I want to do achieve something like this in Objective-C
+(int[10][10])returnArray
{
int array[10][10];
return array;
}
However, this gives an "array initializer must be an initializer list" compiler error. Is this at all possible?
You can't return an array (of any dimension) in C or in Objective-C. Since arrays aren't lvalues, you wouldn't be able to assign the return value to a variable, so there's no meaningful for such a thing to happen. You can work around it, however. You'll need to return a pointer, or pull a trick like putting your array in a structure:
// return a pointer
+(int (*)[10][10])returnArray
{
int (*array)[10][10] = malloc(10 * 10 * sizeof(int));
return array;
}
// return a structure
struct array {
int array[10][10];
};
+(struct array)returnArray
{
struct array array;
return array;
}
Another way you can do it with objective C++, is to declare the array as follows:
#interface Hills : NSObject
{
#public
CGPoint hillVertices[kMaxHillVertices];
}
This means the array is owned by the Hills class instance - ie it will go away when that class does. You can then access from another class as follows:
_hills->hillVertices
I prefer the techniques Carl Norum describes, but wanted to present this as an option that might be useful in some cases - for example to pass data into OpenGL from a builder class.
Related
How can I have a method/function that can return any type? For example sometimes the type will need to be float and sometimes it will need to be NSString* so id won't work because float isn't an id. I am not opposed to doing it in a c or c++ function if it's easier.
The reason why I need a dynamic return type is because I'm using objc/runtime to get an Ivar.
I would like some_type to be able to anything:
- (some_type)getIvarWithName:(const char *)name in:(id)obj
{
Ivar ivar(class_getInstanceVariable(object_getClass(obj),name));
return (some_type)ivar;
}
Return a float wrapped in an NSNumber, then you can use the id return type.
To simplify it, you can even use boxing literals, for example:
return #(1.1f);
The first thing to think about is why you would need a function that can return any type. It really doesn't make sense because you wouldn't be able to assign the value to anything, since you don't know the type. Of course, the situation is different when dealing strictly with Obj-C objects, as the language utilizes unknown objects with the id keyword. Unknown objects are like mixing Honeycrisp apples with Macintosh apples (no pun intended), and what you are trying to do is like mixing Honeycrisp apples with airplanes! However, if you want a certain type returned based off of the parameters (such as returning int for int parameters and float for float parameters), then you can overload the functions. Otherwise, then only way that I know of to return absolutely anything would be a void pointer (void *). This would point to a chunk of data that could really be anything. But back to the original problem. What does it represent and how long is it? Good luck!
UPDATE: As other answers mention, you can wrap simple data types (int, float, etc.) in objects such as NSNumbers or NSValues, which will work for your case. But when extending to more general scenarios with complex types such as structs, these generally can't be wrapped in built-in classes. You would need to make your own class using Obj-C.
There is no polymorphism of that kind in Obj-C.
If you know in advance what will be returned then you could use to methods of course.
Retruning id would work when you use an NSNumber for the float value.
You could even introduce a response object that either carries a number or a string and provides (bool) isNumber and (bool) isString methods for later processing.
But what are you really up to? In which context are you using that and what do you really try to achieve. To me it sounds as if there may be better solutions available.
Ofcourse it's weird solution, but you have weird question.
You need enable objective-c++: rename .m-file to .mm
Then yours code will look something like that:
void weird_function(int a)
{
switch (a)
{
case 0: throw #"Hello";
default: throw a;
}
}
void some_code(int a)
{
try
{
weird_function(a);
}
catch (int a)
{
NSLog(#"Catch int: %d", a);
}
catch (NSString* str)
{
NSLog(#"Catch string: %#", str);
}
}
Yours method can be implemented something like that:
union ValueHolder
{
void* voidPtrValue;
int intValue;
float floatValue;
NSString* nssstringValue;
};
- (void)getIvarWithName:(const char *)name in:(id)obj
{
ValueHolder vh;
Ivar ivar = object_getInstanceVariable(obj,name, &vh.voidPtrValue));
if (NULL == ivar)
return;
const char* encoding = ivar_getTypeEncoding(ivar);
if (0 == strcmp(encoding, #encode(int)))
throw vh.intValue;
if (0 == strcmp(encoding, #encode(float)))
throw vh.floatValue;
if (0 == strcmp(encoding, "#\"NSString\""))
throw vh.nsstringValue;
}
I found that using a template in c++ works to have a custom type
The following code works best for my situation:
template <typename _type>
static inline _type & hookVar(id self, NSString*name)
{
Ivar ivar(class_getInstanceVariable(object_getClass(self),[name UTF8String]));
#if __has_feature(objc_arc)
void *pointer(ivar == NULL ? NULL : reinterpret_cast<char *>((__bridge void *)self) + ivar_getOffset(ivar));
#else
void *pointer(ivar == NULL ? NULL : reinterpret_cast<char *>(self) + ivar_getOffset(ivar));
#endif
return *reinterpret_cast<_type *>(pointer);
}
To call the function I just use something like:
NSWindow *win = hookVar<NSWindow*>(self, #"_window");
I currently have this code:
#interface Matrix4 : NSObject
{
float mat[16];
}
#property (readonly) float mat[1];
I want the property to either give me the mat array or have multiple properties giving me readonly access to mat[1], mat[2], etc.
I current have "Property cannot have array of function type float[1]" as an error message
Arrays cannot be return values, so the property cannot have an array type. Instead you must return a pointer, so declare the property as a pointer to the element type of the array:
#property (readonly) float *mat;
Keep the instance variable as float mat[16] as you have now. Then implement the accessor to return a pointer to the array:
- (float *)mat {
return mat; // array decays to pointer automatically
}
Alternatively, you could have an accessor directly for the individual elements:
- (float)matIndex:(NSUInteger)i {
// maybe check bounds here?
return mat[i];
}
The problem with these approaches is that the information about the size of the array is lost, so you would probably want to put the size of the array in a macro or const variable. If you need something a bit more object-oriented, make the array an NSArray and store NSNumbers in it.
edit: One option would also be to wrap the array in a struct to preserve the size info, though you still probably want to pass it around by reference:
struct matrixf16 {
float f[16];
};
#interface Matrix4 : NSObject {
struct matrixf16 mat;
}
#property (readonly) struct matrixf16 *mat;
(Also, if I'm guessing correctly that the size is 16 because it's meant to hold a 4×4 matrix, why not make the array float f[4][4].)
As the compiler is telling you, properties cannot have array or function type.
You can manually implement the getter, like
#interface Matrix4 : NSObject {
float mat[16];
}
- (float *)mat;
#implementation
- (float *)mat {
return mat;
}
or you can consider using an NSArray instead, depending on your requirements. NSArray is definitely more overweight than a native C array, but it allows you to use properties.
However I suspect you have a design issue: it looks like you are trying to implement a squared matrix, but you are exposing the internal representation, most likely so that the client can set the matrix elements.
You should instead hide the internal representation and only expose methods to perform matrix operations. For instance, you can think of exposing a method which sets the matrix value, as:
- (void)setValue:(float)value forRow:(int)row column:(int)col {
NSParameterAssert(row >= 0 && row < 4 && col >= 0 && col < 4)
mat[row * 4 + col] = value;
}
and one that gives you an element back
- (float)valueForRow:(int)row column:(int)col {
NSParameterAssert(row >= 0 && row < 4 && col >= 0 && col < 4)
return mat[row * 4 + col];
}
and make the mat ivar private. This gives you also the flexibility of changing the internal representation at will, without breaking the client's code.
The above implementation is also very easy to generalize to a squared matrix of size, by providing a dimension parameter and using a NSArray or dynamic memory allocation (since variable-length arrays cannot be ivars).
How to return a C-style array of integers from an Objective-C method? This is what my code looks like so far:
Function call:
maze = [amaze getMaze];
Function:
-(int*) getMaze{
return maze;
}
I just started writing in Objective-C today so this is all new to me.
In C if you need to return an array from a function, you need to allocate memory for it using malloc and then return the pointer pointing to the newly allocated memory.
Once you're done working with this memory you need to free it.
Something like:
#include <stdlib.h> /* need this include at top for malloc and free */
int* foo(int size)
{
int* out = malloc(sizeof(int) * size); /* need to get the size of the int type and multiply it
* by the number of integers we would like to return */
return out; /* returning pointer to the function calling foo().
* Don't forget to free the memory allocated with malloc */
}
int main()
{
... /* some code here */
int* int_ptr = foo(25); /* int_ptr now points to the memory allocated in foo */
... /* some more code */
free(int_ptr); /* we're done with this, let's free it */
...
return 0;
}
This is as C style as it gets :) There are probably other (arguably more suitable) ways to do this in Objective C. However, as Objective C is considered a strict superset of C, this would also work.
If I may further expand on the need to do this by pointers. C-style arrays allocated in a function are considered local, once the function is out of scope they are automatically cleaned up.
As pointed out by another poster, returning a standard array (e.g. int arr[10];) from a function is a bad idea as by the time the array is returned it no longer exists.
In C we get around this problem by allocating memory dynamically using malloc and having a pointer that points to that memory returned.
However unless you free this memory adequately, you may introduce a memory leak or some other nasty behavior (e.g. free-ing a malloc-ed pointer twice will produce unwanted results).
Given you explicitly ask about C-style arrays no suggestions here that you should use NSArray etc.
You cannot return a C-style array directly (see below) as a value in Objective-C (or C or C++), you can return a reference to such an array.
Types such as int, double and struct x can all be passed by value - that is the actual bits representing the value are passed around. Other things; such as C-style arrays, dynamically allocated memory, Objective-C style objects, etc.; are all passed by reference - that is a reference to a location in memory that contains the actual bits the represent the value is passed around.
So to return a C-style array from a function/method you can:
Dynamically (malloc et al) an array and return the reference to the allocated memory;
Pass in a reference to an already existing array and have the function fill it up; or
Wrap the array up as a struct...
The normal choices are (1) or (2) - note you cannot return a reference to a stack allocated array, as in:
int *thisIsInvalid()
{
int myValues[5];
...
return myValues; // will not work, the type is correct but once function
// returns myValues no longer exists.
}
If you really want to return a (small) array by value you can actually do it using (3). Remember that struct values are passed by value. So the following will work:
typedef struct
{
int array[5];
} fiveInts;
fiveInts thisIsValid()
{
fiveInts myValues;
...
myValues.array[3] = ...; // etc.
...
return myValues;
}
(Note that there is no overhead from wrapping the array inside a struct when it comes to reading/writing the array - the cost in the above is copying all the values back - hence only advised for small arrays!)
HTH
- (NSArray *)toArray:(int *)maze {
NSMutableArray *retVal = [[NSMutableArray alloc] init];
for (int c = 0; maze[c] != NULL; c++) {
[retVal addObject:[NSNumber numberWithInt:maze[c]]];
}
return [retVal array];
}
I've never been comfortable passing mutable data in and out of methods and not sure why. If you need to change the values later, send the array a mutableCopy message.
you can do it in this way
- (void)getArray:(int *)array withLength:(NSUInteger)length{
for (int i = 0; i < length; i++)
array[i] = i;
}
int array[3];
[object getArray:array withLength:3];
NSLog(#"%d %d %d", array[0], array[1], array[2]); // 1 2 3
I'd like to create an NSObject subclass that contains a few member vars:
#interface PointMass : NSObject
{
CGPoint mCurPosition;
CGPoint mLastPosition;
CGPoint mAcceleration;
}
-(id) initWithPosition:(CGPoint*) pos;
#import "PointMass.h"
#implementation PointMass
-(id) initWithPosition:(CGPoint*)pos
{
mCurPosition = *pos;
mLastPosition = *pos;
mAcceleration = ccp(0,0);
return self;
}
#end
And I would like to create a C-style array to hold a bunch of them within a cocos2d class:
// mNumPoint declared in interface, I've set it to 100
PointMass *pointMassList;
pointMassList = malloc(sizeof(PointMass*) * mNumPointMass);
for (int = 0; i < mNumPointMass; i++)
{
CGPoint point = ccp(100,100);
PointMass *p = [[PointMass alloc] initWithPosition: &point];
pointMassList[i] = p;
}
But I get an error
Expected method to write array element not found on object of type 'PointMass *'
Do I need to tell the compiler more about my PointMass Object if I want to store pointers to it in a C array?
I'm basically trying to have a play around with some particle math on iPhone without needing to unpack points from an NSArray constantly if it isn't clear what I'm trying to achieve here.
If I've gone about this in a backwards way I'd love to be corrected - it has been a while since I wrote vanilla C and I'm a little rusty!
it has been a while since I wrote vanilla C
You should still be able to make the distinction between a pointer-to-T and a pointer-to-pointer-to-T (T being PointMass in this case). You want to store an array of PointMass *, and not an array of PointMass (which you couldn't do anyway). So change the declaration of pointMassList to
PointMass **pointMassList;
and it will work. However, if you're using Objective-C anyway, why don't you simply store the instances into an NSArray?
Hello stackoverflow fellow members?
Struct Declaration in class A
struct PointSprite
{
GLfloat x;
GLfloat y;
GLfloat size;
Color4f color;
} ParticleSystems[MAXIMUM_PARTICLES_ON_SCREEN];
// I generally put some stuffs in ParticleSystem array.
// for ex) struct PointSprite *ps = &ParticleSystems[index];
// and it works well on the class A, but I want to get class B to access this array.
My question is, how am I suppose be return the array of 'ParticlelSystems' array so that other class can access to it? I have tried below code to return the pointer, but compiler gives me a warning.
- (struct ParticleSystems *) commitParticles
{
struct ParticleSystems *ptr = &ParticleSystems; // it said, assigning incompatible pointer type
return ptr;
}
Or should I need to allocate the 'ParticleSystems' array? Please help ! Thanks
If you are creating the array inside the function then you should dynamically allocate it using new and then return a pointer to it.
You cannot return arrays from a function, you will have to return a pointer to it.
Sample Code:
ParticleSystems* doSomethingInteresting()
{
ParticleSystems *ptr = new ParticleSystems[MAXIMUM_PARTICLES_ON_SCREEN];
//do the processing
return ptr;
}
The caller takes the ownership of the returned dynamically allocated array and needs to deallocate it to avoid memory leaks:
delete []ptr;
You can either return it, after allocating one, or you can fill one passed to you by the user. The latter leaves the responsibility to the user to provide a ParticleSystem to the method which receives the data. It can be a local array, or a malloced one.
- (void) commitParticles: (ParticleSystems *) sprites
{
// set members of the structs here
}
I prefer this kind of passing to returning a malloced array. Your mileage may vary.
You're getting the assigning incompatible pointer type compiler warning because your ptr declaration should be of type PointSprite *, not ParticleSystems *