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I have seen many posts here on Stack Overflow warning Objective-C users not to compare objects with the == operator. For example, the following is always recommended against.
if (string1 == string2) {} // string1 and string2 are both of type NSString *
I understand this warning as the == operator in this case checks for pointer equality, not the actual values of string1 and string2. If this is the case, why do I so often see people comparing their objects to nil with the == operator. For example, I have seen the following several times.
if (obj == nil) {} // obj is of type id, or any other object
If comparing pointers with the == operator is bad practice on things like NSStrings (or NSNumbers or NSArrays etc), then why is it so commonplace with nil.
Is this the standard because there is only one type of nil i.e.: nil always equals nil? Why are direct comaprisons using the == operator frowned upon between objects, but let slip when comparing to nil?
Thank your for your time.
Using == tests for pointer equality, which seems to be what you want to test for in this particular case. You are checking whether some pointer is a nil pointer. Specifically, a pointer that equals zero. Comparing objects is more complicated as you've pointed out. Equality between objects often relies on some internal part of the two objects being "the same".
In the case of strings, "equality" can mean "has the same characters" but it could also mean "has the same characters (ignoring capitalization)". There are also cases where you might want a nil string to equal an empty string or even a string with just whitespace. The point is that you must understand the different methods of determining equality in order to implement your own definition of equality for your objects.
Note that checking for pointer equality is almost never what you want to use when comparing things like strings or arrays. There are some interesting optimizations that can throw you off. For example, all empty NSArray's point to the same singleton object.
nil is like NULL from C or std::nullptr in C++. Basically points to nothing.
When a object in Objective-C is unallocated (haven't been alloc), the pointer will be nil.
So if (obj == nil) will check if obj is allocated (or initialized as they often happen together)
This technique is often used when writing a custom initializer or checking if some object exists or not.
I'm reading a book on Objective-C and the author said that if local variables aren't assigned a value they will be set to nil, but static variables will be set to zero. So, I set up int a and didn't assign it a value. Then NSLog(#"%i", a) to display it and a was displayed as zero. I was a little confused on that and I was wondering if someone could clarify it for me?
With ARC enabled, your Objective-C object pointer variables will be set to nil regardless of where you create them.
Without ARC, and for built in C types, your variables will not be initialized.
Instance variables of Objective-C objects are always set to 0 (or nil) when you allocate an object.
Statics are set to 0.
I've gotten in the habit of always giving a default value to variables, though. It's been a good habit to have.
No2. Just as in "plain" C, local variables are not assigned a default value. (Although you may get lucky the first time part of the stack is used: do not rely on this!.)
Anyway, nil is 01 -- that is, nil == 0 is always true -- so NSLog("#%i", nil) says "hey, log the argument as an integer", which is ... 0.
Happy coding.
1 See nil in gdb is not defined as 0x0? which covers the technical definition, including the Objective-C++ case, in more detail. Note that the type changes depending upon architecture as well so "#%i" could very well be wrong for a particular system.
2 See wbyoung's answer for ARC-specific rules.
This question already has answers here:
Sending a message to nil in Objective-C
(11 answers)
Closed 9 years ago.
I just started reading the Objective-C tutorials, and there is a section on "sending a message to nil":
There are several patterns in Cocoa
that take advantage of this fact. The
value returned from a message to nil
may also be valid:
What does this mean? I can't seem to follow it.
The special treatment of nil means that you can do the following:
SomeClass * someObject;
someObject = nil;
[someObject doSomething];
And you can be assured that nothing will happen.
Now, why is this important?
In Objective-C, sending a message to an object means telling that object to do something, or asking that object for some information. Some examples:
[someObject updateRecords]; // 1
x = [someObject size]; // 2
Line 1 sends someObject a message called updateRecords, and line 2 sends the same object a message called size, which is expected to return a value. These messages boil down to method calls, and the actual code that ends up being run is determined by the Objective-C runtime system, since Objective-C is a dynamically-typed language.
To determine which method to invoke, the runtime system reads information from the address of the object in question (someObject, in the examples above) to work out what class it is an instance of. Using that information, it is able to look up the appropriate method to call, and when all that has been figured out, it executes the code in the method.
If the runtime system did not treat nil as a special case, it would probably crash if you tried to execute the code shown at the top. nil is defined to be zero, so the runtime would start reading information from an address stored at location zero in memory, which is almost gauranteed to be an access violation.
nil is basically a null pointer (i.e. it is the number zero stored in a pointer).
All messages to nil are legal (they won't cause a crash), but they don't do anything.
All messages to nil return nil, or 0, or 0.0, or NO, depending on the return type.
You can send any message to nil. Nothing happens.
What exactly is it you don't understand in those docs?
The great thing about nil messaging compared to other languages like C# is that you can write code that performs multiple method calls without having to test for nil at each step.
id obj1 = [SomeClass object];
id obj2 = [obj1 doSomething];
id obj3 = [obj2 anotherMethod];
id thingICareAbout = [obj3 doSomethingElse];
If you go through several steps to get to thingICareAbout, it saves a lot of unnecessary lines of code to not have to test if obj1, obj2 and so on are nil before using them. You can just check if thingICareAbout is nil once at the end if you need to. Sometimes you don't even have to do that, if your code still works when it's nil (or 0 for primitive values).
In C# you would have had to explicitly check if each object is nil, set up exception handling around that block of code, or just hope none of the intermediate objects are ever nil.
One other thing to keep in mind (that I just learned myself!) is that 10.5 changed this behavior-- it used to be that it was only safe for integers and pointers to objects, not structs or floating point values. You might see some additional error checking when you're looking at other code because of this.
It does what you would expect: nothing.
Currently I understand it as a kind of "empty object". But what's it really?
Objective-C objects
First of all, when you call this:
id someObject = [NSArray array];
someObject isn't the array object directly but only a pointer to it. That means, if someObject is equal to 0x1234 there's an object at that address in the memory.
That's the reason why
id someOtherObject = someObject;
doesn't copy the object. Both pointers point now to the same object.
Pointer to 0x0
So, how is nil defined? Let's take a look at the source code:
objc.h
#define nil __DARWIN_NULL /* id of Nil instance */
_types.h
#ifdef __cplusplus
…
#else /* ! __cplusplus */
#define __DARWIN_NULL ((void *)0)
#endif /* __cplusplus */
Looks like nil is a pointer to the address 0x0.
So what?
Let's see what the Objective-C Programming Reference has to say:
Sending Messages to nil
In Objective-C, it is valid to send a
message to nil—it simply has no effect
at runtime. There are several patterns
in Cocoa that take advantage of this
fact. The value returned from a
message to nil may also be valid: …
The returned values are either nil, 0 or a struct with all variables initialized to 0. Which one it is depends on the expected return type. There is an explicit check in the objective-c runtime for messages to nil, that means it's really fast.
Nil, nil, NULL
Those are the 3 types. Here are all the definitions:
#define Nil __DARWIN_NULL /* id of Nil class */
#define nil __DARWIN_NULL /* id of Nil instance */
#define NULL __DARWIN_NULL
#define __DARWIN_NULL ((void *)0)
As can be seen, they are all exactly the same. Nil and nil are defined by Objective-C, NULL comes from C.
What's the difference then? It's only about style. It makes the code more readable.
Nil is used as a non-existent class: Class someClass = Nil.
nil is used as a non-existent instance: id someInstance = nil.
NULL is a pointer to a non-existent memory part: char *theString = NULL.
Short
nil isn't an empty object but a non-existent one. A method -getSomeObject doesn't return an empty object if it doesn't exist but returns nil which tells the user that there is no object.
Maybe this makes sense: (Both would compile and run.)
if (anObject == nil) { // One cannot compare nothing to nothing,
// that wouldn't make sense.
if (anObject) { // Correct, one checks for the existence of anObject
It's not an empty object, it's the lack of any object at all. The rest of the answers cover the other semantics, so I'll leave it at that :)
nil should only be used with pointers, and nil represents a pointer which points to nothing (it's value is zero)
NSString *myString = nil; // myString points to nothing
int x = nil; // invalid, "x" is not a pointer, but it will compile
It's "nothing". But a "nothing" you can send messages to without getting killed as you would if you were trying to call a method on NULL.
You can look at this question to have more info on NULL vs. nil
It's a null pointer - a pointer to "nothing".
Formally defined, it is as Joshual defined - a pointer to nothing or a pointer to no object at all.
From a practical, implementation perspective, particularly when dealing with data structures and algorithms, a nill often will represent a sentinel in a data structure or an object that represents "nothing" for example, in a red-black tree, technically, all of the leaf nodes are "nill", but they still have the same or similar properties & operations of a leaf node (color, pointer to a parent, etc.) - in those cases, it is really a "nothing object" ... if that makes any sense.
So, formally, it is a pointer to nothing, in practice, it is often treated as a representation of nothing, but it is never ... null.
One useful way of thinking about nil is imagining that it's the empty object that "does nothing".
By "does nothing" I mean, any message you send it won't have side effects.
It's "empty" in the sense that when you ask it for the value of any property, it always returns nil. So it's not holding any values --> it's empty.
(Actually, it's a little more complicated than that, when you ask for the value of a property that returns a type that's NOT an obj-c object. You will get back a pointer-sized 0. So for any scalar values that are no larger than sizeof(void*) you get 0. But if you ask for a struct or a double on a 32 bit system, you get an undefined result. I've written about this here.)
Maybe I'm just missing the obvious but this seems like a semantic question. As in, you can use lots of different words to describe it, but your description works well enough already.
http://en.wikipedia.org/wiki/Null_(computer_programming)
Just to clarify, in Objective-C nil and null are not the same thing. Neither do they represent the same thing in Objective-C as they do in mathematics or other programming languages.
Nil is actually a special memory address of 0x0 (at least the compiler treats it as an address.) Nil is used as the address of an object that is named but not allocated. This allows for test for the existence/allocation of named objects as well as providing a safe way to send messages to objects that might not exist. While in math and some languages you can compare scalar variables to nil, in Objective-c, you should not. You should only compare the address of objects.
NULL by contrast can mean either the standard C defined as an integer value ofNULL==0 or it can represents an actual allocated object of the class NSNull with a specific address in memory. NSNull is however, a singleton object i.e. only one exist for every application. It is used as placeholder for other objects usually in collections. You cannot use it in comparisons with any other object. You can only check if a particular pointer points to the singleton NSNull object. Note however that as an allocated object[NSNull null]!=NULL,
The confusion between nil and NULL arises because assigning an address of NULL assigns it to the nil address of 0x0. Such code works but can cause subtle problem at times. It's best to get in the habit of not confusing the two.
So, in Objective-C, nil, NUll and NSNull are three different but overlapping concepts that are easy to confuse. Nil should be used for addresses, NULL should be used for scalar values and NSNull should be used as a placeholder for allocated objects.
As a Java developer who is reading Apple's Objective-C 2.0 documentation: I wonder what "sending a message to nil" means - let alone how it is actually useful. Taking an excerpt from the documentation:
There are several patterns in Cocoa
that take advantage of this fact. The
value returned from a message to nil
may also be valid:
If the method returns an object, any pointer type, any integer scalar
of size less than or equal to
sizeof(void*), a float, a double, a
long double, or a long long, then a
message sent to nil returns 0.
If the method returns a struct, as defined by the Mac OS X ABI Function
Call Guide to be returned in
registers, then a message sent to nil
returns 0.0 for every field in the
data structure. Other struct data
types will not be filled with zeros.
If the method returns anything other than the aforementioned value
types the return value of a message
sent to nil is undefined.
Has Java rendered my brain incapable of grokking the explanation above? Or is there something that I am missing that would make this as clear as glass?
I do get the idea of messages/receivers in Objective-C, I am simply confused about a receiver that happens to be nil.
Well, I think it can be described using a very contrived example. Let's say you have a method in Java which prints out all of the elements in an ArrayList:
void foo(ArrayList list)
{
for(int i = 0; i < list.size(); ++i){
System.out.println(list.get(i).toString());
}
}
Now, if you call that method like so: someObject.foo(NULL); you're going to probably get a NullPointerException when it tries to access list, in this case in the call to list.size(); Now, you'd probably never call someObject.foo(NULL) with the NULL value like that. However, you may have gotten your ArrayList from a method which returns NULL if it runs into some error generating the ArrayList like someObject.foo(otherObject.getArrayList());
Of course, you'll also have problems if you do something like this:
ArrayList list = NULL;
list.size();
Now, in Objective-C, we have the equivalent method:
- (void)foo:(NSArray*)anArray
{
int i;
for(i = 0; i < [anArray count]; ++i){
NSLog(#"%#", [[anArray objectAtIndex:i] stringValue];
}
}
Now, if we have the following code:
[someObject foo:nil];
we have the same situation in which Java will produce a NullPointerException. The nil object will be accessed first at [anArray count] However, instead of throwing a NullPointerException, Objective-C will simply return 0 in accordance with the rules above, so the loop will not run. However, if we set the loop to run a set number of times, then we're first sending a message to anArray at [anArray objectAtIndex:i]; This will also return 0, but since objectAtIndex: returns a pointer, and a pointer to 0 is nil/NULL, NSLog will be passed nil each time through the loop. (Although NSLog is a function and not a method, it prints out (null) if passed a nil NSString.
In some cases it's nicer to have a NullPointerException, since you can tell right away that something is wrong with the program, but unless you catch the exception, the program will crash. (In C, trying to dereference NULL in this way causes the program to crash.) In Objective-C, it instead just causes possibly incorrect run-time behavior. However, if you have a method that doesn't break if it returns 0/nil/NULL/a zeroed struct, then this saves you from having to check to make sure the object or parameters are nil.
A message to nil does nothing and returns nil, Nil, NULL, 0, or 0.0.
All of the other posts are correct, but maybe it's the concept that's the thing important here.
In Objective-C method calls, any object reference that can accept a selector is a valid target for that selector.
This saves a LOT of "is the target object of type X?" code - as long as the receiving object implements the selector, it makes absolutely no difference what class it is! nil is an NSObject that accepts any selector - it just doesn't do anything. This eliminates a lot of "check for nil, don't send the message if true" code as well. (The "if it accepts it, it implements it" concept is also what allows you to create protocols, which are sorta kinda like Java interfaces: a declaration that if a class implements the stated methods, then it conforms to the protocol.)
The reason for this is to eliminate monkey code that doesn't do anything except keep the compiler happy. Yes, you get the overhead of one more method call, but you save programmer time, which is a far more expensive resource than CPU time. In addition, you're eliminating more code and more conditional complexity from your application.
Clarifying for downvoters: you may think this is not a good way to go, but it's how the language is implemented, and it's the recommended programming idiom in Objective-C (see the Stanford iPhone programming lectures).
What it means is that the runtime doesn't produce an error when objc_msgSend is called on the nil pointer; instead it returns some (often useful) value. Messages that might have a side effect do nothing.
It's useful because most of the default values are more appropriate than an error. For example:
[someNullNSArrayReference count] => 0
I.e., nil appears to be the empty array. Hiding a nil NSView reference does nothing. Handy, eh?
In the quotation from the documentation, there are two separate concepts -- perhaps it might be better if the documentation made that more clear:
There are several patterns in Cocoa that take advantage of this fact.
The value returned from a message to nil may also be valid:
The former is probably more relevant here: typically being able to send messages to nil makes code more straightforward -- you don't have to check for null values everywhere. The canonical example is probably the accessor method:
- (void)setValue:(MyClass *)newValue {
if (value != newValue) {
[value release];
value = [newValue retain];
}
}
If sending messages to nil were not valid, this method would be more complex -- you'd have to have two additional checks to ensure value and newValue are not nil before sending them messages.
The latter point (that values returned from a message to nil are also typically valid), though, adds a multiplier effect to the former. For example:
if ([myArray count] > 0) {
// do something...
}
This code again doesn't require a check for nil values, and flows naturally...
All this said, the additional flexibility that being able to send messages to nil does come at some cost. There is the possibility that you will at some stage write code that fails in a peculiar way because you didn't take into account the possibility that a value might be nil.
From Greg Parker's site:
If running LLVM Compiler 3.0 (Xcode 4.2) or later
Messages to nil with return type | return
Integers up to 64 bits | 0
Floating-point up to long double | 0.0
Pointers | nil
Structs | {0}
Any _Complex type | {0, 0}
It means often not having to check for nil objects everywhere for safety - particularly:
[someVariable release];
or, as noted, various count and length methods all return 0 when you've got a nil value, so you do not have to add extra checks for nil all over:
if ( [myString length] > 0 )
or this:
return [myArray count]; // say for number of rows in a table
Don't think about "the receiver being nil"; I agree, that is pretty weird. If you're sending a message to nil, there is no receiver. You're just sending a message to nothing.
How to deal with that is a philosophical difference between Java and Objective-C: in Java, that's an error; in Objective-C, it is a no-op.
ObjC messages which are sent to nil and whose return values have size larger than sizeof(void*) produce undefined values on PowerPC processors. In addition to that, these messages cause undefined values to be returned in fields of structs whose size is larger than 8 bytes on Intel processors as well. Vincent Gable has described this nicely in his blog post
I don't think any of the other answers have mentioned this clearly: if you're used to Java, you should keep in mind that while Objective-C on Mac OS X has exception handling support, it's an optional language feature that can be turned on/off with a compiler flag. My guess is that this design of "sending messages to nil is safe" predates the inclusion of exception handling support in the language and was done with a similar goal in mind: methods can return nil to indicate errors, and since sending a message to nil usually returns nil in turn, this allows the error indication to propagate through your code so you don't have to check for it at every single message. You only have to check for it at points where it matters. I personally think exception propagation&handling is a better way to address this goal, but not everyone may agree with that. (On the other hand, I for example don't like Java's requirement on you having to declare what exceptions a method may throw, which often forces you to syntactically propagate exception declarations throughout your code; but that's another discussion.)
I've posted a similar, but longer, answer to the related question "Is asserting that every object creation succeeded necessary in Objective C?" if you want more details.
C represents nothing as 0 for primitive values, and NULL for pointers (which is equivalent to 0 in a pointer context).
Objective-C builds on C's representation of nothing by adding nil. nil is an object pointer to nothing. Although semantically distinct from NULL, they are technically equivalent to one another.
Newly-alloc'd NSObjects start life with their contents set to 0. This means that all pointers that object has to other objects begin as nil, so it's unnecessary to, for instance, set self.(association) = nil in init methods.
The most notable behavior of nil, though, is that it can have messages sent to it.
In other languages, like C++ (or Java), this would crash your program, but in Objective-C, invoking a method on nil returns a zero value. This greatly simplifies expressions, as it obviates the need to check for nil before doing anything:
// For example, this expression...
if (name != nil && [name isEqualToString:#"Steve"]) { ... }
// ...can be simplified to:
if ([name isEqualToString:#"Steve"]) { ... }
Being aware of how nil works in Objective-C allows this convenience to be a feature, and not a lurking bug in your application. Make sure to guard against cases where nil values are unwanted, either by checking and returning early to fail silently, or adding a NSParameterAssert to throw an exception.
Source:
http://nshipster.com/nil/
https://developer.apple.com/library/ios/#documentation/cocoa/conceptual/objectivec/Chapters/ocObjectsClasses.html (Sending Message to nil).