Question

I’m trying to write a function for a database class that is basically just a wrapper around a hash_map of objects (say shapes) indexed by ID numbers that will look up an ID and cast it to the appropriate pointer type.

e.g. I’d like to be able to do something like this:

Circle* shapeToLookup = NULL;
int idNum = 12;
database.lookup(idNum, circleToLookup);
if(circleToLookup != NULL)
{
    // Do stuff with the circle.
}

and have the database know the type of its argument. Is there a way to do this without either overloading the function (lookup(int, Circle*), lookup(int, Rect*), ad nauseum)? Can you declare a function like lookup(int, Shape*) and have it know which type it’s given?

Thanks!

Answer 1

You can do it with a template.

Edit: new implementation based on the extra information. If mymap is a std::map<int, Shape*>:

template <typename T>
void lookup(int idNum, T* &ptr) {
    auto it = mymap.find(idNum);
    if (it == mymap.end()) {
        ptr = 0;
    } else {
        ptr = dynamic_cast<T*>(*it); // Shape must have a virtual member function
    }
}

Or you might prefer:

template <typename T>
T* lookup(int idNum) {
    auto it = mymap.find(idNum);
    if (it == mymap.end()) {
        return 0;
    }
    return dynamic_cast<T*>(*it);
}

Then call it like Circle *circle = database.lookup<Circle>(123);

Obviously polymorphic containers are a whole heap of fun in themselves, but I’ll assume you have that sorted. There may well be a shared_ptr in there somewhere that I’ve left out.

Old implementation when I thought the DB might store copies of POD:

template <typename T>
void lookup(int idNum, T* &ptr) {
    void *theresult = // something based on idNum

    // some check needed here that theresult really is the right type.
    // how you do this depends on the database, but suppose that
    // the database gives us some integer "type" which indicates the type
    if (type != type_constant<T>::value) {
        ptr = 0;
    } else {
        ptr = static_cast<T*>(theresult);
    }
}

type_constant is an example of “type traits”, you implement it like:

template <typename T>
struct type_constant {};

template <>
struct type_constant<Circle> {
    static const int value = 1;
};

template <>
struct type_constant<Rectangle> {
    static const int value = 2;
};

// etc...