Using the new operator inside of a method.

[dpb167]

Hello,

I'm working with a program that processes a whole lot of 1024x1024 images that are stored as "ImageReader" objects. I want to write a method that turns the images into a 1024x256 image and returns it. The relevant part of the code is:

void event(){
ImageReader im = new ImageReader(1024,1024);

ImageReader newIm = new ImageReader(1024,256);

newIm = groupLines(imageReader);
}

ImageReader::ImageReader * groupLines(ImageReader * im){
	
	const int NUM_OF_LINES = IMAGE_SIZE/LINE_GROUPING;
	
	ImageReader * dummy = new ImageReader(IMAGE_SIZE,NUM_OF_LINES);
	
	int runningSum;
	
	for (int i = 0; i < IMAGE_SIZE; i++) {
		for (int k = 0; k < NUM_OF_LINES; k++) {
			runningSum = 0;
			for (int j = k*LINE_GROUPING; j < LINE_GROUPING*(k+1); j++) {
				runningSum += im->getPixel(i,j);
			}
			dummy->setPixel(i,k,runningSum);
		}
	}
	
	return  dummy;
}

After I call event() several thousand times, I run out of memory. I think it's because I'm using the new operator in the method without a corresponding delete. I need to return "dummy" though, so I can't delete it before I return it. How is this typically handled? Fairly new to C++, hopefully I've provided enough of the code to answer the question.

Thanks!

[Disch]

After I call event() several thousand times, I run out of memory. I think it's because I'm using the new operator in the method without a corresponding delete

You are correct. That's exactly what's happening.

For every new, you should have a matching delete. It doesn't have to be in the same function, it just is generally easier that way.

You have two options here.

Option 1: groupLines new's an ImageReader and returns it. The calling function is responsible for deleting it.

Option 2: groupLines has an ImageReader passed to it by pointer, rather than newing it itself.


Either one will work in your case. Probably Option 1 would be better if the size of the output image is not fixed. If it is fixed, then option 2 is probably better. But it's just a style thing so pick whichever you like.

Option 1:

void event(){
  ImageReader im = new ImageReader(1024,1024);

  ImageReader newIm = groupLines(im);

  // use 'im' and 'newIm' here

  // when you're done with them:
  delete im;
  delete newIm;
}

ImageReader * groupLines(ImageReader * im){

  ImageReader* dummy = new ImageReader( ... );

  // modify 'dummy' here

  return dummy;
}

Notice that we don't new 'newIm' in event(). It is new'd by groupLines.



Option 2)

void event(){
  ImageReader im = new ImageReader(1024,1024);

  ImageReader newIm = new ImageReader(1024,256);

  groupLines(im,newIm);

  // use 'im' and 'newIm' here

  // when you're done with them:
  delete im;
  delete newIm;
}

void groupLines(ImageReader * im, ImageReader * dummy){

  // modify 'dummy' here
}

In option 2, note that we don't new dummy in groupLines. Instead we just use it as passed. It is new'd by event().

[dpb167]

Thanks!