Question

Is there a function in the IOS Api that is like substringFromIndex excepts uses a lot less memory (I am using VERY VERY large strings). If I would have to make that function how would I go about doing that?

Thanks in advance

Answer 1

Yes, but it’s kind of roundabout and potentially dangerous

Here’s what you do:

NSString *veryLargeString = ...;
NSUInteger startingIndex = ...;
NSData *veryLargeStringData = [veryLargeString dataUsingEncoding:NSUTF8StringEncoding];

const void *bytes = [veryLargeStringData bytes];
const void *subBytes = bytes + startingIndex;
NSUInteger subLength = [veryLargeStringData length] - startingIndex;

NSString *substring = [[NSString alloc] initWithBytesNoCopy:subBytes length:subLength encoding:NSUTF8StringEncoding freeWhenDone:NO];

What is this doing:

1. It’s getting the very large string as a series of bytes; I’m pretty sure this doesn’t actually copy the bytes, but be sure to use Instruments to verify. If you can get this super long string as an NSData to begin with instead of an NSString, then you can be much more confident that nothing’s going to get copied.
2. Get the actual byte buffer from the NSData
3. Index into the byte buffer by using some pointer arithmetic
4. Figure out how much of the buffer is left
5. Create a new NSString with the byte buffer, but telling it to NOT copy the bytes, and to NOT free the buffer when the NSString is deallocated

So, why is this dangerous? Basically, if the byte buffer disappears from underneath the NSStrings, Bad Things™ can happen (likely crashing).

However, if you’re smart about it, this will allow you to create substrings without copying the underlying bytes.

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Devious thought:

You could make veryLargeString a retained associated object on substring (i.e., basically make substring an owner of veryLargeString). This would ensure that veryLargeString lives at least as long as substring does. You’d do that like this:

static char ParentStringKey;

objc_setAssociatedObject(substring, &ParentStringKey, veryLargeString, OBJC_ASSOCIATION_RETAIN);

When substring is deallocated, it’ll also automatically release its retain on veryLargeString.