This question already has answers here:
Why do I get a compile-time error when calling a subclass method using superclass reference?
(6 answers)
Closed 4 years ago.
From a book:
What if you want to call a method that’s defined by a subclass from an object
that’s referenced by a variable of the superclass? Suppose that the SoftBall class
has a method named riseBall that isn’t defined by the Ball class. How can you
call it from a Ball variable? One way to do that is to create a variable of the sub-class and then use an assignment statement to cast the object:"
Ball b = new SoftBall();
SoftBall s = (SoftBall)b;
// cast the Ball to a SoftBall
s.riseBall();
I don't understand this. Why can't I just directly call the method from the variable b? (The variable b in this case holds a Softball object). Why the casting?
How can you call it from a Ball variable?
You can't. The compiler doesn't know that this Ball is a SoftBall as opposed to a BaseBall.
You have to cast explicitly. This is merely a way of saying to the compiler: "I know more about the type of this instance than you". The compiler trusts you (mostly: it wouldn't let you cast it to, say, a String), and lets you deal with the consequences (e.g. a potential ClassCastException)
Related
This question already has answers here:
Sending nil to CGPoint type parameter
(2 answers)
Closed 9 years ago.
I have a CGSize property in a class and I need to check if it has been initialized. I know that a CGSize isn't a object, but generally speaking, is the same idea of checking if an object is different from nil. How to do that?
You can compare it to CGSizeZero or an arbitrary size that you consider invalid.
if (!CGSizeEqualToSize(CGSizeZero, mySize)) {
// do something
}
It depends on what you mean by "in a class". If this is an instance variable, your problems are over, because you are guaranteed that an instance variable will be auto initialized to some form of zero (i.e. CGSizeZero). But if you just mean "in my code somewhere", e.g. an automatic variable, then there is no such test; it is entirely up to you to initialize before use, and until you do, the value could be anything at all (sorry, but that's how C works).
On the whole, your question is itself a "bad smell". If it matters to you at some point in your code whether this value has been initialized, then you are doing it wrong. It's your value; it was up to you to initialize it (e.g. when your overall object was initialized). Or, if for some reason you need to know whether your setter has ever been called, then you need to add a boolean to your setter that tells whether it has ever been called.
CGSize is a C struct. With few exceptions (when used as an iVar), there is no guarantee for its initialized value. It could be anything, especially when created on the stack.
Thus, you are responsible for initializing it properly, and since "zeros" are valid values, there is no guaranteed way to tell if it was set to "zero" on purpose, or it has been uninitialized.
This question already has answers here:
What does this ' ->' mean in c/objective-c?
(7 answers)
What is the difference between '->' (arrow operator) and '.' (dot operator) in Objective-C?
(3 answers)
Dot (".") operator and arrow ("->") operator use in C vs. Objective-C
(5 answers)
Closed 10 years ago.
I have been looking at some code and come across the symbol -> being used like obj->method(argument); I have done a little bit of research and found it basically is the same as [obj method:argument]; but I am unsure what -> actually is or does.
So my question is, what does the -> symbol mean in objective-c?
It means the same as the struct dereference operator does in C, which is used to access fields within the struct via a pointer:
struct mystruct
{
int field;
};
struct mystruct *mystruct = ...;
printf("field=%d\n", mystruct->field);
In Objective-C it can also be used to access fields within Objective-C objects:
#interface MyObj : NSObject
{
#public
int field;
}
#end
MyObj *myObj = [[MyObj alloc] init];
NSLog(#"field=%d", myObj->field);
Note that you can only access these fields externally if they are declared #public.
I have been looking at some code and come across the symbol -> being
used like obj->method(argument); I have done a little bit of research
and found it basically is the same as [obj method:argument]; but I am
unsure what -> actually is or does.
So my question is, what does the -> symbol mean in objective-c?
Exactly the same thing it means in C; it is for accessing an item in a C structure. Way back in the days of yore, Objective-C was implemented purely as a C preprocessor extension + a runtime. Classes were nothing more than concatenated C structures and the preprocessor turned each ivar access into self->ivar.
I.e. ivar and self->ivar do the same thing (in a method of class).
Now, you can use -> to poke at some other object's (#public) ivars. But don't. That breaks encapsulation exactly because Objective-C's line of encapsulation is drawn at the method interface. Always use the setters/getters such that behavior can be either observed or overridden.
Finally, no, there is nothing like obj->method(argument) anymore. There was, once, in a failed experiment called Modern Syntax, but it was abandoned because it was a pointless waste of time. You can't use -> to invoke methods.
This question already has answers here:
Closed 10 years ago.
Possible Duplicate:
How to detect a property return type in Objective-C
Is there any way to get a property's type in Objective-C? I can access the property like this:
objc_property_t* properties = class_copyPropertyList(cl, &count);
And get the name like this:
property_getName(properties[i]);
What I need to do though is get the type. Also, the value will be nil in most cases so I can't just get the value calling object_getClass().
Not in the sense that you seem to be asking — in Objective-C classes are typeless; when dealing with non-C types beyond 'it's a class' the type of properties isn't known at runtime. That's why if you do something silly like the following in a view controller:
[self setValue:#3 forKey:#"view"];
You'll see an exception raised when the controller attempts to send a view message to the NSNumber rather than by the key-value coding mechanisms because you tried to put something that isn't a view into in an inappropriate property.
Parsing property_getAttributes will allow you to go no further than distinguishing the various C literal types from an Objective-C object type.
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 have a C struct that contains a function pointer. Now, I have used this setup within C with no problems, but now I'm using this C struct in Objective-C and I need to pass a function (or selector) pointer that is defined in the Objective-C class.
1. Here is what I have for the Objective-C selector that needs to be passed as a pointer to the C function:
- (void)myObjCSelector:(int*)myIntArray
{
// Do whatever I need with myIntArray
}
2. And here is where I run into a wall, Within Objective-C I'm trying to pass the selector as a pointer to the C function call: In place of "myObjCSelectorPointer" I need the proper syntax to pass the selector as a function pointer in this C function call:
passObjCSelectorPointerToCContext(cContextReference, myObjCSelectorPointer);
I did investigate this issue, but could mainly find several different ways of doing similar things, but I couldn't find anything specific for calling C functions and passing an Objective-C selector pointer.
In objc a selector is not a function pointer. A selector is a unique integer that is mapped to a string in a method lookup table stored by the objc runtime. In the above case your method name would be myObjCSelector: and to get the unique selector for it you would type #selector(myObjCSelector:). However this would be of no use to you because it doesnt represent a particular implementation of a function.
What youre looking for is IMP. Refer to this SO question.
EDIT 2:
IMP myObjCSelectorPointer = (void (*)(id,SEL,int*))[self methodForSelector:#selector(myObjCSelector:)];
Then you can call the method using
myObjCSelectorPointer(self,#selector(myObjCSelector:),myIntArray);
However, what this means you will need to make sure that you add the pointer to self in the c function call passObjCSelectorPointerToCContext.
So it should look like this
passObjCSelectorPointerToCContext(cContextReference, self, myObjCSelectorPointer);
when called from within the object that contains the method.
It is important to note though that using IMP is almost never the right technique. You should try to stick with pure Obj-C. Obj-C is quite efficient after the first call to a message because it uses temporal caching.
EDIT 1:
It's useful to understand why objc works in this way. The Apple documents explain it in depth. However a short explanation is as follows:
When you send a message to an object such as [myobject somemethod] the compiler won't immediately know which particular implementation of somemethod to call because there might be multiple classes with multiple overriden versions of somemethod. All of those methods have the same selector, irrespective of its arguments and return values and hence the decision about which implementation of somemethod is deffered to when the program is running. [myobject somemethod] gets converted by the compiler into a C function call:
objc_msgSend(myobject, #selector(somemethod))
This is a special function that searches each myobject class layout to see whether that class knows how to respond to a somemethod message. If not it then searches that class's parent and so on until the root. If none of the classes can respond to somemethod then NSObject defines a private method called forward where all unknown messages are sent.
Assuming that a class can respond to the somemethod message then it will also have a particular pointer of type IMP that points to the actual implementation of the method. At that point the method will be called.
There is considerably more to this procedure than I have described but the outline should be enough to help you understand what the goal of a selector is.
One final point is that the reason method names are mapped to unique integers via the #selector directive is so that the runtime doesn't have to waste time doing string comparisons.
Basically, the answer is: Objective-C selectors are different from function pointers. You need two pieces of data to perform a selector. That is an object and the selector itself. You will need some glue to accomplish your task.
Check this question.
Do you have to use a function pointer? In Objective-C, you can get the function pointer to an arbitrary method implementation (known as an IMP), but this is extremely uncommon, and usually not a good idea. Calling objc_msgSend() directly is also not the greatest idea, because there are several different variants of objc_msgSend(), and the compiler automatically chooses different ones to use based on the return type of the method. Methods that return an object go through objc_msgSend(), but objects that return structs might go through objc_msgSend() or they might go through objc_msgSend_stret(). And if the method returns a double, then it goes through objc_msgSend_fpret()...
Documentation: Objective-C Runtime Reference: Sending Messages
Instead, I might recommend using a target-action pair, or using a block. Then you might do something like:
myContextRef->target = anObjcObject;
myContextRef->action = #selector(invokeMe:);
And when you're done, do:
[myContextRef->target performSelector:myContextRef->action withObject:someReturnInformation];
Or maybe use a block:
myContextRef->completionHandler = [^(id returnInformation) {
[anObjcObject invokeMe:returnInformation];
} copy];
And then when you're done, do:
myContextRef->completionHandler(someReturnInformation);
(and don't forget to -release the block when you free the context)