I'm attempting to understand Objective C arrays. What I want to do is create a multidimensional NSInteger instance array.
Does the code immediately below create an array equivalent to initiating an array and running a double for loop containing NSNull at each point?
#interface Builder: UIView {
NSInteger superDuperArray[5][4];
}
How do I add/replace specific data (NSIntegers) considering this is not an NSMutableArray? I need to do so elsewhere in the file, in several different methods.
The instance variable you've declared does indeed create a multidimensional array that is accessible from any method in the object. It does not, however, contain NSNulls, because that's an object and NSInteger is a primitive type. It contains 0 in every slot (all ivars are initialized to their appropriate zero-value at the object's allocation).
If you don't need to change the size of the array, you're pretty much done. You access the array inside your object just like you'd access an array anywhere else, by subscripting: superDuperArray[2][1], e.g.
Here's a full sample:
#import <Foundation/Foundation.h>
#interface Mutli : NSObject
- (NSInteger)numberAtRow:(NSInteger)row column:(NSInteger)col;
- (void)setNumber:(NSInteger)newNum atRow:(NSInteger)row column:(NSInteger)col;
#end
#implementation Mutli
{
NSInteger numbers[5][4];
}
-(void)setNumber:(NSInteger)newNum atRow:(NSInteger)row column:(NSInteger)col
{
numbers[row][col] = newNum;
}
- (NSInteger)numberAtRow:(NSInteger)row column:(NSInteger)col
{
return numbers[row][col];
}
#end
int main(int argc, const char * argv[])
{
#autoreleasepool {
Mutli * m = [Mutli new];
[m setNumber:10 atRow:2 column:1];
NSLog(#"%ld", [m numberAtRow:2 column:1]);
// This may not crash!
[m setNumber:10 atRow:100 column:34];
}
return 0;
}
N.B. That last line. There's no bounds checking inherent to primitive arrays. Accesses outside the bounds you've set may not crash or cause any immediately-noticeable problem. Instead, you'll get garbage for reads, and will corrupt memory when you write. (Technically, you're invoking Undefined Behavior.) You should really include bounds checking in your accessor methods.
Also, a style note: it's no longer good ObjC form to put ivars in an object's interface. Hide it in the #implementation block as I've done.
An NSInteger is simply a C 'int' (or 64-bit long on 64-bit platforms - the size is immaterial though).
So what you've declared is nothing but a C array, of integers. Since they're simple integers, there's no NSNull.
If you'd declared an NSArray (of NSArray for 2D) then you'd be looking at NSNull and NSNumber in the elements.
Related
I am trying to dynamically instantiate a c array of blocks, load it and then run them and could use some help.
// Definitions ===========================================
typedef void (^MorphC)(ScratchC* scratch);
#property (nonatomic) MorphC __strong * morphCs;
// Building up the Morph Registry ========================
static NSMutableDictionary* morphs_;
+ (void) initialize {
morphs_ = [[NSMutableDictionary alloc] init];
[MathC hydrate];
}
+ (void) hydrate {
[MathC registerMorph:#"sin" execute:^(ScratchC* scratch) {
AEScratchPush(scratch, sin(AEScratchPop(scratch)));
}];
}
+ (void) registerMorph:(NSString*)name execute:(MorphC)execute {
[morphs_ setObject:execute forKey:name];
}
+ (MorphC) morphFromKey:(NSString*)key {
return [morphs_ objectForKey:key];
}
// Loading up a temporary NSMutableArray* _compiling =====
- (void) applyTag:(NSString*)tag stack:(Stack*)stack {
[_compiling addObject:[MathC morphFromKey:tag]];
}
// Initializing C Array and loading from NSMutableArray ==
- (void) build {
_morphCs = (MorphC __strong *)malloc(_compiling.count*sizeof(MorphC));
i = 0;
for (MorphC morph in _compiling)
_morphCs[i++] = morph; // Currently, getting a bad ACCESS here
}
// Executing the Morphs ==================================
- (CGFloat) evaluateFloat:(VarsC*)vars {
if (![_morphs count]) return NAN;
AEScratchLoadVariables(_scratch, vars);
for (int i=0;i<[_morphs count];i++)
_morphCs[i](_scratch);
return AEScratchPop(_scratch);
}
I'm currently getting a EXC_BAD_ACCESS while building up the C Array, but I suspect I have a number of issues. I don't totally understand the need for the __strong at the morphCs definition, but the compiler complains with out. Should the property have a strong indicator also?
Do I need to be doing [morph copy] in one or more places?
Is there anything else I'm messing up?
You can't malloc an array of strong pointers.
Think about the semantics of a strong pointer: When it is declared, it's value is initialized to nil. When the variable goes out of scope, it releases its existing value. Therefore, the compiler must be able to keep track of strong pointers to be able to carry this out. If you have an array of strong pointers of unknown length, when it goes out of scope for example, how can the compiler know how many pointers to release? It can't.
In C++ terminology, strong references are "non-POD" types - they have nontrivial constructors and destructors. Therefore, they cannot be allocated with malloc.
It is mentioned here in the ARC specification:
It is undefined behavior if a managed operation is performed on a
__strong or __weak object without a guarantee that it contains a primitive zero bit-pattern, or if the storage for such an object is
freed or reused without the object being first assigned a null
pointer.
In other words, the only way you can use malloc and free is if you guarantee that every time after you call malloc you zero the memory of all the pointers you allocated, before using them. And every time before free you guarantee to first assign nil to each strong pointer in the array.
However, in Objective-C++, you can use new[] and delete[] to dynamically allocate arrays of strong pointers.
These requirements are followed automatically in Objective-C++ when
creating objects of retainable object owner type with new or new[] and
destroying them with delete, delete[], or a pseudo-destructor
expression.
This question already has answers here:
declaring array of an object X with unknown size objective c
(2 answers)
Closed 8 years ago.
I have a class named Calculator. This class accepts a bunch of test scores, and will store each of them into an array. This array is called scoreArray.
I want to declare the array like this, but I'm having trouble with the scope:
int scoreArray[numTestScores];
If I put that code into the #implementation, it doesn't know what numTestScores is, because it hasn't been passed to the class yet.
And if I try to do this:
-(id)init:(int)numTestScores_
{
if (self = [super init])
{
int scoreArray[numTestScores_];
}
return self;
}
then the array gets created, but the rest of the class doesn't know what scoreArray is.
How can I make it so that scoreArray is created with length "numTestScores" and has the same scope as if I had put it in the implementation block?
Using a native C array is an unnecessary pain. I'd rather use a NSMutableArray, declaring it as a property.
#property (nonatomic, copy) NSMutableArray *scores;
NSMutableArray automatically manages its memory, so you don't need to declare its size in advance.
Just initialize it as
_scores = [NSMutableArray array];
and then add values to it
[self.scores addObject:#(aResult)]; //assuming that aResult is an integer expression
#(...) wraps the value in a NSNumber since NSArray can only hold objects.
To retrieve a score, you can do
int score = [self.scores[0] intValue];
VLAs (variable-length arrays) only work in contexts where... um... where they make sense. In this case, you will rather want to utilize dynamic memory allocation and an instance variable:
#interface MyClass: NSObject {
int *array;
size_t size;
}
// ... etc ...
- (id)initWithSize:(size_t)n
{
if (self = [super init]) {
size = n;
array = malloc(size * sizeof array[0]);
}
return self;
}
// free the allocated memory upon destruction
- (void)dealloc
{
// ...
free(array);
// ...
[super dealloc];
}
As to why it doesn't really make sense to use a variable-length array as an instance variable: instance variables are part of an object. If you declared a VLA inside an object, then the size of the instance would depend on its initialization. That is not something immediately easy to implement, and it is not the way the Objective-C runtime works. (I'm not saying it's impossible, but it would be very, very impractical.)
All classes have their instance size deduced at compile time (well, mostly... nowadays it's rather the initialization of the runtime system), and it can't be changed later. As a consequence, the size of an object cannot vary from initialization to initialization.
On the assumption that you're writing a properly contained object oriented class, the implementation of your set of scores is not important to the outside world. If that is indeed the case, don't create an int array, create an NSMutableArray instead (and if you want to pre-fill numTestScores_ entries to make things easier later, do that, but there shouldn't be a need for it really).
If you must have an array, you will have to allocate it dynamically by declaring scoreArray to be an int * and using malloc. Be careful here though -- you will have to create a dealloc method in your class to free() the array if it has been created.
i have two Objective-C classes, say ParentLayer and ChildLayer. in my child instance, i want to access a C-Array in my parent instance. so i have something like this in my cocos2d code:
#define kNumOfElements 10
#implementation ParentLayer{
int array[kNumOfElements];
}
-(id)init{
//...
for(int i=0;i<kNumOfElements;i++){
array[i] = i;
}
[self addChild:childLayer];
[childLayer initializeValues];
//...
}
-(int *)getArray{
return array;
}
#end
//meanwhile in my child layer...
//...
-(void)initializeValues{
int *arr = [(ParentLayer *)[self parent] getArray];
//NSLog(#"%d",arr[0]); <------- this gives you bad exec access point, and looks like it's 0x00 for memory address
}
//...
what's the proper way to do this?
maybe i dont understand the right memory management behind C Arrays.
i was under the impression that C Arrays didn't need to be allocated,
and that they could be passed by value, on the stack?
also, shouldn't my parent instance still be around? i thought if i
put a C Array as an ivar of my parent, it shouldn't get destroyed
any help is appreciated. thanks!
what's the proper way to do this?
Ideally, you should never pass a C-style array pointer outside of the object that owns it. You open yourself up to all sorts of problems if a piece of code tries to use the array after the object is deallocated, or writes past the end, or something else. It is easier to guarantee that none of this happens if you can make sure the reference never leaves the object's source file.
maybe i dont understand the right memory management behind C Arrays. i was under the impression that C Arrays didn't need to be allocated, and that they could be passed by value, on the stack?
It is not that simple.
A C-style array is just a memory address. That's it. It doesn't carry around the other useful information that an object might, such as number of elements, retain count.
If you declare an array like this:
int array[100];
Then the memory is allocated in either the stack or the heap, depending on where you put the declaration. If it's a local variable inside a function or method, it's on the stack. If it's in global scope or a member variable of an object, it's on the heap.
Furthermore, if it's an instance variable, you're actually setting aside 100 ints worth of memory inside the block of memory allocated to hold the object. It isn't a separate thing.
Since array is just a memory address, you are basically passing it around by reference. Technically, you are passing the address by value, but any changes you make to the memory will be seen by anyone looking at the same address, so it acts like pass by reference.
also, shouldn't my parent instance still be around? i thought if i put a C Array as an ivar of my parent, it shouldn't get destroyed
The way you have coded it, that array will be valid as long as the parent object is around. Once the parent gets deallocated, that memory could be reclaimed. Since the array variable is just a memory address, however, you have no way of knowing whether the data it points to is valid or not. This is the danger of using C-style arrays rather than objects.
Since the last line is giving you NULL (0) address, my guess is that [self parent] is nil. That would put a 0 in arr; when you try to dereference NULL, you will get an exception.
In Objective C, you can use property for this.
#define kNumOfElements 10
#interface ParentLayer: NSObject
{
int *array;
}
#property(nonatomic, assign) int *array;
#end
#implementation ParentLayer
-(id)init{
//...
self.array =(int*)malloc(sizeof(int) * kNumOfElements);
for(int i=0;i<kNumOfElements;i++){
self.array[i] = i;
}
[self addChild:childLayer];
[childLayer initializeValues];
//...
}
//-(int *)getArray{
// return array;
//}
-(void)dealloc
{
if(self.array)
{
free(self.array); self.array = NULL;
}
[super dealloc];
}
#end
-(void)initializeValues{
ParentLayer *player = (ParentLayer *)[self parent] ;
int *arr = player.array;
//NSLog(#"%d",arr[0]); <------- this gives you bad exec access point, and looks like it's 0x00 for memory address
}
can't seem to add a reply to benzado's post. but depending on how to declare your object, it might be automatically deallocated. to ensure that it is retained, use a retain keyword.
[obj retain];
especially using the cocos2d framework, they have quite a number of auto release objects. typically initWith shouldn't be auto release.
I want to make some NSNumber constants via the same style used for NSStrings in this topic. That is, I'm creating separate constants.h/.m files and importing them into classes that need to access them.
The trouble with doing this is that there isn't such a thing as a compile-time constant NSNumber. Only NSString gets that distinction. NSNumbers are always created dynamically. You can fake it by using a function that runs at your program's startup to initialize the variables. Your options:
Create a class with a +load method that performs the initialization.
In the file with the constants, include a function with __attribute__((constructor)). So, for example:
// Constants.m
NSNumber *someGlobalNumber;
__attribute__((constructor))
static void InitGlobalNumber() {
someGlobalNumber = [[NSNumber numberWithInteger:1] retain];
}
But of course then you can't reliably use these numbers in any other functions which are run that early in the startup process. This usually isn't a problem, but is worth keeping in mind.
The other option, which I've seen crop up a few times, is to have a class with accessors for the numbers instead of giving raw access to the variables. It's a bit of a heavier design, but it also feels less voodooish, which has its charms.
Unfortunately you cannot currently generate NSNumber constants in the same way you can generate NSString constants. When you try to do you will get a compiler error
NSNumber * const kNumberConstant = #2; // This doesn't work.
However, you can use primitives instead.
NSInteger const kSomeIntValue = 10;
You can basically achieve close to what you want in three parts:
.h file:
extern NSNumber *MyFirstConstant;
.m file
NSNumber *MyFirstConstant;
AppDelegate.m
+(void)initialize
{
MyFirstConstant = #5;
...
}
AppDelegate is guaranteed to run before any of your other code, and the initialize is the first method that would be called on AppDelegate, so you can essentially insure all your constants are setup for you before your app runs.
update:
Years later, I just realized it is possible to create a NSNumber constant for integers... but it's a hack:
#define CONST_INT_NSNUMBER( x ) ((__bridge NSNumber * const)(void * const)(( x << 8 ) | 0x27))
NSNumber * const number = CONST_INT_NSNUMBER(123) ;
This works because certain integer NSNumbers are stored as tagged pointers.
original answer:
You can't do it.
NSNumber * const mynumber = #5.5;
gives:
Initializer element is not a compile-time constant
Implying the compiler has a special feature specifically for creating compile-time constant NSString objects, but not any other type of object.
You could do this, however:
.h:
extern NSNumber * kConstantNumber ;
.m:
NSNumber * kConstantNumber ;
#implementation NSNumber (InitializeConstants)
+(void)load
{
kConstantNumber = #42;
// ... and the rest ...
}
#end
I was wondering if there is any sample code out there for objective-C for implementing a NSMutableArray of type struct. Inside, I need there to be 2 mutable arrays (via NSMutableArray also) declared in the struct. All the code samples in my book show me how to make an array of defined size via C array syntax (with the brackets), but I don't know how to get one going with NSMutableArray. Has anyone else done this before? Here's my code so far...It compiles fine I have defined the size of the arrays (2 and 5 are used in my code below as an example, but I need to set it so I can have them mutable. I can work with simple structs when they just have some of the "easier-to-understand" data types like int, double, short, long, BOOL (you get the idea). When it gets into pointers though, this is where I become lost (I can use pointers fine, but knowing how to put them in a struct is the difficult part). Once the code is working with NSMutableArray's, would I put "network" in the interface as a pointer to type "Network"? I tried this before, but I got errors. In the end, I basically want to be able to write
network.input[2].list[1].value = 5.0;
on an arbitrarily defined array of type "Network". Could anyone offer suggestion or links to information about making a NSMutableArray of type "Network" which includes a struct of two NSMutableArray's? Thanks for any help!
SomeFile.h
#import <Foundation/Foundation.h>
struct lists{
double value;
};
// supporting structs
struct inputs{
struct lists list[2];
};
struct Network {
struct inputs input[5];
struct lists output[5];
}
#interface SomeFile : NSObject {
}
#end
SomeFile.m
#import "SomeFile.h"
#implementation SomeFile
#end
NSArray and NSMutableArray can only contain Objective-C objects, so you can't store structs in them. If the contents must be structs and you want something similar to NSArray, use NSPointerArray, available in 10.5 and later.
You can store pointers to Objective-C objects (like NSPointerArray* or id) inside a struct just like any other pointer type. For example, you could declare a struct for a doubly-linked list node that stores an Objective-C object like this:
typedef struct DoublyLinkedListNode {
id object;
__strong struct DoublyLinkedListNode *next;
__strong struct DoublyLinkedListNode *prev;
} DoublyLinkedListNode;
The __strong attribute is used in connection with garbage collection in Objective-C 2.0, since pointers to Objective-C objects act as strong references, but C pointer types do not by default. This way, as long as one node in the list is referenced from a __strong reference, the list won't disappear. (Read the Garbage Collection Programming Guide for details, and particularly the second half of Using Core Foundation with Garbage Collection.) You'll probably want to consider doing this for your structs.
As far as your desired syntax, I may not have fully understood your question, but you won't be able to use the bracket syntax to access objects in a Cocoa collections like an NSPointerArray. (Also, odds are you'll have to use the "->" operator instead of "." for the structs, since they're likely to be allocated on the heap. All Objective-C objects must be, and I assume you'll want to store these structs outside of the local scope of a method.)
Since Objective-C doesn't have generics, you also can't "implement [an] NSMutableArray of type struct". In fact, one of your previous SO questions has more detail on the subject. If that's not what you meant, feel free to clarify.
This is not be a complete answer; I’m not sure what you mean by not knowing how to put pointers in structs. I’m going to proceed by assuming you want to model an network of multiple inputs and outputs with a dynamic number of both.
You have a few choices here:
Use value objects instead of structs to store your values:
[[[network inputs] objectAtIndex:2] replaceObjectAtIndex:1 withObject:[NSNumber numberWithDouble:5.0]];
Model your Network with an object:
#interface Network : NSObject {
// ivars
}
- (void) setInput:(double)value atIndex:(NSInteger)valueIndex ofInputAtIndex:(NSInteger)inputIndex;
- (double) outputAtIndex:(NSInteger)index;
#end
Just use structs like you’re already doing; if you need to change the size up-front, use your friend malloc:
struct Network_value {
double value;
}
struct Network {
struct Network_value **inputs;
struct Network_value *outputs;
};
void Network_alloc(Network *n, unsigned inputs, unsigned input_values, unsigned outputs) {
n->outputs = calloc(sizeof(Network_value), outputs);
n->inputs = calloc(sizeof(Network_value *), inputs);
while (inputs --> 0) {
n->inputs[inputs] = calloc(sizeof(Network_value), input_values);
}
}
void Network_free(Network *n, unsigned inputs) {
free(n->outputs);
while (inputs --> 0) {
free(n->inputs[inputs]);
}
free(n->inputs);
}
Network network;
Network_alloc(&network, 5, 2, 5);
network.inputs[2][1].value = 5.0;
Network_free(&network, 2);
Combine ideas 2 and 3 by presenting a Network object but internally store the values with structs. This is probably a good idea if the number of inputs and outputs is very large.