Okay, this is admittedly a bit of a best practice question, but I want to get it right, so hopefully someone can enlighten me:
Scenario is pretty standard, but has one twist:
I have a class in a framework I write that inherits directly from NSObject. It has a designated initializer with quite a few arguments, most of which are nonnull. Since the thing is part of a framework, I explicitly use the NS_DESIGNATED_INITIALIZER macro (which I don't always do in smaller, personal apps).
The problem is that this leads to XCode warning me to also override init, i.e. the superclass's designated initializer. But additionally it demands that I call my designated initalizer from it, which I can't do, because I simply lack meaningful defaults for its arguments.
I don't really want to throw an exception in the "small" init, I'd much rather like to return nil.
To get rid of the warning, I added init as a second designated initalizer in my class's extension like so:
#interface MyClassName ()
// some other stuff not relevant`
-(nullable instancetype)init NS_DESIGNATED_INITIALIZER;
#end
Now I can safely just return nil; in the overriden init method like I wanted.
That means my documentation (I'm using appledoc) and by extension XCode's code completion won't tell someone using my framework that init is actually also a designated initializer (so they won't accidentally use it), but it is still there (in unit tests, for example, this could come in handy).
My question is: Are there any dangers to this besides from someone actually using it in production, happily sending messages to nil afterwards without realizing? Would this be one of the few cases where throwing an exception in init would be preferable?
Rather than just returning nil from init (and maybe adding a comment saying you shouldn't call it) – you should mark it as unavailable.
Not only will this dismiss the warning about you not overriding NSObject's designated initialiser – it will also generate a compile-time error if anyone tries to call init instead of your designated initialiser.
To do this, you can either use the NS_UNAVAILABLE macro, or use the unavailable __attribute__ as shown by this answer. The advantage of using the __attribute__ is you can specify a message that the compiler will present to the user.
For example:
#interface Foo : NSObject
-(instancetype) init __attribute__((unavailable("You cannot create a foo instance through init - please use initWithBar:")));
-(instancetype) initWithBar:(Bar*)bar NS_DESIGNATED_INITIALIZER;
#end
...
Foo* fooA = [[Foo alloc] init]; // ERROR: 'init' is unavailable: You cannot create a foo instance through init - please use initWithBar:
Foo* fooB = [[Foo alloc] initWithBar:[[Bar alloc] init]]; // No error
Related
I believe the image is pretty descriptive of my problem. If I define only a custom getter as below:
- (UIImage *) secondaryIconImage {
if (_secondaryIconImage) return _secondaryIconImage;
return nil;
}
then I get an error: 'cannot pair a synthesized setter with a user defined getter'. However, if I try to declare the setter as well my code won't compile at all:
- (void) setSecondaryIconImage:(UIImage *)secondaryIconImageVar {
_secondaryIconImage = secondaryIconImageVar;
}
In particular this gives me the compile time error 'Use of undeclared identifier _secondaryIconImage'
I thought perhaps once I declared the setter I would have to synthesize the instance variable myself, so I tried adding:
#synthesize _secondaryIconImage;
This didn't help. What's going on here and how can I fix it?
p.s. I have a custom getter because I will eventually need to do something fancier, but I didn't see a reason to complicate the example since even this basic example is giving me a problem.
As it notes, you cannot define the getter without the setter. (Your getter makes no sense of course, so I assume your actual getter is more complex?)
If you define the getter and the setter, nothing will be synthesized for you (since you've written everything, there's nothing to synthesize), and no ivar will be defined for you. You need to define it yourself if you need it.
You need to explicitly request an ivar via your #synthesize (thanks to Richard Ross for setting me straight):
#synthesize secondaryIconImage = _secondaryIconImage;
Your code asks for a property called _secondaryIconImage, so the ivar would be __secondaryIconImage.
Wherever I've read, it's written to never use an object without calling it's initializer function. And initializer functions always "have to" start with "init".
Is there a reason behind this naming (Does naming a method starting with init do something special)? What happens if I do not call the initializer function i.e. if I just do [MyClass alloc] and start using the object?
Alloc is called to allocate space in memory for the data type you are specifying. Whether it is NSString or NSNumber, calling Alloc will reserve the most efficient space in memory as possible for that data type (small or large).
Init is called to instantiate the class and superclass's important variables. These variables could include the Rect to recalculate a certain size in order to layout subviews, or perhaps instantiate with a delegate of some kind to perform some protocol upon creation. If it all becomes too much for you Objective-C does allow you to instantiate objects like MyClass *myObject = [MyClass new];
It may all seem redundant and a waste of time, but fortunately Swift has cut down tremendous amounts of redundancies like this in the new programming language. Now all you have to do is var myObject = MyClass() and if there are any custom initializers they would likely be writen like so var myObject = MyClass(frame: CGRectZero)
Happy coding!
I recommend reading the Apple Documentation on Object Initialization, and Initialization.
Is there a reason behind this naming (Does naming a method starting with init do something special)?
It is the convention, beginning a method name with init does not do anything special.
There are some minor quirks, such as if a method beginning with init returns type id, the compiler will convert the return type to instancetype, but these are barely worth mentioning.
What happens if I do not call the initializer function i.e. if I just do [MyClass alloc] and start using the object?
If you're using a standard SDK class, you'll likely run into a crash / exception. Initialization is used to set up the initial state of the instance variables, and without doing this can lead to undefined behaviour.
For a small example, if you called [[MyObject alloc] vegetable]; with the following class, you'd get nil returned because _vegetable hasn't been assigned.
#interface MyObject : NSObject
-(NSString*)vegetable;
#end
#implementation MyObject {
NSString *_vegetable;
}
-(instancetype)init {
self = [super init];
if (self) {
_vegetable = #"Cabbage";
}
return self;
}
-(NSString*)vegetable {
return _vegetable;
}
#end
It is the way you create objects in Objective-C. It is a hard requirement of the language. Creating an object in Objective-C is a 2 step process: alloc and init.
Under the covers, the reason you must call init is, wait for it, initialization.
The call to alloc creates a block of memory for your object and zeros it out. Calling init allows the object and it's ancestors to set things up so the object is ready to function. It initializes the object's memory and does other housekeeping that set the object up.
Further, every object's init method needs to call super init, so the initialization goes all the way up the object chain, all the way to NSObject. All the ancestor classes are designed to assume that their init method is called.
+ (id)alloc;
and
- (id)init;
are methods from NSObject.h
The alloc does
+ (id)alloc {
return _objc_rootAlloc(self);
}
id
_objc_rootAlloc(Class cls)
{
#if 0 && __OBJC2__
// Skip over the +allocWithZone: call if the class doesn't override it.
// fixme not - this breaks ObjectAlloc
if (! ((class_t *)cls)->isa->hasCustomAWZ()) {
return class_createInstance(cls, 0);
}
#endif
return [cls allocWithZone: nil];
}
It does memory allocation, and return a class Instance.
But when I came to the init method, this is the implementation
- (id)init {
return _objc_rootInit(self);
}
id
_objc_rootInit(id obj)
{
// In practice, it will be hard to rely on this function.
// Many classes do not properly chain -init calls.
return obj;
}
It only return self object (NSObject) without doing any initialization.
The documentation also says the same thing.
"The init method defined in the NSObject class does no initialization; it simply returns self."
If that is the case, alloc method alone is sufficient.
Init method is only required for overridding.
Any explanation here?
This is the implementation source NSObject.mm
http://www.opensource.apple.com/source/objc4/objc4-532/runtime/NSObject.mm
alloc is to do with memory allocation while init (or initX etc., the init family) is to do with configuring that allocated memory as needed when an object is created - whether any particular class, now or in the future following some revision, needs to do any work in init is dependent on the semantics of that class. However as you don't know for any arbitrary class whether it's init needs to do any work you must call it, and as any arbitrary class does not know whether its superclass needs to do any initialisation to must call its superclasses init within its own init. For this chain to work NSObject must have an init, it so happens that it (currently, who knows in the future) does no work. NSObject's init is the end of the chain and the only one that does not need to call another init.
Confusion arises for some as many languages combine the two operations, allocation and initialisation, into one indivisible operation, e.g. new in Java. Indeed Cocoa has a new as well which is defined as alloc followed by init.
And Apple should really have written:
The init method defined in the NSObject class currently does no initialization; it simply returns self.
or simply said nothing.
HTH
CRD explained it pretty well.
I will state it more strongly, however.
The way you create a new object in Objective C is to use alloc/init.
You should always initialize every object, and custom init methods should always call [super init]. Consider failure to call init on ALL objects an error, and also consider calling [super init] in your custom init methods an error.
I am following Big Nerd Ranch iOS Programming by Joe Conway and am kinda puzzled when I saw the following code.
WebViewController.h
#import <Foundation/Foundation.h>
#interface WebViewController : UIViewController
#property (nonatomic, readonly) UIWebView *webView;
#end
WebViewController.m
#import "WebViewController.h"
#implementation WebViewController
- (void)loadView
{
// Create an instance of UIWebView as large as the screen
// Tell web view to scale web content to fit within bounds of webview
}
- (UIWebView *)webView
{
return (UIWebView *)[self view];
}
#end
Shouldn't one synthesize the property declared in .h file? Xcode didn't give an warning either (which it usually does when I declare a property with synthesizing).
By the way, in the book, he also mentioned
In WebViewController.h, add a property (but not an instance variable)
Doesn't declaring a property automatically generate an instance variable for you? Let me know what I missed. Thanks.
This is a good question, and one to which I think the answer is a bit subtle and often not immediate to people starting out with Objective-C. First of all let's dispel a myth about properties. The myth is this:
Objective-C properties are related to instance variables.
This is not necessarily true. It is often true, but not true in every case. Here's what is true:
An Objective-C property declares a getter and a setter method
(Or in the case of a readonly property, just a getter). In particular when you declare a property called foo (for the sake of discussion, let's assume it's not readonly), the only thing you are really doing is telling the compiler that the class you're writing has a method called -foo and a method called -setFoo:. By adding an #synthesize foo; declaration into the .m file, you are telling the compiler to generate those methods itself. Here you are saying, "Compiler, create an instance variable (also called foo) and generate implementations for my -foo and setFoo: methods that access this instance variable. In particular, return the value of this variable in -foo and set the value of the variable in -setFoo:.
Note, it is not necessary to have the compiler synthesize the methods for you. Rather, you can write -foo and -setFoo: yourself. Furthermore, you are not in any way required to create an instance variable to support the implementation of these methods. You can write them however you wish.
To summarize: the #property only tells the compiler about the existence of the getter and setter methods. You can then either: #synthesize the methods in your .m or write the methods yourself.
This is because the "webView" getter method is implemented in the .m file and because of that, "#synthesize" isn't necessary.
If a "webView" method wasn't explictly created in code, then the compiler would complain about the property not being synthesized. Synthesizing a "read only" property, in this case, would only create a "getter" method which would do roughly the same thing you see in the code up there.
And yes, according to the Apple docs on declared properties, it says this about "#synthesize":
You use the #synthesize directive to tell the compiler that it should synthesize the setter and/or getter methods for a property if
you do not supply them within the #implementation block. The
#synthesize directive also synthesizes an appropriate instance
variable if it is not otherwise declared.
I want to force user to use my own init method (for example -(id)initWithString:(NSString*)foo;) and not the basic [[myObject alloc]init];.
how can I do that?
All other answers here are outdated. There is a way to do this properly now!
While it is easy to just crash at runtime when somebody calls your method, compile-time checking would be far preferable.
Fortunately, this has been possible in Objective-C for a while.
Using LLVM, you can declare any method as unavailable in a class like so
- (void)aMethod __attribute__((unavailable("This method is not available")));
This will make the compiler complain when trying to call aMethod. Great!
Since - (id)init is just an ordinary method, you can prohibit calling of the default (or any other) initializer in this way.
Note, though, that this will not insure against the method being called using the dynamic aspects of the language, for instance via [object performSelector:#selector(aMethod)] etc. In the case of init, you won't even get a warning, because the init method is defined in other classes, and the compiler doesn't know enough to give you an undeclared selector warning.
So, to ensure against this, make sure that the init method crashes when being called (see Adam's answer).
If you want to disallow - (id)init in a framework, make sure to also disallow + (id)new, as this will just forward to init.
Javi Soto has written a small macro to forbid using the designated initializer faster and easier and to give nicer messages. You can find it here.
tl; dr
Swift:
private init() {}
Since all Swift classes include an internal init by default, you can change it to private to keep other classes from calling it.
Objective C:
Put this in your class's .h file.
- (instancetype)init NS_UNAVAILABLE;
This relies on an OS define that prevents the method named from being called.
The accepted answer is incorrect - you CAN do this, and it's very easy, you just have to be a bit explicit. Here's an example:
You have a class named "DontAllowInit" which you want to prevent people init'ing:
#implementation DontAllowInit
- (id)init
{
if( [self class] == [DontAllowInit class])
{
NSAssert(false, #"You cannot init this class directly. Instead, use a subclass e.g. AcceptableSubclass");
self = nil; // as per #uranusjr's answer, should assign to self before returning
}
else
self = [super init];
return nil;
}
Explanation:
When you call [super init], the class that was alloc'd was the SUBCLASS.
"self" is the instance - i.e. the thing that was init'd
"[self class]" is the class that was instantiated - which will be SUBCLASS when the SUBCLASS is calling [super init], or will be the SUPERCLASS when the SUPERCLASS is being called with plain [[SuperClass alloc] init]
So, when the superclass receives an "init" call, it just needs to check whether the alloc'd class is the same as its own class
Works perfectly. NB: I don't recommend this technique for "normal apps" because usually you INSTEAD want to use a Protocol.
HOWEVER ... when writing Libraries ... this technique is VERY valuable: you frequently want to "save (other developers) from themselves", and its easy to NSAssert and tell them "Oops! you tried to alloc/init the wrong class! Try class X instead...".
-(id) init
{
#throw [NSException exceptionWithName: #"MyExceptionName"
reason: #"-init is not allowed, use -initWithString: instead"
userInfo: nil];
}
-(id) initWithString: (NSString*) foo
{
self = [super init]; // OK because it calls NSObject's init, not yours
// etc
Throwing the exception is justified if you document that -init is not allowed and therefore using it is a programmer error. However, a better answer would be to make -init invoke -initWtihString: with some suitable default value i.e.
-(id) init
{
return [self initWithString: #""];
}
Short answer: you can't.
Longer answer: the best practice is to set your most detailed initializer as the designated initializer, as described here. 'init' will then call that initializer with sane, default values.
Another option is to 'assert(0)' or crash in another way inside the 'init', but this isn't a good solution.
I actually voted up Adam's answer, but would like to add some things to it.
First, it is strongly encouraged (as seem in auto-generated init methods in NSObject subclasses) that you check self against nil in inits. Also, I don't think class objects are guaranteed to be "equal" as in ==. I do this more like
- (id)init
{
NSAssert(NO, #"You are doing it wrong.");
self = [super init];
if ([self isKindOfClass:[InitNotAllowedClass class]])
self = nil;
return self;
}
Note that I use isKindOfClass: instead because IMHO if this class disallows init, it should disallow its descendants to have it as well. If one of its subclass want it back (which doesn't make sense for me), it should override it explicitly by calling my designated initializer.
But more importantly, whether you take the above approach or not, you should always have appropriate documentation. You should always clearly state which method is your designated initializer, try as best as you can to remind others not to use inappropriate initializers in documentation, and put some faith in other users/developers, instead of trying to "save everybody else's asses" with clever codes.