I recently discovering these classes like NSMapTable and NSPointerArray, which work like the traditional collections, but also let you store weak references or plain old C pointers. Unfortunately it looks like you can't use the for...in syntax to iterate over non-NSObject pointers. For example:
typedef struct Segment {
CGPoint bottom, top;
} Segment;
...
NSPointerArray *segments = [[NSPointerArray alloc]
initWithOptions:NSPointerFunctionsOpaqueMemory];
...
Segment *s = malloc(sizeof(Segment));
[segments addPointer: s];
...
for (Segment *s in segments) { // nope...
The compiler does not like that last line. The error:
Selector element type 'Segment *' (aka 'struct Segment *') is not a valid object
So, do I need to do this?
for (int i=0, len=segments.count; i<len; i++) {
Segment *seg = [segments pointerAtIndex:i];
...
That's not the end of the world, but I just want to make sure.
(This might be more of theoretical interest.)
NSPointerArray does conform to the NSFastEnumeration protocol, it is only the
for (id object in collection) language construct that cannot be used with arbitrary pointers which
are not Objective-C pointers.
But you can get a whole bunch of pointers from the array by calling the NSFastEnumeration
method countByEnumeratingWithState:objects:count: directly. This is a bit tricky because
that method need not fill the supplied buffer (as explained here: How for in loop works internally - Objective C - Foundation).
Here is a simple example how this would work:
__unsafe_unretained id objs[10];
NSUInteger count = [segments countByEnumeratingWithState:&state
objects:objs count:10];
// Now state.itemsPtr points to an array of pointers:
for (NSUInteger i = 0; i < count; i++) {
Segment *s = (__bridge Segment *)state.itemsPtr[i];
NSLog(#"%p", s);
}
So this does not help to make the code simpler and you probably want to stick with
your explicit loop.
But for large arrays it might improve the performance because the pointers are "fetched"
in batches from the array instead of each pointer separately.
the for (... in ...) syntax won't work in this case because Segment is a struct, not an Objective C object. Your second for loop should work.
Related
How do I print the name of the method I am in, in Objective-C?
In Java/C# I could do this using reflection. Is similar possible with Objective-C?
For just printing method name, use
NSLog(#"%#", NSStringFromSelector(_cmd));
To printing method arguments is a complex task.
every message in ObjectiveC is converted to objMsgSend (id self, SEL, arg0...)
We need to walk through stack, and print them out, as we see after _cmd + sizeof(SEL) will give us address to arg0 (but the size and type for arg0 is unknown).
Method method = class_getInstanceMethod([self class], _cmd);
unsigned nargs = method_getNumberOfArguments(method);
void *start = _cmd;
for(unsigned i = 0; i< nargs ; i++) {
char *argtype = method_copyArgumentType(method, i);
//find arg size from argtype
// walk stack given arg zie
free(argtype);
}
Basic implementations should look like this.
So if the arguments are objects, that will be great (as we know the size of argument are 32/64 bit). Otherwise we need to do type encoding, and move cursor by argument's size. Have look https://github.com/holtwick/HOLog , that does exactly what you want, but it works under Simulator only.
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
a.H:
-(NSArray *) returnarray:(int) aa
{
unsigned char arry[1000]={"aa","vv","cc","cc","dd"......};
NSArray *tmpary=arry;
return tmpary;
}
a.c:
#include "a.H"
main (){
// how do I call returnarray function to get that array in main class
}
I need that array in main and I need to retain that array function in separate class.
Can someone please provide a code example to do this?
These lines:
unsigned char arry[1000]={"aa", "vv", "cc", "cc", "dd", ...};
NSArray *tmpary=arry;
Should instead be:
unsigned char arry[1000]={"aa", "vv", "cc", "cc", "dd", ...};
NSMutableArray * tmpary = [[NSMutableArray alloc] initWithCapacity: 1000];
for (i = 0; i < 1000; i++)
{
[tmpary addObject: [NSString stringWithCString: arry[i] encoding:NSASCIIStringEncoding]];
}
This is because a C-style array (that is, int arr[10]; for example) are not the same as actual NSArray objects, which are declared as above.
In fact, one has no idea what an NSArray actually is, other than what the methods available to you are, as defined in the documentation. This is in contrast to the C-style array, which you are guaranteed is just a contiguous chunk of memory just for you, big enough to hold the number of elements you requested.
C-style arrays are not NSArray's so your assignment of arry (the definition of which has some typos, at least the unsighned part) is not valid. In addition, you call arry an array of char, but you assign it an array of null-terminated strings.
In general you need to loop and add all the elements of the C-style array to the NSArray.
I'm not sure why you must do it in main. If you want a global you can do it by declaring a global in another file. That said, you CANNOT assign a plain C data array to an objective C NSArray, which is different in nature entirely.
As part of a unit test framework, I'm writing a function genArray that will generate NSArrays populated by a passed in generator block. So [ObjCheck genArray: genInt] would generate an NSArray of random integers, [ObjCheck genArray: genChar] would generate an NSArray of random characters, etc. In particular, I'm getting compiler errors in my implementation of genArray and genString, a wrapper around [ObjCheck genArray: genChar].
I believe Objective C can manipulate blocks this dynamically, but I don't have the syntax right.
ObjCheck.m
+ (id) genArray: (id) gen {
NSArray* arr = [NSMutableArray array];
int len = [self genInt] % 100;
int i;
for (i = 0; i < len; i++) {
id value = gen();
arr = [arr arrayByAddingObject: value];
}
return arr;
}
+ (id) genString {
NSString* s = #"";
char (^g)() = ^() {
return [ObjCheck genChar];
};
NSArray* arr = [self genArray: g];
s = [arr componentsJoinedByString: #""];
return s;
}
When I try to compile, gcc complains that it can't do gen(), because gen is not a function. This makes sense, since gen is indeed not a function but an id which must be cast to a function.
But when I rewrite the signatures to use id^() instead of id, I also get compiler errors. Can Objective C handle arbitrarily typed blocks (genArray needs this), or is that too dynamic?
Given that blocks are objects, you can cast between block types and id whenever you want, though if you cast the block to the wrong block type and call it, you're going to get unexpected results (since there's no way to dynamically check at runtime what the "real" type of the block is*).
BTW, id^() isn't a type. You're thinking of id(^)(). This may be a source of compiler error for you. You should be able to update +genArray: to use
id value = ((id(^)())(gen))();
Naturally, that's pretty ugly.
*There actually is a way, llvm inserts an obj-c type-encoded string representing the type of the block into the block's internal structure, but this is an implementation detail and would rely on you casting the block to its internal implementation structure in order to extract.
Blocks are a C-level feature, not an ObjC one - you work with them analogously to function pointers. There's an article with a very concise overview of the syntax. (And most everything else.)
In your example, I'd make the gen parameter an id (^gen)(). (Or possibly make it return a void*, using id would imply to me that gen generates ObjC objects and not completely arbitrary types.)
No matter how you declare your variables and parameters, your code won't work. There's a problem that runs through all your compiler errors and it would be a problem even if you weren't doing convoluted things with blocks.
You are trying to add chars to an NSArray. You can't do that. You will have to wrap them them as some kind of Objective C object. Since your only requirement for this example to work is that the objects can be inputs to componentsJoinedByString, you can return single-character NSStrings from g. Then some variety of signature like id^() will work for genArray. I'm not sure how you parenthesize it. Something like this:
+ (id) genArray: (id^()) gen;
+ (id) genString {
...
NSString * (^g)() = ^() {
return [NSString stringWithFormat:#"%c", [ObjCheck genChar]];
};
...
}
NSString * is an id. char is not. You can pass NSString * ^() to id ^(), but you get a compiler error when you try to pass a char ^() to an id ^(). If you gave up some generality of genArray and declared it to accept char ^(), it would compile your call to genArray, but would have an error within genArray when you tried to call arrayByAddingObject and the argument isn't typed as an id.
Somebody who understands the intricacies of block syntax feel free to edit my post if I got some subtle syntax errors.
Btw, use an NSMutableArray as your local variable in genArray. Calling arrayByAddingObject over and over again will have O(n^2) time performance I imagine. You can still declare the return type as NSArray, which is a superclass of NSMutableArray, and the callers of genArray won't know the difference.
I have cpp code where the struct objects are put into the CLISTS. I am porting this code into Objective C.
CLIST is similar to a doubly linked list with .RemoveAt , .GetAt , .InsertBefore , .GetNext , .GetHeadPosition functions.
How to implement the same in Objective C.
Do I need to implement doubly linked list in Objective C.Is there any other predefined methods to use it.
A CLIST is presumably circular? Hence the GetHeadPosition?
In any case, NSArray (or, NSMutableArray in this case, since you want to be inserting) is the normal way to keep ordered lists in Objective-C.
For RemoveAt, use removeObjectAtIndex:. For GetAt, use objectAtIndex:. For InsertBefore you're probably going to want to write a little something like:
- (void)insert:(id)objectToInsert before:(id)referenceObject
{
int index = [array indexOfObject:referenceObject];
if(index == NSNotFound) return; // or whatever you'd expect.
// Maybe object is just inserted at the end?
index = index - 1;
if(index < 0) index = [array count];
[array insertObject:objectToInsert atIndex:index];
}
(which would probably go better in an NSArray category, but you get the point)
For GetNext and GetHeadPosition you probably want to keep your array position in a separate variable. So for GetNext:
arrayPosition = (arrayPosition + 1)%[array count];
return [array objectAtIndex:arrayPosition];
And for GetHeadPosition, just:
return arrayPosition;
EDIT: for iterating through an NSArray, the easiest way is actually to ignore anything explicit and just use:
for(ObjectType *object in array)
{
/* do something with object */
}
That generally means you don't really need an analogue of GetNext, but you can't mutate the array while in that loop so it's not always usable.