How can I increase the performance in a map lookup with key type std::string?

Question

I\'m using a std::map (VC++ implementation) and it\'s a little slow for lookups via the map\'s find method.

The key type is std::string.

Answer 1

As Even said the operator used in a set is < not ==.

If you don't care about the order of the strings in your set you can pass the set a custom comparator that performs better than the regular less-than.

For example if a lot of your strings have similar prefixes (but they vary in length) you can sort by string length (since string.length is constant speed).

If you do so beware a common mistake:

struct comp {
    bool operator()(const std::string& lhs, const std::string& rhs)
    {
        if (lhs.length() < rhs.length())
            return true;
        return lhs < rhs;
    }
};

This operator does not maintain a strict weak ordering, as it can treat two strings as each less than the other.

string a = "z";
string b = "aa";

Follow the logic and you'll see that comp(a, b) == true and comp(b, a) == true.

The correct implementation is:

struct comp {
    bool operator()(const std::string& lhs, const std::string& rhs)
    {
        if (lhs.length() != rhs.length())
            return lhs.length() < rhs.length();
        return lhs < rhs;
    }
};