Why is there a "potential leak"? - ios

Question

Xcode's analyser is complaining that there is a "potential leak of an object". The first line within the following method is highlighted:
- (void)retrieveBeginRestoreData {
self.restoreContext = [self.image newARGBBitmapContext];
if (!self.restoreContext) self.restoreData = nil;
CGRect rect = {{0,0},self.image.size};
CGContextDrawImage(self.restoreContext, rect, self.image.CGImage);
self.restoreData = CGBitmapContextGetData(self.restoreContext);
}
I have a property declared as such:
#property (nonatomic, assign) CGContextRef restoreContext
The newARGBBitmapContext is defined by the following:
- (CGContextRef)newARGBBitmapContext {
CGContextRef context = NULL;
CGColorSpaceRef colorSpace;
void * bitmapData;
size_t bitmapByteCount;
size_t bitmapBytesPerRow;
// Get image width, height. We'll use the entire image.
size_t pixelsWide = CGImageGetWidth(self.CGImage);
size_t pixelsHigh = CGImageGetHeight(self.CGImage);
// Declare the number of bytes per row. Each pixel in the bitmap in this
// example is represented by 4 bytes; 8 bits each of red, green, blue, and
// alpha.
bitmapBytesPerRow = (pixelsWide * 4);
bitmapByteCount = (bitmapBytesPerRow * pixelsHigh);
// Use the generic RGB color space.
// colorSpace = CGColorSpaceCreateWithName(kCGColorSpaceGenericRGB);
colorSpace = CGColorSpaceCreateDeviceRGB();
if (colorSpace == NULL)
{
fprintf(stderr, "Error allocating color space\n");
return NULL;
}
// Allocate memory for image data. This is the destination in memory
// where any drawing to the bitmap context will be rendered.
bitmapData = malloc( bitmapByteCount );
if (bitmapData == NULL)
{
fprintf (stderr, "Memory not allocated!");
CGColorSpaceRelease( colorSpace );
return NULL;
}
// Create the bitmap context. We want pre-multiplied ARGB, 8-bits
// per component. Regardless of what the source image format is
// (CMYK, Grayscale, and so on) it will be converted over to the format
// specified here by CGBitmapContextCreate.
context = CGBitmapContextCreate (bitmapData,
pixelsWide,
pixelsHigh,
8, // bits per component
bitmapBytesPerRow,
colorSpace,
(CGBitmapInfo)kCGImageAlphaPremultipliedFirst);
if (context == NULL)
{
free (bitmapData);
fprintf (stderr, "Context not created!");
}
// Make sure and release colorspace before returning
CGColorSpaceRelease( colorSpace );
return context;
}
I managed to resolve this issue by instead declaring restoreContext as an instance variable in the header file; the "potential leak" warning goes away.
Questions:
What was the issue in the first place?
How was the issue fixed when I stopped declaring restoreContext as a property?
What is the correct way to fix the issue with restoreContext being declared as a property?

Answer 1

This line
self.restoreContext = [self.image newARGBBitmapContext];
does the following:
It (potentially) creates an instance object of CGContext.
Since the method name starts with new, an ownership transfer is applied. That means that the receiver (your code) is responsible for releasing it.
When the line of code is run a second time, the reference of the already existing instance of CGContext is overridden without releasing the instance, it points to. The older instance will leak.