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Closed 10 years ago.
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How to wrap a Struct into NSObject
Can the new Clang Objective-C literals be redirected to custom classes?
I have a custom struct:
typedef struct {
float f1;
float f2;
} MYCustomStruct;
that I need to add to an NSArray. I've already written a category to create NSValues of these structs, which I then add to the NSArray, however I'd like to simplify that even further using boxed expressions, if possible. I'd love to be able to do this:
#[#(instanceOfMYCustomStruct)];
however, I'm confronted with the following error:
Illegal type 'MYCustomStruct' used in a boxed expression
Is there a way to use boxed expressions with custom structs?
I would use a NSValue to box a struct, as it has built-in support for it. Unfortunately, you cannot use objective-c's cool literals for it, though:
struct foo myStruct;
NSValue *val = [NSValue valueWithBytes:&myStruct objCType:#encode(typeof(myStruct))];
// to pull it out...
struct foo myStruct;
[val getValue:&myStruct];
While this may be unwieldy & ugly amidst other objc code, you have to ask yourself - why are you using a struct in the first place in Objective-C? There are few speed performances gained over using an object with a few #property(s), the only real reason I could see is if you are integrating with a C library for compatibility with memory layouts, and even then, the structure of an objective-c object's memory layout is well-defined, so long as the superclass doesn't change.
So what is your real purpose in boxing a struct? If we have that, we can help you further.
Related
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NSMutableArray - force the array to hold specific object type only
(12 answers)
Closed 9 years ago.
I'm programming on Objective C for the first time, coming from C++ (so far I like
the latter much better!). I have a question regarding mutable arrays, namely I want to create one with the specific type of one of my objects, 'CMParticle', instead of the generic ID type. To access data in my object from my mutable array, I have to cast it as one of my objects each time (which is I believe cumbersome) like so:
rij[0] = ((CMParticle *)particles[*pi]).crds[0] - ((CMParticle *)particles[*pj]).crds[0];
where 'particles' is my mutable array of CMParticle objects. I would rather do this
rij[0] = particles[*pi].crds[0] - particles[*pj].crds[0];
Prior to this I declare my mutable array like so:
particles = [NSMutableArray array];
It would be nice if I could declare this array with my type somehow so I don't have to typecast every time. Is there a way to do this?
What you're trying to do doesn't actually make sense in Objective C.
C++ containers are homogenous, but generic. You can have a vector<CMParticle>, or a vector<int>, and they're different types.
ObjC containers are heterogeneous. You just have an NSArray, and it can hold CMParticle objects, NSNumber objects, or anything else, all mixed up in one big array.
You generally don't need these casts at all. If you want to send a message to my_array[3], just do [my_array[3] doSomething:15]. Just like a higher-level language (Python, Ruby, Smalltalk, Javascript, etc.).
The only problem is that (unlike Python, etc.), there are a few cases where you do need the cast. Most critically (and annoyingly), if you want to access members directly, you have to cast first. This is one of the reasons that ObjC (unlike Python, etc.) encourages you to use #property and/or explicit accessors instead of directly accessing members. (Also, as a more minor annoyance, because variables have declared types, you can't just write tempval = my_array[3];, you have to specify the type, like: CMParticle *tempval = my_array[3].)
Another way to look at this: C++ extends C's static, weak type system to give you a stronger static type system; ObjC instead bolts on a separate dynamic type system (unfortunately leaving the existing C stuff unchanged, which is where the occasional problems come in).
You can pretty easily write your own NSMutableArray subclass that's generic (taking the class at runtime, unlike C++'s compile time, of course) and homogenous, but all that does is add restrictions; the elements will still be id everywhere. The only way around that is to write a custom class for each array: MutableCMParticleArray, MutableNSNumberArray, etc.
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.
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Closed 10 years ago.
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objective c difference between id and void *
why most of the objects we create in iphone are pointers
According to Stanford university course, 2010/2011
Lecture 3
The guy made something strange there (at least for me), which is that
NSString *digit = sender.titlelabel.text;
Why is digit a pointer?
The type of your digit is id, which is just basically just a C pointer to a certain struct. All references to objects in Objective-C have this primitive type, regardless of the Class of the object. So the answer to your question is, unfortunately, because that's the way Objective-C works.
So whether you're declaring an NSString*, or an UITableViewController*, or MyClass*, your variable has type id. This is the primary means by which the language implements polymorphism. So, for example, the following declarations are equivalent:
NSString *digit;
id digit;
And it's true of method prototypes as well. These are equivalent:
-(UITableViewCell *)tableView:(UITableView)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath
-(id)tableView:(id)tableView cellForRowAtIndexPath:(id)indexPath;
A variable of type id is not an object itself, it is a pointer to an object. It is the handle with which you manipulate an object. Objective-C does all of the class compatibility work at runtime.
Hope this helps. Any questions?
Updates
That's right: int, float, double, char, void, and the pointer combinations, are all C primitive types. You can and will still use these quite a bit, and they are just what they are in a C program. But Objective-C adds the id type as a way to bridge the gap between the primitive typing of C and the very high-level typing of objects by the Objective-C system. id itself is typedef'd as a pointer to a simple struct in objc.h. At the level of the compiler and the language itself, there really isn't too much meaning to the type. For example, you'll almost never declare an array of ids, certainly never perform any arithmetic with them.
In fact, it's not too far a stretch to say that Objective-C is just plain vanilla C with some added syntax (particularly, the square-bracket notation for method invocation), a few extra primitive types (id, IMP, SEL), and a big runtime library. It's this runtime library that handles all things Object-Oriented.
Anyway, to answer your question, when you're actually programming, you will most often (99% of the time) just use class names to declare your variables - NSString *, NSData *, UITableViewController *, and so on. And the compiler will know what you're talking about, and issue a warning if you write code that clearly tries to put an NSString* where an NSData* is expected. But the actual meaning of those types really exists only at runtime.
I've digressed a little, but I'm not sure where your understanding is failing you, so I thought I'd just explain things a bit. You might want to read Apple's The Objective-C Programming Language to get a feel for the language.
NSString is an Objective-C class and all object references in Objective-C are pointers. I would suggest reading through some of the documentation such as Learning Objective-C A Primer:
Notice the * in the first declaration. In Objective-C, object
references are pointers. If this doesn’t make complete sense to you,
don’t worry—you don’t have to be an expert with pointers to be able to
start programming with Objective-C. You just have to remember to put
the * in front of the variable names for strongly-typed object
declarations. The id type implies a pointer.
It's not a digit, it's the "text" from the label, which is (I'm guessing) a string of integers and such to express the time.
So, all NSString types are declared as pointers in Obj-c.
sender.titlelabel.text;
Returns a NSString *
Remember, it's the same as:
NSString *str = [sender.titlelabel getText];
Because text is too. Or more preceisly, because the getText message returns a pointer.
You can find an intersting about why it has to be a pointer:
NSString and Pointers
I Hope it will help you to understand it in a Objective-C way.
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 have a game object which processed in two completely different places. In Contact Listener i check some conditions and if they occur i must save one or more portions of complex data. So i decided to use struct. For example:
struct SomeStruct
{
int value1;
int value2;
CGPoint value3;
b2Vec2 value4;
};
typedef SomeStruct SomeStruct;
In Game Scene i go through all game objects and if its the stack/array not empty, do some stuff and wipe it.
In Contact Listener it repeats from the beginning.
I must use this architecture because of strict order of execution (method must be called after other methods).
I suspect that i need something like vector or NSMutableArray (i think it will not work with struct), so vector may the the only way.
But don't understand how to achieve it. May you help me with some code/pseudocode or link to the book/article where i can found a solution?
Cocoa provides NSValue class for that purpose:
This creates an object that you can add to NSMutableArray:
NSValue *someObj = [NSValue valueWithBytes:&myStruct objCType:#encode(SomeStruct)];
You can use [someObj pointerValue] to access a void* representing the address of the structure that you put in NSValue.
There is a lot of solutions for this problem.
Don't use struct. An obj-c class is practically the same thing as a struct.
Use CFArray (CFArrayCreateMutable) and put it there as a pointer.
Use a C++ class with STL vector.
Use a C array (SomeStruct[]) and increase its length when you need it.
Use a classic implementation of a stack, with a linked list (every struct has a pointer to the next value).