Can't use dispatch_sync in c++ class with Objective-C++ code

Question

I need to use Objective-C++ code in inherited C++ class which works with video recording from iPhone camera (getting CMSampleBufferRef through an other native-objective-c class Wrapper with CMSampleBufferDelegate).

The AVCaptureVideoOutput that i have works in its own dispatch_queue_t callbackQueue, so, when i want to get the last frame from my Wrapper class, I need to lock the callbackQueue to make it wait till the copying will be done.

As i know, it's done with dispatch_sync, syncing the captureOutput.callbackQueue. But i can't get this code working:

// .mm
frame_t MyCppClass::getLastFrame()
{
    dispatch_sync(pCaptureVideoDataOutput.sampleBufferCallbackQueue, ^{ // error: no matching function for call to 'dispatch_sync'

        CVImageBufferRef imageBuffer = CMSampleBufferGetImageBuffer(wrapperInstance->currentBuffer);
        CVPixelBufferLockBaseAddress(imageBuffer,0);

        // doing copying frame data from buffer...

        CVPixelBufferUnlockBaseAddress(imageBuffer, 0);
    }); // error: control may reach end of non-void block

    return frame;
}

// .h
@interface Wrapper : NSObject <AVCaptureVideoDataOutputSampleBufferDelegate> {
  CMSampleBufferRef currentBuffer;
}
@end

// .mm
@implementation Wrapper
- (void)captureOutput:(AVCaptureOutput *)captureOutput didOutputSampleBuffer:(CMSampleBufferRef)sampleBuffer fromConnection:(AVCaptureConnection *)connection {
    // doing copying to currentBuffer
}
@end

EDIT:

When i changed to

dispatch_sync(pCaptureVideoDataOutput.sampleBufferCallbackQueue, (dispatch_block_t)^{

it fixed the first error, but the second is still here..

Got stuck with this.. any help is appreciated!

Answer 1

The error:

error: no matching function for call to 'dispatch_sync'

indicates that the function prototype may not be visible. Make sure you are including the header for libdispatch in your .mm file...

#include <dispatch/dispatch.h>

The second error:

error: control may reach end of non-void block

is because your function is declared to return a frame_t but it has no return statement.

Answer 2

I figured it out!

I've got some emergency return statements inside the block. I thought that it will return the function, but it returns the block... so compiler was right.