In the following example a std::map structure is filled with 26 values from A - Z (for key) and 0 – 26 for value. The time taken (on my system) to lookup the last entry (10000000 times) is roughly 250 ms for the vector, and 125 ms for the map. (I compiled using release mode, with O3 option turned on for g++ 4.4)

But if for some odd reason I wanted better performance than the std::map, what data structures and functions would I need to consider using?

I apologize if the answer seems obvious to you, but I haven't had much experience in the performance critical aspects of C++ programming.

#include <ctime>
#include <map>
#include <vector>
#include <iostream>

struct mystruct
{
    char key;
    int value;

    mystruct(char k = 0, int v = 0) : key(k), value(v) { }
};

int find(const std::vector<mystruct>& ref, char key)
{
    for (std::vector<mystruct>::const_iterator i = ref.begin(); i != ref.end(); ++i)
        if (i->key == key) return i->value;

    return -1;
}

int main()
{
    std::map<char, int> mymap;
    std::vector<mystruct> myvec;

    for (int i = 'a'; i < 'a' + 26; ++i)
    {
        mymap[i] = i - 'a';
        myvec.push_back(mystruct(i, i - 'a'));
    }

    int pre = clock();

    for (int i = 0; i < 10000000; ++i)
    {
        find(myvec, 'z');
    }

    std::cout << "linear scan: milli " << clock() - pre << "\n";

    pre = clock();

    for (int i = 0; i < 10000000; ++i)
    {
        mymap['z'];
    }

    std::cout << "map scan: milli " << clock() - pre << "\n";

    return 0;
}