Implementing a QHash-like lookup with multiple keys

Question

I'm trying to find the best way to implement a QHash-like lookup table that uses multiple keys to return one value. I have read that the Boost library has similar functionality, but I would like to avoid this if possible.

An example of what I would like to do is as follows (obviously the following pseudo-code is not possible):

//First key (int) - Engine cylinders
//Second key (int) - Car weight
//Value (int) - Top speed
MyLookup<int, int, int> m_Lookup
m_Lookup.insert(6, 1000, 210);
m_Lookup.insert(6, 1500, 190);
m_Lookup.value(6, 1000); //Returns 210

My first (and extremely slow) idea was to just create a Struct, and iterate over a list until I find a qualifying item.

Struct Vehicle {
   int cyl;
   int weight;
   int speed'
}

QList<Vehicle> carList; //Assume this is populated
for(i = 0; i < carList.length; ++i) { 
if(carList[i].cyl == 6 && carList[i].weight == 1000) {
return carList[i].speed; } }

My other idea was to concatenate the two keys into one key, and implement a couple functions to combine and separate the two keys when needed. While this would work, and probably be a lot faster than a full iteration, it seems a bit hacked together.

QHash<QString, int> m_Lookup;
m_Lookup.insert("6|1000", 210);

Does anyone know of a better way to try and achieve this?

Answer 1

Option 1

Use a QPair as your key:

QHash<QPair<int, int>, int> m_Lookup;

m_Lookup.insert(QPair<int, int>(6, 1000), 210);
m_Lookup.insert(QPair<int, int>(6, 1500), 190);

qDebug("Value is %d", m_Lookup.value(QPair<int, int>(6, 1000)));

Option 2

Create a class to represent your desired vehicle characteristics (complete with equality/inequality operators) and create an implementation of qHash for your class:

class Vehicle
{
public:
   Vehicle(short cylinders, short weight) 
      : m_Cylinders(cylinders), m_Weight(weight) { }

   short cylinders() const { return m_Cylinders; }
   short weight() const { return m_Weight; }

   bool operator==(const Vehicle& other) const 
      { return (m_Cylinders == other.m_Cylinders && m_Weight == other.m_Weight); }
   bool operator!=(const Vehicle& other) const 
      { return !operator==(other); }

private:
   short m_Cylinders;
   short m_Weight;
};

inline uint qHash(const Vehicle& key)
{
   uint k = static_cast<uint>(key.cylinders()) << 16 | static_cast<uint>(key.weight());
   return k;
}

int main(int argc, char** argv)
{
   QHash<Vehicle, int> m_Lookup;

   m_Lookup.insert(Vehicle(6, 1000), 210);
   m_Lookup.insert(Vehicle(6, 1500), 190);

   qDebug("Value is %d", m_Lookup.value(Vehicle(6, 1000)));

   return 0;
}