i am viewing this video on objective c. the guy show a method which takes multiple arguments and it looks like this
- (void) setTo: (int) n over: (int) d
{ .... }
to use it he shows:
[myFraction setTo: 100 over:200];
how would that bracket notation look in dot noation? andi dont understand what that over
means, would anyone know? thnx
The dot notation is a shorthand notation for property access only. The compiler translates it to the appropriate setter/getter method call. It is just syntactic sugar.
So given this property access using dot notation:
myFraction.numerator=100;
The compiler replaces it with the following equivalent code:
[myFraction setNumerator:100]
Now it should be clear why you cannot use dot notation for sending a normal message to an object. I can't even think of a way how that should even look like.
There is a lot of discussion concerning dot vs. bracket notation going on. One of the arguments against dot notation is the confusion it generates especially for beginners. Other languages do of course use methods for property accessors too, however they hide this fact more consistently than Objective-C.
You cannot pass multiple arguments with dot notations. A setter usable by dot-notation must have the prototype
-(void)setXxxx:(type)value;
However, you can create an auxiliary struct to group all arguments into one:
struct Fraction { int n, d; };
struct Fraction MakeFraction(int n, int d) {
struct Fraction r;
r.n = n;
r.d = d;
return r;
}
...
-(void)setValue:(struct Fraction)f { ... }
...
myFraction.value = MakeFraction(100, 200);
(and a "over" b means a / b.)
From the looks of it, this method is setting two instance variables/properties of the object the numerator (n) and the denominator (d). You cannot do this using dot notation unless you break it into two calls:
myFraction.numerator=100;
myFraction.denominator=200;
Note that dot notation is only for accessing instance variables/properties and not an alternative to message sending.
In Objective-C, we can name methods with arguments in the middle. So when he writes [myFraction setTo:100 over:200];, the over could mean... well, whatever he wants. It's part of the method name he chose. In this case, he was probably trying to make the method sound like English. ("Set this fraction to 100 over 200." We read fractions as "numerator over denominator" often in normal speech.)
Some methods, called "accessors", we write very frequently: these are methods of the form - (int)variable (called "getters"), and - (void)setVariable:(int)newValue (called "setters"). My examples here would assumedly return, or change, respectively, an instance variable called variable. Here's what the method implementations might look like:
- (int)variable
{
return variable;
}
- (void)setVariable:(int)newValue
{
variable = newValue;
}
It's common to have accessors like this for almost every instance variable your class has. At some point, someone got tired of writing [myInstance setVariable:20]; and such, and decided they'd rather it look like many other languages out there, myInstance.variable = 20;.
Therefore, Objective-C 2.0 added dot notation, which allows you to write...
myInstance.variable, which is exactly equivalent to [myInstance variable] in most circumstances (and does NOT access the instance variable variable directly!), and...
the special case myInstance.variable = 20;, which is exactly equivalent to [myInstance setVariable:20];. Again, note that this does not access variable directly, it sends a message to myInstance. So if we'd written some other code in setVariable, it would still be accessed if we used dot notation.
Dot notation is designed to be used for accessors. You could also theoretically use it for any method that returns a value and takes no arguments (myArray.count, for example). Using it for anything else (myInstance.doSomeAction) is extremely poor style. Don't do it. Therefore, as I'm sure you can guess by now, [myFraction setTo:100 over:200] has no dot notation equivalent, as it takes 2 arguments, and isn't an accessor.
[myFraction numerator]
Could be written as
myFraction.numerator
but you can also assign values such as
instance.property = value
But you cannot pass multiple arguments in dot notation.
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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 diving into iOS programming and I'm having difficulty getting my head around the idea of Dot Notation and Method Notation.
As far as I understand it, Dot Notation can be used to invoke setters/getters on properties and is much more cleaner to write/read. Method Notation is used to send messages to objects to manipulate them etc.
Could someone give me a simple explanation as to why the following two statements are essentially different and one will compile but the other will instead fail due to a syntax error.
- (IBAction)digitPressed:(UIButton *)sender
{
NSString *digit = [sender currentTitle];
self.display.text = [self.display.text stringByAppendingFormat:digit];
self.display.text = self.display.text.stringByAppendingFormat:digit;
}
Thanks.
You're entering into Objective-C development at an interesting time where old syntax is being used with new syntax. Dot syntax is syntactic sugar and there are some cases where you can use it but you should not.
The following is invalid syntax. Anything where you'd use a colon (besides setters or getters), you won't use dot notation.
self.display.text = self.display.text.stringByAppendingFormat:digit;
Also, you would use stringByAppendingString, not stringByAppendingFormat
You use dot notation for accessing variables, not for calling actions that will have effects.
Correct:
self.foo.attributeOfMyClass
Incorrect:
self.foo.downloadSomethingFromAWebsite
Ensuring you always use dot notation for accessing property values and you always use bracket notation (even when you don't have to) for calling action methods, your code will be much clearer upon a glance.
Dot notation is just shorthand for a specific kind of method--namely, accessors. You may use it in the following cases:
When setting a property: foo.bar = 3; is equivalent to [foo setBar:3];.
When requesting a property: in any case except the one above, foo.bar is equivalent to [foo bar].
Dot notation is only shorthand--there is nothing magic about its relationship to properties. You could theoretically use dot notation to send any message that takes no arguments (foo.doSomething), but this would be very very bad style, as dot notation is intended for properties. Also note that if dot notation vs. square brackets is confusing you while you're learning, it's a perfectly valid choice to avoid dot notation altogether. It's just one shortcut you may use for accessors, if you like.
Actually, your second statement is not correct. Objective C way to invoke methods (messages) is using the [instance message] syntax.
As you said, the dot notation is just to call getters and setters on class properties, but not messages, that's why your second statement is not correct. The two lines you may wanted to compare are:
self.display.text = [self.display.text stringByAppendingFormat:digit];
[[self display] setText:[[[self display] text] stringByAppendingFormat:digit]];
Note that the message stringByAppendingFormat has to be called the normal way.
The dot notation is just to write faster and not so many brackets, but it will execute exactly the same instructions once compiled.
Another reason for using selector notation rather than dot notation is due to the dynamic language features in Objective C. As an example, consider the following:
NSString *s = #"Hello World!";
NSLog(#"Length is %d", s.length);
This works as we would expect. However, objects in Objective C may be passed around with type id. Consider the following:
id s = #"Hello World!";
NSLog(#"Length is %d", s.length);
This won't compile, as id doesn't have a property called length. The following will work, however:
id s = #"Hello World!";
NSLog(#"Length is %d", [s length]);
The reason this works is that Objective C knows about NSString, and so knows that there is some object type that responds to the selector length. Of course, if you try the following:
id s = [[UIView alloc] init];
NSLog(#"Length is %d", [s length]);
Your code will compile correctly, but a runtime exception will occur (unrecognized selector sent to instance) as UIView does not have a length selector.
Let's say we have the class Class with the variable variableOne we are going to use both notations.
Dot notation is the purest way to access a variable. It is also the way that bracket notation is most likely doing it behind the scenes. By typing Class.variableOne... variableOne is a part of Class and the "." after the class tells the compiler that it would like to access a part of the class--either a variable or a method.
Bracket notation is uses a method to access the variable. Let's say...
-(int) setVariable:x {
self.variableOne = x;
}
-(int) showVariable {
return self.variableOne
}
So when you're using bracket notation to set the variable [variableOne setVariable:5] or displaying the variable [variableOne showVariable] it calls the appropriate method.
This is a very simple way to think of the difference, I realize another answer has already been accepted but perhaps this answer will explain it for someone who didn't understand another answer.
When your code gets compiled, clang actually first takes all of your dot notation and turns it into method/bracket notation, so self.display and [self display] are exactly the same. Dot notation is actually fairly new as of Objective-C 2.0. It's simply for convenience.
Dot notation can only be used for properties, because doing something like you tried to do (which will not compile) gets cumbersome:
self.display.text.stringByAppendingFormat:digit;
It also wouldn't work for methods that take multiple arguments since you'd need to put spaces between arguments and suddenly the line of code would look awkward and hard to read.
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.
I've tried to use id to create duck typing in objective-c. The concept looks fine in theory but failed in practice. I was unable to use any parameters in my methods. The methods were called but parameters were wrong. I was getting BAD_ACESS for objects and random values for primitives. I've attached a simple example below.
The question:
Does any one knows why the methods parameters are wrong?
What is happening under the hood of the objective-c?
Note: I'm interest in the details. I know how to make the example below work.
An example:
I've created a simple class Test that is passed to an other class using property id test.
#implementation Test
- (void) aSampleMethodWithFloat:(float) f andInt: (int) i {
NSLog(#"Parameters: %f, %i\n", f, i);
}
#end
Then in the class the following loop is executed:
for (int i=0; i < 10; ++i) {
float f=i*0.1f;
[tst aSampleMethodWithFloat:f andInt:i]; // warning no method found.
}
Here is the output that I'm getting. As you can see the method was called but the parameters were wrong.
Parameters: 0.000000, 0
Parameters: -0.000000, 1069128089
Parameters: -0.000000, 1070176665
Parameters: 2.000000, 1070805811
Parameters: -0.000000, 1071225241
Parameters: 0.000000, 1071644672
Parameters: 2.000000, 1071854387
Parameters: 36893488147419103232.000000, 1072064102
Parameters: -0.000000, 1072273817
Parameters: -36893488147419103232.000000, 1072483532
Update:
I've found out by accident that when I add a declaration of aSampleMethodWith... to the class with for loop the warning disappears and the method on the Test class is called correctly.
Update 2:
As pointed out by JeremyP the direct cause of the problem is that the floats are treated as doubles. But anyone knows why? (following the 5why principle :) ).
According to #eman the call is translated to simple C function call and compiler directive to get the SEL. So the #selector gets confused. But why? The compiler have all necessary type informations in the first method call. Does any one knows a good source of information about the Objective-C internals I've search The Objective-C Programming Language but i didn't find the answer.
By default floating point values are passed as doubles, not floats. The compiler does not know, at the point where [tst aSampleMethodWithFloat:f andInt:i]; occurs that it is only supposed to pass a float, so it promotes f to a double. This means that, in the method, when the compiler does know it is dealing with a float, f is the float formed by the first four bytes of the double passed to the method and i is an int formed from the second four bytes of the double passed.
You can fix this by either
changing the first parameter of aSampleMethodWithFloat:andInt: to a double
importing the interface declaration of Test into the file where you use it.
NB there is no gain except a small amount of space when using floats in C. You might as well use doubles everywhere.
I think JeremyP is correct about the problem being about doubles vs floats. As for implementation details, message dispatch in Objective-C uses the objc_msgSend(id theReceiver, SEL theSelector, ..) C function (for some deep nitty-gritty, see here). You can simulate the same results of method dispatch like so:
SEL theSelector = #selector(aSampleMethodWithFloat:andInt:);
objc_msgSend(self.test, theSelector, 1.5f, 5);
SEL is just a number that corresponds to a function (that is dynamically determined based on the method signature). objc_msgSend then looks up the actual function pointer (of type IMP) of the method and invokes it. Since objc_msgSend has a variable number of arguments, it will just use as many as you pass in. If you were to do:
objc_msgSend(self.test, theSelector, 1.5f);
It would use 1.5f correctly and have junk for the other variable. Since the method signature typically denotes the number of arguments, this is hard to do under normal usage.
You can make the warning go away by making a category like this:
#interface NSObject (MyTestCategory)
- (void) aSampleMethodWithFloat:(float) f andInt: (int) i;
#end
Without a signature available at the calling point, it isn't known what type the parameters are supposed to have. Undefined methods will be assumed to take ... as parameters, which isn't what yours does. If there is any interface seen by the compiler at this point, where the method in question exists, that definition will be used.
The trouble here is with the dividing line between C and Objective-C. The id type specifies any object, but ints and floats are not objects. The compiler needs to know the C type of all the arguments and the return type of any method you call. Without a declaration, it assumes that a method returns id and takes an arbitrary number of id arguments. But id is incompatible with int and float, so the value doesn't get passed correctly. That's why it works correctly when you provide a declaration — then it knows your int is an int and your float is a float.