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NSMutableDictionary Vs std::tr1::unordered_map

From: David Chisnall
Subject: NSMutableDictionary Vs std::tr1::unordered_map
Date: Wed, 6 Jul 2011 18:38:36 +0100

Hi Everyone,

I'm not sure if this is interesting to anyone else, but I thought I'd share it 

One of the interesting things about ARC is that it makes it trivial to use C++ 
collections with Objective-C objects: the compiler will insert the relevant 
read / write barriers on every assignment, including ones in templates.  This 
means that you can do things like std::map<int, __weak id> and get a map from 
integers to zeroing weak references to objects.  

With this in mind, I thought I'd see how GNUstep's NSMutableDictionary 
implementation held up against the C++ TR1 / C++0x unordered_map, as 
implemented in GNU libstdc++.  I generated a large array of NSNumber instances 
containing pseudorandom values, then populated each of the collections with 
them (same key and value) and then searched for each one.

The three collections I tested were:

- The stock NSMutableDictionary, as returned by +new
- A new NSMutableDictionary subclass (CXXMap), which is a thin wrapper around 
- Using unordered_map directly.

I tested on a couple of machines.  Unfortunately, modern CPUs are rubbish for 
benchmarking, because there is no longer a strong correlation between CPU 
seconds and work done.  My rough tests showed:

- Inserting into CXXMap was roughly 20-30% faster than inserting into 
- Searching CXXMap was about 10% faster than searching NSMutableDictionary
- The performance difference between CXXMap and using unordered_map directly 
was negligible.

I've attached my CXXMap code at the end of this email.  Feel free to use it in 
your own workloads and see if you see anything different.


-- Sent from my IBM 1620

namespace {
struct eqid
    bool operator()(const id s1, const id s2) const
        return (s1 == s2) || [s1 isEqual: s2];

struct hashid
    size_t operator()(const id s1) const
        return (size_t)[s1 hash];

@interface CXXMap : NSMutableDictionary
    std::tr1::unordered_map<id, id, hashid, eqid> map;
@implementation CXXMap
- (void)setObject: (id)obj forKey: (id)key
    key = [key copy];
    map[key] = obj;
- (void)removeObjectForKey:(id)aKey
- (NSUInteger)count
    return (NSUInteger)map.size();
- (id)objectForKey: (id)key
    return map[key];

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