I can add bytes to a NSMutableData instance easially by using the AddData method, however I do not see any similar method for removing data? Am I overlooking something, or do I need to create a new object and copy over only the bytes I need?
+4
A:
Since NSMutableData is toll-free bridged with CFMutableDataRef, you can use the CFDataDeleteBytes() function:
NSMutableData *data = ...
CFDataDeleteBytes((CFMutableDataRef)data, CFRangeMake(3, 4));
RJ
2010-02-27 00:21:00
Be careful. Down this path lies performance woes. If you are editing relatively few times or working with a small bit of data, no big deal. Anything larger or frequent or performance sensitive, consider using an optimized data structure of some type.
bbum
2010-02-27 02:33:28
Thanks, this worked great. How do you know that NSMutable data is 'toll-free bridged' with CFMutableDataRef? I do not see any mention of this in the NSMutableData docs?
Zenox
2010-02-27 11:47:29
If you look at the "Overview" section of the NSMutableData docs http://developer.apple.com/mac/library/DOCUMENTATION/Cocoa/Reference/Foundation/Classes/NSMutableData_Class/Reference/NSMutableData.html you'll see it mentioned in the second paragraph
RJ
2010-03-01 17:50:20
Why using CoreFoundation, if you can use the NSMutableData method replaceBytesInRange, which according to documentation removes the bytes in range, if the replacement byte pointer is NULL and the replacement length is also 0?
Mecki
2010-08-03 21:59:40
+1
A:
If the data you want to remove is at the end, you can use
[NSMutableDataInstance setLength:[NSMutableDataInstance length] - n];
or with the obj-c 2.0 syntax
NSMutableDataInstance.length -= n;
for anything more complicated than that I'd recommend manipulating the raw data.
Jared P
2010-02-27 00:24:34
+4
A:
I don't quite understand why RJ is suggesting to use CoreFoundation, when NSMutableData has actually a method for exactly that. Please read the documentation of
- (void)replaceBytesInRange:(NSRange)range withBytes:(const void *)replacementBytes length:(NSUInteger)replacementLength
Apple clearly says the following:
If the length of range is not equal to replacementLength, the receiver is resized to accommodate the new bytes. Any bytes past range in the receiver are shifted to accommodate the new bytes. You can therefore pass NULL for replacementBytes and 0 for replacementLength to delete bytes in the receiver in the range range. You can also replace a range (which might be zero-length) with more bytes than the length of the range, which has the effect of insertion (or “replace some and insert more”).
To remove 10 byte from the end, use:
[data setLength:[data length] - 10];
It could also be done via replaceBytesInRange, but it's in fact much faster, because the bytes are not really removed. Instead only the internal size variable is changed and NSMutableData will behave as if the bytes were removed. IOW, this is a O(1) operation (that means it will always take equally long to perform, regardless of how many bytes you remove), and it is very fast.
To remove 10 byte from front, use:
[data replaceBytesInRange:NSMakeRange(0, 10) withBytes:NULL length:0];
To remove 10 bytes in the middle (e.g. after 20 bytes), use:
[data replaceBytesInRange:NSMakeRange(20, 10) withBytes:NULL length:0];
replaceBytesInRange is a O(n) operation, though. That means no matter how long it takes to remove 100 byte, it will take twice as long to remove 200 bytes and so on. It is still pretty fast and only limited by the throughput of your computer's memory (RAM). If you have 10 MB of data and you remove 1 MB from front, 9 MB are copied to fill the gap of the just removed MB. So the speed of the operation depends on how fast can your system move 9 MB of RAM from one address to another one. And this is usually fast enough, unless you deal with NSMutableData objects containing hundreds of MB.
Mecki
2010-08-03 21:58:37