declaration?
For the most part, it shouldn't be necessary, since #synthesize will create a variable if it is not predefined. Also, you can create your own variable with #synthesize as well, using:
#synthesize aVar = _theVariableName;
That will create a getter method (aVar), a setter method (setAVar), and an instance variable named _theVariableName which you can use for memory management purposes.
However, I don't know if #dynamic will do the same, and if implementing your own setters and getters, I think you need the declaration...