Using NSGlyph and memory allocation - cocoa

Question

in a method to track line breaks frequently, for a NSTextView visibleRect, i am allocating memory for NSGlyph to use NSLayoutManager getGlyphs:range:.
should/can i find out how much memory this should be since i have a reference for the range (without affecting layout), and also, what kind of cleanup should happen -- running with ARC ?
the code (which runs on a main queue) :
NSLayoutManager *lm = [self.textView layoutManager];
NSTextContainer *tc = [self.textView textContainer];
NSRect vRect = [self.textView visibleRect];
NSRange visibleRange = [lm glyphRangeForBoundingRectWithoutAdditionalLayout:vRect inTextContainer:tc];
NSUInteger vRangeLoc = visibleRange.location;
NSUInteger numberOfLines;
NSUInteger index;
NSGlyph glyphArray[5000]; // <--- memory assigned here
NSUInteger numberOfGlyphs = [lm getGlyphs:glyphArray range:visibleRange];
NSRange lineRange;
NSMutableIndexSet *idxset = [NSMutableIndexSet indexSet];
for (numberOfLines = 0, index = 0; index < numberOfGlyphs; numberOfLines++) {
(void)[lm lineFragmentRectForGlyphAtIndex:index effectiveRange:&lineRange withoutAdditionalLayout:YES];
[idxset addIndex:lineRange.location + vRangeLoc];
index = NSMaxRange(lineRange);
}
self.currentLinesIndexSet = idxset;

Answer 1

With the NSGlyph glyphs[5000] notation, you're allocating the memory on the stack. But instead of 5000 glyphs it only has to hold visibleRange.length + 1 glyphs:
glyphArray
On output, the displayable glyphs from glyphRange,
null-terminated. Does not include in the result any NSNullGlyph or
other glyphs that are not shown. The memory passed in should be large
enough for at least glyphRange.length+1 elements.
And because it is on the stack, you don't have to worry about freeing the memory—because never malloced memory; it is freed automatically when leaving the function—even without ARC
So it should work, if you write it like this:
NSLayoutManager *lm = ...
NSRange glyphRange = ...
NSGlyph glyphArray[glyphRange.length + 1];
NSUInteger numberOfGlyphs = [lm getGlyphs:glyphArray range:glyphRange];
// do something with your glyphs