I'm sorry if this is a bit of a C-noob question: I know I need to swot up on my pointers. Unfortunately I'm on a deadline so don't have time to work through a whole book chapter, so I'm hoping for a bit more targeted advice.
I want to store some objective-C objects in a C array. I'm using ARC. If I were on the Mac I'd be able to use NSPointerArray instead, but I'm on iOS and that's not available.
I'll be storing a three-dimensional C array: conceptually my dimensions are day, height, and cacheNumber. Each element will either be a pointer to an objective-C object, or NULL.
The number of caches (i.e. the size of the cacheNumber dimension) is known at compile time, but the other two are not known. Also, the array could be very large, so I need to dynamically allocate memory for it.
Regarding ownership semantics, I need strong references to the objects.
I would like the whole three-dimensional array to be an instance variable on an objective-C object.
I plan to have a method that is - tableForCacheNumber:(int)num days:(int*)days height:(int*)height. That method should return a two-dimensional array, that is one specific cache number. (It also passes back by reference the size of the array it is returning.)
My questions:
What order should I put my dimensions so that I can easily return a pointer to the subarray for one specific cache number? (I think it should be first, but I'm not 100%.)
What should the return type of my method be, so that ARC doesn't complain? I don't mind if the returned array has an increased reference count or not, as long as I know which it's doing.
What type should my instance variable that holds the three dimensional array be? I think it should just be a pointer, since that ivar just represents the pointer to the first item that's in my array. Correct? If so, how do I specify that?
When I create the three-dimensional array (for my ivar), I guess I do something like calloc(X * Y * Z, sizeof(id)), and cast the result to the type for my ivar?
When accessing items from the three-dimensional array in the ivar, I believe I have to dereference the pointer each time, with something like (*myArray)[4][7][2]. Correct?
Will the two-dimensional array I return from the method be similarly accessed?
Do I need to tag the returned two-dimensional array with objc_returns_inner_pointer?
I'm sorry once again that this is a bit of a bad Stack Overflow question (it's too long and with too many parts). I hope the SO citizens will forgive me. To improve my interweb karma, maybe I'll write it up as a blog post when this project has shipped.
First off: while you don't have NSPointerArray, you do have CFMutableArrayRef and you can pass any callbacks you want for retain/release/description, including NULL. It may be easier (and performance is something you can measure later) to try that first.
Taking your points in order:
you should define your dimensions as [cacheNumber][days][height], as you expect. Then cache[cacheNumber] is a two-dimensional array of type id *[][]. As you've said performance is important, be aware that the fastest way to iterate this beast is:
for (/* cacheNumber loop */) {
for (/* days loop */) {
for (/* height loop */) {
//...
}
}
}
it should be of type __strong id ***: that's a pointer to a pointer to a pointer to id, which is the same as array of (array of (pointer to id)).
your ivar needs to be __strong id **** (!), because it's an array of the above things.
you guess incorrectly regarding allocating the array.. If you're using a multidimensional array, you need to do this (one dimension elided for brevity):
- (__strong id * * *)someArray {
__strong id * * *cache = (__strong id * * *)malloc(x*y*sizeof(void *));
id hello = #"Hello";
cache[0] = (__strong id * *)malloc(sizeof(void *)); //same for cache[1..x-1]
cache[0][0] = &hello; // for all cache[x][y]
return (__strong id * * *)cache;
}
correct, that is how you use such a pointer.
yeah, the two-D array works in the same way, sans the first dimension.
I don't think so, you're handing out __strong object pointers so you should be grand. That said, we're at about the limit of my ability with this stuff now so I could well be wrong.
Answering my own question because this web page gave me the missing bit of info I needed. I've also upvoted Graham's answer, since he was very helpful in getting my head round some of the syntax.
The trick I was missing is knowing that if I want to refer to items in the array via the array[1][5][2] syntax, and that I don't know the sizes of my array at compile time, I can't just calloc() a single block of data for it.
The easiest to read (although least efficient) method of doing that is just with a loop:
__strong Item ****cacheItems;
cacheItems = (__strong Item ****)calloc(kMaxZooms, sizeof(Item ***));
for (int k = 0; k < kMaxZooms; k++)
{
cacheItems[k] = (__strong Item ***)calloc((size_t)daysOnTimeline, sizeof(Item **));
for (int j = 0; j < daysOnTimeline; j++)
{
cacheItems[k][j] = (__strong Item **)calloc((size_t)kMaxHeight, sizeof(Item *));
}
}
I'm allocating a three dimensional array of Item *s, Item being an objective-C class. (I have of course left out the error handling code in this snippet.)
Once I've done that, I can refer to my array using the square brackets syntax:
cacheItems[zoom][day][heightToUse] = item;
The web page I linked to above also describes a second method for performing the memory allocations, that uses only one call to calloc() per dimension. I haven't tried that method yet, as the one I've just described is working well enough at the moment.
I would think of a different implementation. Unless it is a demonstrable (i.e. you have measured and quantified it) performance issue, trying to store Objective-C objects in plain C arrays is often a code smell.
It seems to me that you need an intermediate container object which we will call a Cache for now. One instance will exist for each cache number, and your object will hold an NS(Mutable)Array of them. Cache objects will have properties for the maximum days and height.
The Cache object would most easily be implemented with an NSArray of the objects in it, using simple arithmetic to simulate two dimensions. Your cache object would have a method -objectAtDay:Height: to access the object by its coordinates.
This way, there is no need at all to worry about memory management, ARC does it for you.
Edit
Given that performance is an issue, I would use a 1D array and roll my own arithmetic to calculate offsets. The type of your instance variable would be:
__strong id* myArray;
You can only use C multilevel subscripts (array[i][j][k]) if you know the range of all the dimensions (except the first one). This is because the actual offset is calculated as
(i * (max_j * max_k) + j * max_k + k) * sizeof(element type)
If the compiler doesn't know max_j and max_k, it can't do it. That's precisely the situation you are in.
Given that you have to use a 1D array and calculate the offsets manually, the Apple example will work fine for you.
Related
I've taken a look at several other questions that all deal with this error, but all of the answers basically just say "don't do pointer math". The code I have is as follows:
MyObject * __strong * array = (MyObject * __strong *)calloc(sizeof(MyObject*), 5);
array[0] += 4;
I want to know why I can't do pointer math. According to the error, the MyObject* values that are pointed to are of different sizes, but that shouldn't matter, right? It's just an array of pointers - who cares how big the stuff they point to is?
I'm guessing I probably just don't understand all the nuances of ARC.
Your variable "array" is a pointer to a pointer and yes, you can consider it as an array of pointers. The size of your object "MyObject" is not constant obviously and the compiler is right not allowing you do such operation.
It is not clear what your wanted to do with your variable.
array[0] += 4;
Takes "MyObject *" from the first element of your array and want to add to it offset of 4, but to calculate such offset compilers wants to know the size of the object.
May be you wanted to do something like
array = array + 4;
or
array += 4;
I was looking around and couldn't find anything, and I'm starting to think it's not possible with objective-c.
I have a NSMutableArray *myMutableArray and the size varies depending on what csv file is loaded. Since I do not set a size of myMutableArray I can't do:
if (c == 5){
myMutableArray[q] = [[NSNumber numberWithFloat:myOtherArray] stringValue];
q = q + 1;
c = 0;
}
Else {
c = c + 1;
}
Since myMutableArray is technically of size nil I guess I can't add objects to it.
In cases, q can be between 1500 and 2500.
My question is, how do I make `myMutableArray' change size on every loop.
If this isn't possible, I guess I will have to make myMutableArray very large - but I need the values in myMutableArray for a graph. If I do myMutableArray = [[NSMutableArray alloc] initWithCapacity:5000]; and don't use the 5000 memory locations, will these locations be nil, or 0? (sorry if the technical words are wrong, by memory locations I mean the memory given to myMutableArray)
Thank you, if there is anything else I can add to this please feel free to let me know.
EDIT: What I'm trying to achieve is adding data to the array, and with that data create a graph
You can't have a sporadically populated array. Creating an array with capacity 5000 is just a hint as to how much data you might be going to store into the array. Consider using an array of dictionaries where the dictionary contains the q value (presumably one coordinate) and the associated value. You can then sort the array based on the q values if you need to (for plotting). Then you can just add the dictionaries to the array as usual (addObject:).
The NSMutableArray class declares the programmatic interface to objects that manage a modifiable array of objects. This class adds insertion and deletion operations to the basic array-handling behavior inherited from NSArray.
If you
arrayWithCapacity:
Creates and returns an NSMutableArray object with enough allocated memory to initially hold a given number of objects.
Mutable arrays expand as needed. When declaring them, we can init them like this:
+ (instancetype)arrayWithCapacity:(NSUInteger)numItems
Here numItems simply establishes the object’s initial capacity.
Later to add more data, i.e. to expand mutable array, use this
addObject:
What it does is, it inserts a given object at the end of the mutable array.
- (void)addObject:(id)anObject
It's important to note that:
The object to add to the end of the array's content. This value must not be nil. It raises an NSInvalidArgumentException if anObject is nil.
Out of pure curiosity, I started playing with array's in ways that I have never used before. I tried making a data structure array, and set it equal to another:
typedef struct _test {
float value;
} test;
Simple enough struct, so I tried this:
test struct1[10];
test struct2[20];
struct1 = struct2;
I didn't think this would work, and it didn't even compile. But, this interests me a lot. Is it possible to take an array of 10 and increase the size to 20, while copying the data?
Objective-C
I am actually doing this with Objective-C, so I'd like to hear from the Objective-C people as well. I want to see if it is possible to change the size of struct1 in this file.
#interface Object : NSObject {
test struct1;
}
Remember: This is only out of curiosity, so everything is open to discussion.
Something else that is not exactly pertinent to your question but is interesting nonetheless, is that although arrays cannot be assigned to, structs containing arrays can be assigned to:
struct test
{
float someArray[100];
};
struct test s1 = { /* initialise with some data*/ };
struct test s2 = { /* initialise with some other data */ };
s1 = s2; /* s1's array now contains contents of s2's array */
This also makes it possible to return fixed-length arrays of data from functions (since returning plain arrays is not allowed):
struct test FunctionThatGenerates100Floats(void)
{
struct test result;
for (int i = 0; i < 100; i++)
result.someArray[i] = randomfloat();
return result;
}
As others have said, arrays allocated like that are static, and can not be resized. You have to use pointers (allocating the array with malloc or calloc) to have a resizable array, and then you can use realloc. You must use free to get rid of it (else you'll leak memory). In C99, your array size can be calculated at runtime when its allocated (in C89, its size had to be calculated at compile time), but can't be changed after allocation. In C++, you should use std::vector. I suspect Objective-C has something like C++'s vector.
But if you want to copy data between one array and another in C, use memcpy:
/* void *memcpy(void *dest, const void *src, size_t n)
note that the arrays must not overlap; use memmove if they do */
memcpy(&struct1, &struct2, sizeof(struct1));
That'll only copy the first ten elements, of course, since struct1 is only ten elements long. You could copy the last ten (for example) by changing &struct2 to struct2+10 or &(struct2[10]). In C, of course, not running off the end of the array is your responsibility: memcpy does not check.
You can also you the obvious for loop, but memcpy will often be faster (and should never be slower). This is because the compiler can take advantage of every trick it knows (e.g., it may know how to copy your data 16 bytes at a time, even if each element is only 1 byte wide)
You can't do this in C with static arrays, but you can do it with dynamically allocated arrays. E.g.,
float *struct1, *struct2, *struct3;
if(!(struct1 = malloc(10 * sizeof(float))) {
// there was an error, handle it here
}
if(!(struct2 = realloc(struct1, 20 * sizeof(float))) {
// there was an error, handle it here
// struct1 will still be valid
}
if(!(struct3 = reallocf(struct2, 40 * sizeof(float))) {
// there was an error, handle it here
// struct2 has been free'd
}
In C, I believe that's a good place to use the realloc function. However, it will only work with dynamically allocated arrays. There's no way to change the memory allocated to struct1 by the declaration test struct1[10];.
In C arrays are constants, you can't change their value (that is, their address) at all, and you can't resize them.
Clearly if you declare your array with a fixed size, test struct1[10] then it cannot be resized. What you need to do is to declare it as a pointer:
test *struct1;
Then you must use malloc to allocate the array and can use realloc to resize it whilst preserving the contents of the original array.
struct1 = malloc(10*sizeof(*struct1));
//initialize struct1 ...
test *struct2 = realloc(struct1, 20*sizeof(*struct1));
If you're using Objective C, you know you can just use NSMutableArray, which automatically does the realloc trick to reallocate itself to store however many objects you put in it, up the limit of your memory.
But you're trying to do this with struct? What would that even mean? Suppose you increase the amount of memory available to struct1 in Object. It's still a struct with one member, and doesn't do anything more.
Is the idea to make Object be able to contain an expanded struct?
typedef struct _test2 {
float value;
NSObject *reference;
} test2;
But then you still can't access reference normally, because it's not a known part of Object.
Object *object2;
...
NSLog(#"%#", object2.struct1.reference); // does not compile
If you knew you had one of your modified objects, you could do
Object *object2;
...
NSLog(#"%#", ((test2)(object2.struct1)).reference);
And also you could still presumably pass object2 to anything that expects an Object. It only has any chance of working if struct1 is the last member of Object, and don't mess with subclassing Object either.
Some variety of realloc trick might then work, but I don't think realloc in particular, because that's intended to be used on objects that are allocated with malloc, and the details of what C function is used to allocate objects in not exposed in Objective C, so you shouldn't assume it's malloc. If you override alloc then you might be able to make sure malloc is used.
Also you have to watch out for the fact that it's common in Objective C for more than one pointer to an object to exist. realloc might move an object, which won't be semantically correct unless you correct all the pointers.
I've been programming for a while in objective-c, but I've unfortunately never delved very deeply into C and memory pointers, although I do have a rudimentary understanding of them. I'm working with an array of CLLocationCoordinate2D structures, and I'm trying to figure out how to append to the array. First of all, I get the
NSString *aString; //a bunch of coordinates
CLLocationCoordinate2d *coordinates;
int length;
doSomethingCool(aString, &coordinates, &length);
after I do something cool, I want to preserve it in a class variable. If I simply do something like
points = newPoints
points contains the appropriate contents. However, if I try to do something like this:
points = malloc(sizeof(CLLocationCoordinate2D) * length);
points[0] = *newPoints;
points ends up with contents different from newPoints.
Ultimately my goal is to be able to append to points based on length, but I'm not going to be able to do that if I can't get the above code to work. What am I doing wrong?
Your code simply copies the first value of newPoints into the first value of points (*newPoints is equivalent to newPoints[0]).
One situation is to make a new array, copy all values, switch the arrays, and free() the old one. For example:
int* newvals = malloc(sizeof(int) * newcount);
memcpy(newvals, vals, sizeof(int) * oldcount);
free(vals);
vals = newvals;
You can also use realloc - its behavior is similar to the above (though it can fail!), but at times may be more efficient.
Note that you simply can't change the underlying pointer's size in a safe and portable fashion. You will need to update your instance ("class") variable with the new pointer.
The idea would be to copy all of the array into a temporary array, resize the original, and then copy them back. However, managing this could get hairy. You'd be better off using an std::vector and just appending it.
EDIT: I just realized you're using C, not C++. Disregard the second half of this.
Right now I have this setup:
An NSMutableArray which stores two NSMutableArray instances.
A for-loop walks over a set of data and writes values into these arrays. It's for a big diagram / chart which is going to be displayed with OpenGL ES.
The NSMutableArray containing two other NSMutableArray objects is returned by a method, and the caller assigns it to a retaining property.
The pitty is this: There can be up to 2.000 values, and I don't like to create all those NSNumber objects.
Now I hope there's a lightweight way to do this with C.
Before I walk the loop I know the number of data points.
Now I want to refactor this so that I get lightweight C-arrays that hold just plain old float values. I know how to create a C-array of float values, but not really dynamically:
CGFloat values[ ] = {
0, 2.5f,
30.2f, 2.5f,
50.95f, 200.55f,
930.2f, 122.1f,
};
Questions:
1) How can I create an array like this dynamically in a loop?
2) How would I put two of those arrays into one array?
3) What about the memory managament? The method returns that multidimensional C-array, and the receiver needs to assign that to an instance variable (property). It needs to be kept around for a while. How could I create an instance variable to hold such an C-array, without knowing it's exact size in advance?
For your first question, you can dynamically create an array using malloc(), and use a pointer variable to store a reference to the first element of it. For example, this will create an array of 8 CGFloats:
CGFloat *a = malloc(8 * sizeof a[0]);
If a is non-NULL, you can then access a[0] through a[7] in the usual way. You can return this CGFloat * value to another function, and when you are done with the array, you must pass this value to free().
For second question, you can create an array of two pointers to the two dynamic arrays:
CGFloat *a[2] = { NULL, NULL };
a[0] = malloc(8 * sizeof a[0][0]);
a[1] = malloc(16 * sizeof a[1][0]);
You can now access a[0][0] through a[0][7] and a[1][0] through a[1][15] (as long as a[0] and a[1] are not NULL).
However, there is a wrinkle here: You cannot directly return arrays in C, so you cannot return a from a function anymore. You could instead use two levels of dynamic arrays, but it probably makes more sense to store the array of pointers within the retaining object in the first place, and pass a reference to this array to the function that fills it in.
This means that your containing object would include the CGFloat *a[2] field, and your function that allocates the dynamic arrays and fills them would include a CFloat *dest[] parameter. That function would start with something like:
dest[0] = malloc(8 * sizeof dest[0][0]);
dest[1] = malloc(16 * sizeof dest[1][0]);
...and then fill in the values as normal.
For the third question, the array that is created by malloc() will live until that pointer is later passed to free(). As mentioned, you can happily return that value and store it elsewhere, as long as you don't free() it until you're done with it.
This will get you a new array:
CGFLoat* myArray = malloc(numberOfFloat * sizeof(float));
You can do things like myArray[6] = 0;.
When you're done with it, you have to call free(myArray), or you will have a memory leak.
With this in mind, your instance variable will just be a CGFloat*.