Question

I have several GUIDs and I’d like to implement a hash table to quickly retrieve them. How would I do that?

If I treat the GUIDs as hash codes I need to do something like

index = GUID % prime_number_that_covers_all_GUID_bits

but I’m unsure if this is the right way to do it. How should I do to realize such a hash table?

Answer 1

You could use std::unordered_map, which takes a Key type (GUID) in your case, and a Value type, which could be some user info or program info (depending on your app). Storing is as simple as calling the member functions insert() or emplace() and looking up a stored value is done by calling find().

The example below uses std::string as the underlying type for your keys, and implicitly std::hash<std::string> as the hash function. For other GUID types, you might need to roll your own hash function object and pass that as a template parameter to the hash table.

#include <iostream>
#include <ios>
#include <string>
#include <unordered_map>

typedef std::string GUID;

class UserInfo
{
public:
    UserInfo(bool b): is_genius_(b) {}
    bool is_genius() const { return is_genius_; }

private:
    bool is_genius_;
    // your stuff here
};

int main()
{
     std::unordered_map<GUID, UserInfo> table;      
     GUID x = "Johnny Pauling";

     // insert into table
     table.emplace(x, UserInfo(true));

     // lookup in table
     auto it = table.find(x);

     // if found, print it
     if (it != table.end())
         std::cout << std::boolalpha << it->second.is_genius();
}

Output on LiveWorkSpace