Hash table faster in C# than C++?

Question

Here's a curiosity I've been investigating. The .NET Dictionary class performs ridiculously fast compared to the STL unordered_map in a test I keep running, and I can't figure out why.

(0.5 seconds vs. 4 seconds on my machine) (.NET 3.5 SP1 vs. Visual Studio 2008 Express SP1's STL)

On the other hand, if I implement my own hash table in C# and C++, the C++ version is about twice as fast as the C# one, which is fine because it reinforces my common sense that native machine code is sometimes faster. (See. I said "sometimes".) Me being the same person in both languages, I wonder what tricks was the C# coder from Microsoft able to play that the C++ coder from Microsoft wasn't? I'm having trouble imagining how a compiler could play such tricks on its own, going through the trouble of optimizing what should look to it to be arbitrary function calls.

It's a simple test, storing and retrieving integers.

C#:

const int total = (1 << 20);
int sum = 0;
Dictionary<int, int> dict = new Dictionary<int, int>();
for(int i = 0; i < total; i++)
{
    dict.Add(i, i * 7);
}

for(int j = 0; j < (1 << 3); j++)
{
    int i = total;
    while(i > 0)
    {
        i--;
        sum += dict[i];
    }
}
Console.WriteLine(sum);

C++:

const int total = (1 << 20);
int sum = 0;
std::tr1::unordered_map<int, int> dict;
for(int i = 0; i < total; i++)
{
    dict.insert(pair<int, int>(i, i * 7));
}

for(int j = 0; j < (1 << 3); j++)
{
    int i = total;
    while(i > 0)
    {
        i--;
        std::tr1::unordered_map<int, int>::const_iterator found =
            dict.find(i);
        sum += found->second;
    }
}
cout << sum << endl;

Answer 1

the two versions are not equivalent , your are constructing an iterator in each pass of your C++ while loop. that takes CPU time and throws your results.