Objective-C Blocks, C++ Variables and async code

Question

I'm having an issue while glueing together c++ vars with obj-c async code on iOS.

The real problem is located in the async code, I'm using third-party libraries built in C++ that expect object references, e.g.:


- (void) processFrame:(cv::Mat &)mat;

My problem real problem is how to call this ? I need to create the c++ object on a different thread and pass it to the async code, a bit like this:


__block cv::Mat mat(videoRect.size.height, videoRect.size.width, CV_8UC4, baseaddress, 0);
dispatch_async(dispatch_get_main_queue(), ^{
            [self processFrame:mat];
        });

Which give an error (Bad access), the problem is (I guess) the object is destroyed before the method runs, so I tried creating the object in the heap:


__block cv::Mat *mat= new cv::Mat(videoRect.size.height, videoRect.size.width, CV_8UC4, baseaddress, 0);
dispatch_async(dispatch_get_main_queue(), ^{
            [self processFrame:(*mat)];
        });

And still:


__block cv::Mat *mat= new cv::Mat(videoRect.size.height, videoRect.size.width, CV_8UC4, baseaddress, 0);
dispatch_async(dispatch_get_main_queue(), ^{
            [self processFrame:mat];
        });

I get keeping "Bad access" errors all the time

Any ideas ?

score 2 · Answer 1 · answered Dec 07 '11 at 18:30

2

The __block qualifier tells the compiler to not copy the object for the block so that is why it is failing. If you are not reassigning mat or are not trying to prevent an unnecessary retain/copy operation then you should remove __block.

cv::Mat mat(videoRect.size.height, videoRect.size.width, CV_8UC4, baseaddress, 0);
dispatch_async(dispatch_get_main_queue(), ^{
            [self processFrame:mat];
        });

The examples where you create a new cv::Mat may be failing because you may be deleting mat too soon.

__block cv::Mat *mat= new cv::Mat(videoRect.size.height, videoRect.size.width, CV_8UC4, baseaddress, 0);
dispatch_async(dispatch_get_main_queue(), ^{
            [self processFrame:mat];
            //mat would need to be deleted here
        });
delete mat; //If you are doing this, you will likely get an EXC_BAD_ACCESS

answered Dec 07 '11 at 18:30

Joe

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Thank you for your anwser, I still have the same issues, even when I create the object in the heap... How should I pass a "heap object" to a method that expect an object reference (like cv::Mat & mat) ? – user361526 Dec 08 '11 at 18:19
For a heap object pass it like this `[self processFrame:*mat];`, then you can `delete mat;` right after that call if you no use for it any longer. – Joe Dec 08 '11 at 18:42
Tried that before, still have the same error :( I will just try to use another library that lets me avoid mixing C++ and Obj-C :) Thanks for the help – user361526 Dec 08 '11 at 20:47
This should work maybe try posting more of the relevant code. – Joe Dec 08 '11 at 21:53

Objective-C Blocks, C++ Variables and async code

1 Answers1