objective-c complexity reference

Question

For the c++ STL, there is a de-facto standard location (besides the de-jour standard , I mean) to find information about the complexity guarantees of standard container operations.

Is there an analogous, web-accessible document listing complexity guarantees for NSArray, NSDictionary, etc.?

For example, I cannot find a reference that gives complexity for [NSArray count]

Answer 1

Correct. There isn't one. C++ / the STL (based on my limited understanding) have a significant performance focus. Objective-C / Foundation basically don't.

NSArray , NSDictionary and friends are interfaces. They tell you how to use them, not how they behave. This gives them the freedom to switch implementation under the hood for performance reasons. The point is, you don't need to care, and this won't be specified in the API so you can't even if you want to ;)

For a really good read on this subject, highlighting implementation switches, and with a rough comparison between Foundation classes and STL / C data structures, check out the Ridiculous Fish (by someone on the Apple AppKit team) blog post about "Our arrays, aren't"

Answer 2

Is there an analogous, web-accessible document listing complexity guarantees for NSArray, NSDictionary, etc.?

No. If you understand what the different containers do, you'll have a pretty good idea of how they behave (eg dictionary == map -> nearly constant-time lookups). But don't assume that you know exactly how these structures behave, because they may change their behavior based on circumstances . In other words, a class like NSArray may not be (certainly isn't) implemented as an actual array in the sense of a C-style array even though it has that same "ordered sequence of elements" behavior.

You can, of course, analyze the complexity of your own code: your own binary search through an NSArray is always going to take O(log n) operations any way you slice it. Just don't assume that inserting an element into an NSMutableArray is going to require moving all the subsequent elements, because your "array" might really be a linked list or something else.