I'm reading objc/runtime source code. I noticed there are fixedUp getter and setter in struct method_list_t. What does that mean?
Edit 1:
I have read the 2 answers to the question What is `objc_msgSend_fixup`, exactly?. I don't think these have answered my question because they did't explain what fix_up really means.
Edit 2:
Here is the source code I was reading. https://github.com/xdxu1126/objc4
The struct mentioned (struct method_list_t) is located at FILE objc-runtime-new.h LINE 586.
static uint32_t fixed_up_method_list = 3;
fix_up mean is equal 3
3 = (alloc + init + currentMethod)
this is my own idea
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What does a type name in parentheses before a variable mean?
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Closed 8 years ago.
I'm learning objective-C and I was looking at some sample code from:
https://developer.apple.com/library/ios/samplecode/UsingPhotosFramework/Listings/SamplePhotosApp_AAPLAssetGridViewController_m.html#//apple_ref/doc/uid/TP40014575-SamplePhotosApp_AAPLAssetGridViewController_m-DontLinkElementID_8
I'm confused about this line of code here:
CGSize cellSize = ((UICollectionViewFlowLayout *)self.collectionViewLayout).itemSize;
I understand that it's trying to get the itemSize property and store it into cellSize, but I have no idea what ((UICollectionViewFlowLayout *)self.collectionViewLayout) is all about. Can someone break it down for me? Is there another way to write this line of code?
What it means is:
Cast self.collectionViewLayout to be of UICollectionViewFlowLayout type. Then of self.collectionViewLayout get the itemSize property. Finally, save everything is a property of type CGSize.
I believe is a elegant and concise way of writing it.
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Checking the size of an object in Objective-C
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Closed 8 years ago.
I'm new to obj-c . During these day's practices I noticed that every class based on NSObject can't have an entity like : NSObject en; in c++ but NSObject* en instead.
But, sometimes I need to know the Size of an Object.I can't simply write sizeof(en) because en is a pointer var.I can't simply use sizeof(NSObject) neither for the compiler telling me Application of sizeof to interface 'XXXX' is not supported on this architecture and platform.
I want to know if there is a way to get sizeof(NSObject) .If not,what the syntax is designed this for & any other ways to get the size.
From doc
class_getInstanceSize
Returns the size of instances of a class.
size_t class_getInstanceSize(Class cls)
Parameters cls A class object.
Return Value The size in bytes of instances of the class cls, or 0 if
cls is Nil.
But I doubt this is what you really want. Because I never found it useful and can't think a case it may be useful. (other than learning memory layout of objects and low level implementation details)
First, you should import malloc.h
If you use Non-ARC:
malloc_size(myObject);
if you are using ARC:
malloc_size((__bridge const void *) myObject));
This linker is a question similar to yours.
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I want to ask that what is that id? I don't understand what this id is. I got this code from a book and it says a generic type that's used to refer to any kind of object. Can anyone help me with this? I read it few times. Still can't get it.
void drawShapes (id shapes[], int count){
for (int i = 0; i < count; i++) {
id shape = shapes[i];
[shape draw];
}
} // drawShapes
id is an alias for an unknown Objective-C object. It can be used to declare any Objective-C object value.
In the example you have it is using an id rather than a specific class so that the code is not dependent on the class of shape.
Strictly speaking id is defined as a pointer to an objc_object struct.
typedef struct objc_object {
Class isa;
} *id;
In practical terms this means any Objective-C object.
However don't confuse this with NSObject *. While in many cases the equivalence may hold, there are classes which do not descend from NSObject but are still valid Objective-C objects (and therefore whose type can be id). One notable example is NSProxy.
In the code you posted, the id stands for the type of items that will be stored in a C static Array. In particular, the id type indicates any Objective-C object.
Anyway, I would not recommend to use C static arrays in Objective-C to contains objects of unknown type, when you can achieve the same result by using an instance of NSArray.
id means "a reference to some random Objective-C object of unknown class" an example is when you make an at property for a uibutton sometimes it will come up as id but setting it as uibutton will help xcode fill in blanks for you while your typing because now xcode knows exactly what this object is. in your situation shape could be a string or a number or something else if you were just looking at that line of code though passing anything into it could give another line a error later on.
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Ok... I understand what pointers are and how they point to a memory location where the variable is stored.
However, I still can't fully get my head wrapped around when the pointer (asterisk) is to be used and when it isn't. It's still seemingly random to me.
Does anyone have any recommendations for good tutorials on the web or book chapters that really go into detail on pointers?
Thanks!
Edit: What I'm really looking for is a detailed guide that is specific to Objective-C either in a deep website tutorial or book chapter(s). I don't think there is enough space here to fully explain it for me.
Being aware that this is tagged as Objective-C..
Take a look at here.
(Un)fortunately, the asterisk * has many different meanings depending on context. So, let's look at the relevant ones. Let us write T for some arbitrary type (say int):
T x; // variable of type x, stored somewhere in memory
T * pt; // a pointer to a variable of type x -- doesn't have a value just yet
pt = &x; // the value of pt is now the address of the variable x
So far so good. We have used the asterisk to designate a new type, namely T*, which is a "pointer to T".
But what do we do with a pointer? We can dereference it to get to the value of the variable at the address pointed to by the pointer:
T y; // another variable of type T
y = *pt; // equivalent to y = x;
*pt = 81; // equivalent to x = 81;
So, if the asterisk is part of the typename, then it designates a pointer type. If it comes before a variable name (which is itself of pointer type), then it dereferences the pointer.
[Clarification:] In C, pointers naturally go hand-in-hand with the "address-of" operator &, which is used to actually obtain a pointer to something. In Obj-C, a pointer is obtained as the result of object allocation (+ initailization): T * pt = [T new];
Beware that Obj-C offers an alternative syntax to the traditional C and C++ pointer syntax, so you may encounter pointers in other guises.[/Clarification]
(The asterisk can also be used as a binary operator of course, so you can have something like this: int x = 5; int * p = &x; int y = *p * *p;.)
Peter Hosey of Growl and Adium has posted a guide to pointers on his blog. He calls it, "Everything you need to know about pointers in C," but that's because he regards the ugly pointer aspects as belonging to C rather than Obj-C. Check it out.
I don't know of any reference beyond the Clang ARC reference that describes all the crazy modifiers you can now put on pointers in Obj-C, though. And that's not a good learning resource if you're already confused.
Assuming that C pointers have no secret to you, basically in Objective-C there are two caveats to avoid:
The NS- prefix does not always denote a class. Sometimes this is only a typedef. You have to check the reference manuals.
Dotted notation introduced in Objective-C 2.0. Despite the fact that every object is a pointer, sometimes instance variables can be accessed by myObj.myVar, instead of (more consistent for newcomers) myObj->myVar.
The ultimate guide to pointers is in Kernighan & Ritchie. They're the same in Objective-C as they are in C.
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Possible Duplicate:
Constants in Objective C
I'm designing a controller and I'm gonna need some constants inside it (locally, just for that controller). Looking at some sample code provided by Apple, I can see these lines:
#import "Constants.h"
#define kTextFieldWidth 260.0
static NSString *kSectionTitleKey = #"sectionTitleKey";
static NSString *kSourceKey = #"sourceKey";
static NSString *kViewKey = #"viewKey";
const NSInteger kViewTag = 1;
Can anyone explain to me what the difference between them is? Which style should I use? Are they dependent on the type of object/value you assign to them? Meaning use: static NSString * for strings, #define for floats and NSInteger for integers? How do you make the choice?
The #define keyword is a compile time directive that causes the define'd value to be directly injected into your code. It is global across the entire program and all linked libraries. So you can strike that off the list, based on your desire to create a constant for the controller only.
The main difference between static and const is that static variables can be changed after initialization, const ones cannot. If you want to be able to modify your variable after initialization then you should use the static keyword.
Hope that helps.
As Scott and benzado pointed out that is the best way to define your constant values. However as far as defines go it is harder to debug using defines as you can usually not easily see the expanded value in a debugger. You will only need to add an extern declaration to the header file of your class if your intentions are to expose the variable globally. And the next thing to remember is to put the const declaration after the pointer (*) or else you will get warnings of discard qualifiers from pointer in most uses.