I've seen this operator pop up quite a few times in example code in "Learn Objective C on the Mac."
I believe it's an operator in the C language which Objective C inherits.
I tried Googling and searching Stack Overflow and oddly nothing came up.
Does it have an English name?
It has to do with structures.
When we have a struct available locally on the stack, we access its members with the . operator. For example:
CGPoint p = CGPointMake(42,42);
NSLog(#"%f", p.x);
However, if we instead have a pointer to a structure, we have to use the -> operator:
CGPoint *p = malloc(1*sizeof(CGPoint));
p->x = 42.0f;
NSLog(#"%f", p->x);
free(p);
-> is not specific to Objective-C. It's a C operator.
Now that's cleared, it's the member access operator, equivalent to a pointer dereference and then using the dot operator on the result.
Say you had a struct like this:
typedef struct Person {
char *name;
} Person;
Person *jacob = malloc(1*sizeof(Person));
So this statement:
jacob->name = "Jacob";
Is equivalent to this statement:
(*jacob).name = "Jacob";
Of course, don't forget the free:
free(jacob);
In C
a->b
is a shortcut for
(*a).b
which is for dereferencing of members of a struct that is pointed to.
This is useful, because of . binds stronger than the dereferencing operator * . So by using -> you avoid having to use these ugly parentheses.
It's a member selection (or access) equivalent to a pointer de-reference (as pointed out in comments)
a->member is equivalent to (*a).member in C/C++
The same thing that it means in C. It can be used to access the instance variables of objects directly, but generally this is not the best practice. The dot notation you're referring to is a property, not the usual C dot notation.
It's the "indirect member access operator". It's a C operator, which both Objective-C and C++ inherited.
This
a->b
is equivalent to:
(*a).b
but is less typing. (The parens are necessary in the expanded form due to precedence of * and ..)
a->b is equivalent to (*a).b, and designates member b of the object pointed to by a.
In the C standard, it is called the "structure/union pointer operator," which isn't really the catchiest name.
It's all been said, it is a shortcut for accessing members of a struct pointer, but just a note to add that you can access ivars using the same syntax due to the way the Objective-C runtime works:
#interface Foo : NSObject {
NSString *something;
}
/* ... SNIP ... */
NSLog(#"something = %#", foo->something); // Where foo is an instance of the Foo class
This is handy when you need to access ivars that aren't exposed by methods; specifically when you need to implement copyWithZone: on your objects.
Related
Help me :( i dont know why?
#interface RootViewController : UITableViewController {
BOOL isSearchOn;
}
self->isSearchOn = NO; ( no error)
self.isSearchOn = NO; ( error)
Dot operator . is used to access property.
Arrow operator -> is used to acces instance variable.
So you use
self->isSearchOn = NO; // which is an instance not a property
You can also use this way by skipping self->
isSearchOn = NO;
-> is the traditional C operator to access a member of a structure referenced by a pointer. Since Objective-C objects are (usually) used as pointers and an Objective-C class is a structure, you can use -> to access its members, which (usually) correspond to instance variables.
or
When applied to pointer arrow operator is equivalent to applying dot operator to pointee (ptr->field is equivalent to (*ptr).field)
or
pSomething->someMember
is equivalent to
(*pSomething).someMember
. is used when you have a object and want to reference a attribute or a method, while you must use -> when holding the pointer only. instead of using -> you could aswell go for *. but I guess that you should in any case read about pointers in C.
There are essentially three operators here, two of which are inherited from C and the third which is from Objective-C.
In C, . and -> do the same thing—they retrieve a value from a struct—but - automatically dereferences a pointer to a struct, so the following two expressions are equivalent:
(*a).b
a->b
The second operator is simply syntactic sugar. Importantly, however, . doesn't make any sense when applied to a pointer—you have to use ->.
Objective-C allows the use of . with an object pointer (and only with a pointer, since all Objective-C objects are pointers) to reference a property value, a much higher level construct than the C operators provide.
Therefore, you may use . on objects to retrieve properties defined in the interface with #property, but you will need to use -> to retrieve ivars (instance variables) directly, which isSearchOn is in your example. However, it is usually better practice to expose variables using properties in Objective-C rather than refer to them directly using C's -> operator.
This question already has answers here:
Closed 10 years ago.
Possible Duplicate:
Arrow operator (->) usage in C
Dot (“.”) operator and arrow (“->”) operator use in C vs. Objective-C
I'm a newbie looking at a freeware/open-source program last updated in 2008, and I don't recognize the -> in the following notation:
- (id)copyWithZone:(NSZone *)zone
{
GFIPGeniusItem * newItem = [[[self class] allocWithZone:zone] init];
newItem->_stringValue = [_stringValue copy];
newItem->_imageURL = [_imageURL copy];
newItem->_webResourceURL = [_webResourceURL copy];
newItem->_speakableStringValue = [_speakableStringValue copy];
newItem->_soundURL = [_soundURL copy];
return newItem;
}
I'm assuming it's allowing some sort of shortcut, but I'd love to specifically what it does.
It's a way to directly access an instance variable within an ObjC object from outside that object. (The syntax and -> is borrowed from C structs, behaving as if the reference were a pointer-to-structure).
This access mechanism is almost vestigial at this point, and very uncommonly seen in modern ObjC code, because good encapsulation requires the use of accessors and properties, not touching instance variables directly. It's legitimate in some very special cases, though, and this is one of them:
When copying an object, you want to get a resulting copy that matches exactly the state of the current self. The cleanest way of achieving this is often to set the copy's ivars explicitly, to prevent any side-effects that the init overloads or accessors might cause. It's "safe" because the code doing it is still located within the class that's in question, so if you needed to change or add ivars, you could update this code as well with the knowledge of anything else that might require.
Doing this from outside the class in question is bad form, and there's no good reason to do it.
In Objective-C you have some kind of two variable type accessors. The one everybody should know is the "." one (e.g. Class.variable). This type calls either the appropriate getter or setter.
Now, the other type - the one you asked for - is for in-class usage. Obviously, as the getter or setter gets called automatically with the "." notation you need a way to set the variable without a setter (calling the setter in the setter itself results in an endless loop). Therefore, this "->" notation is used -> simply, it is the direct-access mode.
Usually, Objective-C the variable name for both notations is the same but some prefer to have the in-class notation variable name beginning with "_". This is achieved by editing the #synthesize variable line to #synthesize variable = _variable.
That's a pointer indirection operator. a->b means the same thing as (*a).b (where the . is the structure member access operator, not Objective-C's property dot syntax).
When you say:
newItem->_stringValue
you're directly accessing the _stringValue instance variable of the object to which newItem points.
The -> operator is very common in C++, but not so much in Objective-C.
In Objective C, like in C++, the p->m notation is equivalent to (*p).m This is, the dereference of the pointer to the base type followed by a call to the corresponding method or property.
So in your case, using the other notation it would look like this:
(*newItem)._stringValue = [_stringValue copy];
(It's more common to use the -> operator)
I'm trying to wrap my head around some of the differences in usage and syntax in C vs. Objective-C. In particular, I want to know how (and why) the usage differs for the dot operator and the arrow operator in C vs. Objective-C. Here is a simple example.
C Code:
// declare a pointer to a Fraction
struct Fraction *frac;
...
// reference an 'instance' variable
int n = (*frac).numerator; // these two expressions
int n = frac->numerator; // are equivalent
Objective-C Code:
// declare a pointer to a Fraction
Fraction *frac = [[Fraction alloc] init];
...
// reference an instance variable
int n = frac.numerator; // why isn't this (*frac).numerator or frac->numerator??
So, seeing how frac is the same in both programs (i.e. it is a pointer to a Fraction object or struct), why are they using different syntax when accessing properties? In particular, in C, the numerator property is accessed with frac->numerator, but with Objective-C, it is accessed using the dot operator, with frac.numerator. Since frac is a pointer in both programs, why are these expressions different? Can anyone help clarify this for me?
frac is actually not the same in both programs.
A C Fraction is a struct, which is a base type with no overloaded operators and is only really able to be constructed and destructed by default. If you define functions or fields on the struct, the way to access those properties in C is with the dot (.) operator. Objective-C maintains this operator when you use structs. For convenience, you can perform a dereference-and-dot operation using the arrow (->) operator (the two equivalent expressions you mention). Objective-C also preserves this when accessing structs.
An Objective-C Fraction in your example, however, is probably (one would assume) a pointer of at least type id, which is simply a classname and pointer to the instance of that class under the hood. It's also very likely to be a subclass of NSObject or NSProxy. These Objective-C classes are special in that they have a whole layer of predefined operations on top of just a C struct (if you really want to dig into it then you can take a look at the Objective-C Runtime Reference). Also important to note, an Objective-C class is always a pointer.
One of the most basic operations is objc_msgSend. When we operate on these types of objects, the Objective-C compiler interprets a dot (.) operator or the square bracket syntax ([object method]) as an objc_msgSend method call. For more detailed info about what actually happens here, see this series of posts by Bill Bumgarner, an Apple engineer who oversees the development of the Obj-C runtime.
The arrow (->) operator is not really supposed to be used on Objective-C objects. Like I said, Objective-C class instances are a C struct with an extra layer of communication added, but that layer of communication is essentially bypassed when you use the arrow. For example, if you open up Xcode and type in [UIApplication sharedApplication]-> and then bring up the method completion list, you see this:
Here you can see a bunch of normal fields which we generally access with square bracket syntax (like [[UIApplication sharedApplication] delegate]). These particular items, however, are the C fields that store the values of their respective Objective-C properties.
So, you can roughly think of it like this:
Dot operator on a C object
(at run time) Return value of the field
Arrow operator on a C object (pointer)
Dereference pointer
Return value of the field
Dot operator/square brackets on an Objective-C object (pointer)
(at compile time) Replace with call to objc_msgSend
(at run time) Look up Obj-C class definition, throw exception if something went wrong
Dereference pointer
Return value of the field
Arrow operator on an Objective-C object (pointer)
(at run time) Dereference pointer
Return value of the field
Now I'm definitely oversimplifying here, but to summarise: the arrow operators appear to do basically the same thing in both cases, but the dot operator has an extra/different meaning in Objective-C.
Dot-notation is a design choice. Since we always deal with pointers to objc instances, I'd guess the designers wanted something familiar, which also would not break existing programs. It was introduced in ObjC 2 - just a few years ago. Before that, you always had to use brackets for messaging.
Dot notation makes a difference though - it is not direct access, but a message.
That is:
obj.property = val;
// is the same as:
[obj setProperty:val];
// and not:
obj->property = val;
val = obj.property;
// is the same as:
val = [obj property];
// and not:
val = obj->property;
You can still write obj->ivar to access a pointer to object's members (if visible).
In your first example, Fraction is a struct.
In your second example, Fraction is an Objective-C class (and in iOS would likely be a subclass of NSObject).
C++ does not allow overloading of operator .. Therefore without additional information you can deduce that the dot notation you're seeing is an additional language construct integrated into Objective-C, rather than a C/C++ defined or overloaded operator.
As it happens, the dot notation is simply a design feature the implementors chose as shorthand for property access, entirely equivalent to the square bracket getter:
myObjCVar.prop == [myObjCVar prop];
The dot operator on objects is a special syntax for accessing objects' properties. It calls the property's getter or setter behind the scenes. So, for example, [#"hello" length] and #"hello".length are equivalent*. For all other types, the dot is the same as the C dot, and the arrow is always the same.
* Note: The accessor method won't always be named the same as the property. If it's a declared property and the declaration designates a special getter or setter method, that one will be used instead.
The dot and arrow notation are equally the same in C as it is in Objective-C (strict superset of ). I think the fundamental difference that needs to be distinguished is the difference between a struct and an Objective-C object.
The dot notation used for objects in Objective-C are used for properties that was introduced in Objective-C 2.0. However, with structs, the -> and dot notation between Objective-C and C are the same.
I'm a relative newbie to Objective-C (only studied Arron Hillegras's book) and am confused by the following snippit of code I've found in one of Apple's code examples, in particular what does the -> mean?
-(id) initWithNumbers:(NSArray *)numbers
{
self = [super init];
if (self != nil)
{
self->_numbers = [numbers copy];
}
return self;
}
In the header file _numbers is declared as
NSNumber * _number;
( the underscore has some significance from what I recall reading somewhere but that too eludes me at the moment.
Thanks
Robin
-> is a normal C operator for accessing the members of a pointer to a struct; the . operator is for accessing members of a struct. Thus:
a->b
is translated to
(*a).b
Since Objective-C objects are pointers to structs underneath it all, this works for accessing instance variables.
You use -> when you want to access an ivar. Like C structures you will use a . or -> (in pointers to structs) in Objective-C objects you can use -> but is not necessary since you can access them directly.
Hence:
self->_numbers = [numbers copy];
and
_numbers = [numbers copy];
are the same
You want to use -> when you want to access that ivar explicitly.
Be aware that in Objective-C you can use . but only when its a property.
You can use -> regardless that.
It's usually called the 'arrow' operator. It allows you to access the instance variables of an object (or of a struct) using a reference or pointer to the instance. It's common syntax with C and C++.
I'm struggling to find a nice write up, but you might find this one informative.
As to the underscore - commonly they mean "private"; according to the Coding Guidelines for Cocoa - Naming Basics:
Avoid the use of the underscore
character as a prefix meaning private,
especially in methods. Apple reserves
the use of this convention. Use by
third parties could result in
name-space collisions; they might
unwittingly override an existing
private method with one of their own,
with disastrous consequences.
That indicates a reference to -in this case- an instance variable of an object. Self refers to the object itself, and by writing self->_numbers, you refer to the variable that is part of the class instead of a global variable named _numbers.
Are you sure this is not mentioned in your book?
It's an operator which roots from C Programming Language. And since Objective-C is compatible with C programming, you may see developers using traditional C-style programming.
"->" is used to access a elements of a pointer. And since the "objects" in Objective-C are just pointers (denoted by *), e.g. NSNumber *number; , you can use this notation to access their elements.
I never used this notation in Objective-C since the dot notation in Objective-C is accepted. If "self" has a synthesized property "number" then self.number should be the same as self->number (this is true ONLY in Objective-C.
Is it true, that the Asterisk always means "Hey, that is a pointer!"
And an Pointer always holds an memory adress?
(Yes I know for the exception that a * is used for math operation)
For Example:
NSString* myString;
or
SomeClass* thatClass;
or
(*somePointerToAStruct).myStructComponent = 5;
I feel that there is more I need to know about the Asterirsk (*) than that I use it when defining an Variable that is a pointer to a class.
Because sometimes I already say in the declaration of an parameter that the Parameter variable is a pointer, and still I have to use the Asterisk in front of the Variable in order to access the value. That recently happened after I wanted to pass a pointer of an struct to a method in a way like [myObj myMethod:&myStruct], I could not access a component value from that structure even though my method declaration already said that there is a parameter (DemoStruct*)myVar which indeed should be already known as a pointer to that demostruct, still I had always to say: "Man, compiler. Listen! It IIISSS a pointer:" and write: (*myVar).myStructComponentX = 5;
I really really really do not understand why I have to say that twice. And only in this case.
When I use the Asterisk in context of an NSString* myString then I can just access myString however I like, without telling the compiler each time that it's a pointer. i.e. like using *myString = #"yep".
It just makes no sense to me.
an * is actually an operator to de-reference a pointer. The only time it means "hey i'm a pointer" is during variable declaration.
Foo* foo // declare foo, a pointer to a Foo object
&foo // the memory address of foo
*foo // de-reference the pointer - gives the Foo object (value)
mmattax well covered the distinction between declaration (as a pointer) and dereferencing.
However, as to your point about:
(*myVar).myStructComponentX = 5;
to access a member of an instance of a C struct (as this is) you can do what you did , or more commonly you use the -> notation:
myVar->myStructComponentX = 5;
Objective-C is a little confusing here because it recently (in ObjC 2.0) introduced property syntax, which is a short cut for:
int val = [myObject someIntProperty];
and can now be written as:
int val = myObject.someIntProperty;
This is Objective C (2.0) syntax for accessing a property which you have declared (not an actual member variable), whereas your example was accessing a member of a C struct.
Make sure you are clear on the difference.
As I said in my answer of your previous question, #"yep" is already a pointer, so there is no need of * before myString which is also a pointer. In this case, you assign pointers not values.