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Obj-C #synthesize
Synthesized property and variable with underscore prefix: what does this mean?
Whats the use of having synthesize statement like this
#synthesize popOverTableData = _popOverTableData;
Thanks!
It simply states that the backing variable for popOverTableData is _popOverTableData. Normally without this, and just doing #synthesize popOverTableData means that the backing variable is called the same.
Related
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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.
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How does an underscore in front of a variable in a cocoa objective-c class work?
Prefixing property names with an underscore in Objective C
When we declare a property and then synthesize it like this for example:
#synthesize name = _name;
So, the _name is an instance variable for the name property we are gonna use in the following implementation.
My question is why we need this ivar and what would happen if i didn't create the _name ivar?
Thank you.
My understanding is that there is a default ivar name, which is the same as the property name. One of the reasons for specifying your own, with an underscore, is to eliminate ambiguity in your setter:
-(void) setFoo:(id) foo {
// here, foo should ONLY refer to the passed-in var
// If your ivar is the same, it is ambiguous.
// If your ivar is _foo, then there is clarity.
}
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Prefixing property names with an underscore in Objective C
When synthesizing properties I found out that someone is doing:
#synthesize myVar = _myVar;
what is "_myVar" and which is the difference with simply doing :
#synthesize myVar;
Lastly when I should prefer the first solution to the last one?
Thanks
Luca
What _myVar really is in your example, is the name of the ivar that is backing your property. By default, when you synthesize a property, an ivar of the same name is created for you. So, you can use your property to set your ivar through setter/getter or the _myVar to directly access your variable (bypassing KVC/KVO of course).
EDIT:
From Apple's Coding Guidelines for Cocoa
...In many cases, when you use a declared property you also synthesize
a corresponding instance variable.
Make sure the name of the instance variable concisely describes the
attribute stored. Usually, you should not access instance variables
directly, instead you should use accessor methods (you do access
instance variables directly in init and dealloc methods). To help to
signal this, prefix instance variable names with an underscore (_)...
If you want to use some existing data member in setter and getter then it can be specify like that.
e.g. #synthesize personName=pName;
by this we can use pName instead of personName as per our convenience.
It the name of the private variable.
Se my answer on an other post: answer
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How does an underscore in front of a variable in a cocoa objective-c class work?
I'm currently learning Objective-C and I'm following a tutorial that uses variables a little funny (to me at least!).
Basically, class variables are declared like this:
#interface .. {
UITextField *_titleField;
UIImageView *_imageView;
}
#property (retain) IBOutlet UITextField *titleField;
#property (retain) IBOutlet UIImageView *imageView;
and then synthesized like this:
#synthesize titleField = _titleField;
#synthesize imageView = _imageView;
So basically, whats the purpose of this?
titleField is the synthesized property and _titleField is the backing field for it.
Maybe http://mutelight.org/articles/the-objective-c-retain-property-pattern will help you understand it better
Often people use the _ character as a marker for an instance variable (a.k.a. in other languages a ‘field’, or ‘member’, etc). Some people put an underscore after the variable name, some people put it before, some people don't use it at all, some people use different prefixes. The idea is that it helps you distinguish at a glance what are instance variables and what aren't.
Of course, if you do decide to name your instance variables in a particular way, but still want your properties to have ‘normal-looking’ names, you need to map the ‘normal-looking’ name to the ‘instance variable’ name.
A property is just a marker for a pair of methods - 'foo' and 'setFoo' (unless the property is readonly, in which case only 'foo' will by synthesized). A variable (those are instance, not class variables, BTW) is the actual memory store. Properties can be associated with a memory store - this is what #synthesize does - but don't have to. The point being, properties and instance variables often go together but are distinct.
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What does #synthesize window=_window do?
How come when you #synthesize something like buttonPressed , You need to do this:
#synthesize buttonPressed = buttonPressed_;
I've been following some tutorials and this keeps coming up. Why?
You do not have to do it that way.
By default, #synthesize variableName does work, if your synthesized accessors shall have the same name as your instance variable.
In your example, the instance variable is called buttonPressed_ but your accessor methods will omit the _ and thus just be called setButtonPressed and buttonPressed.