How could NSArray take ownership of a c++ object - c++

Question

As I know, c++ object in iOS is allocated in the stack memory. Now I have to add the c++ object into a NSArray, however NSArray cannot store c++ objects directly.
The question is how could I store a c++ object in a NSArray and let the NSArray take ownership of the c++ object so that I could use the c++ object whenever I want.
Thanks for any help!
=========EDIT========
Here are some code snippet:
std::list<ImageTextContent> msgs = body->getMsgs();
std::list<ImageTextContent>::iterator itmsgs0;
for(itmsgs0 = msgs.begin();itmsgs0 != msgs.end();itmsgs0++)
{
ImageTextContent *tmpmsgs = &(*itmsgs0);
ImageTextContentWrapper *itc = [[ImageTextContentWrapper alloc] initWithImageTextContent:tmpmsgs];
[self.dataSource addObject:itc];
}
[self.tableview reloadData];
the cellForRow method:
- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath
{
........
ImageTextContentWrapper *itc = (ImageTextContentWrapper *)[_dataSource objectAtIndex:indextPath.row];
NSString *title = [NSString stringWithCString:(itc.imageTextContent->getTitle()).c_str() encoding:NSUTF8StringEncoding];
cell.titleLabel.text = title;
........
}
ImageTextContentWrapper class:
#interface ImageTextContentWrapper : NSObject
#property (nonatomic, assign) ImageTextContent *imageTextContent;
- (id)initWithImageTextContent:(ImageTextContent *)imageTextContent;
#end
#implementation ImageTextContentWrapper
- (id)initWithImageTextContent:(ImageTextContent *)imageTextContent
{
if (self = [super init])
{
self.imageTextContent = imageTextContent;
}
return self;
}
#end

Answer 1

Create an Objective-C class owning your C++ object, and store objects of this class in NSArray.

Answer 2

A small code example how to wrap a C++ object in a Objective-C class:
For a C++ class CPP which could look like
class CPP {
public:
std::string aName;
CPP(std::string aName) : aName(aName) { }
};
you can create a Objective-C wrapper class ObjectiveC:
// ObjectiveC.h
#interface ObjectiveC : NSObject
{
CPP *cpp;
}
- (id)initWithString:(NSString*)string;
- (void)print;
#end
//ObjectiveC.mm <- this .mm tells Xcode to use the right compiler
#import "ObjectiveC.h"
#implementation ObjectiveC
- (id)initWithString:(NSString*)string
{
self = [super init];
if (self)
{
cpp = new CPP([string UTF8String]);
}
return self;
}
- (void)print
{
NSLog(#"%s", cpp->aName.c_str());
}
#end
Now you can create a NSArray containing indirectly containing the C++ objects:
// main.mm for example (.m -> .mm !)
ObjectiveC *oc1 = [[ObjectiveC alloc] initWithString:#"oc1"];
ObjectiveC *oc2 = [[ObjectiveC alloc] initWithString:#"oc2"];
ObjectiveC *oc3 = [[ObjectiveC alloc] initWithString:#"oc3"];
NSArray *array = [[NSArray alloc] initWithObjects:oc1, oc2, oc3, nil];
for (ObjectiveC *oc in array) {
[oc print];
}
A small remark: You have to change all implementation files containing and interacting with this wrapper files from .m to .mm. The second option is to change the compiler in the Xcode settings.

Answer 3

You're off the mark a little. There are two ways to create C++ objects. On the stack (by declaring it as a local, non-pointer variable) or on the heap (e.g. using operator new or by declaring it as a non-pointer instance variable in an ObjC object).
If you actually have one on the stack, there is no way to (safely) reference it from an ObjC class. In ObjC, all objects are on the heap, so everyone who retains it is guaranteed that it will stay around even if the function in which it is declared returns. And the last one who releases it causes its memory to actually be freed.
You can't give that guarantee with stack objects, as they must go away the moment the function in which they are a local variable returns.
But if you could create your C++ object with operator new and your code would still work, instead of a stack object, then you can just create an ObjC class in an ObjC++ file (.mm suffix instead of straight .m), and put that in an array. If you want to learn more about ObjC++, I recommend you listen to my guest spot on the NSBrief podcast, where I talk about ObjC++ in detail.
Alternately, what reason prevents you from just putting your C++ object in a std::vector instead of an NSArray? What are you hoping to use this NSArray full of C++ objects for?
PS - For completeness' sake, let me also mention that there is +NSNumber numberWithNonretainedPointer: (or maybe it was "unretained") which you could use as a pre-made object for keeping a pointer in an array, but this is not recommended, as it does not call delete on the pointer when it is released, so you have to be very careful or you'll leak C++ objects. Better to make your own class.