Question

I hope the headline isn’t too confusing. What I have is a class StorageManager containing a list of objects of classes derived from Storage. Here is an example.

struct Storage {};                         // abstract

class StorageManager
{
private:
    map<string, unique_ptr<Storage>> List; // store all types of storage

public:
    template <typename T>
    void Add(string Name)                  // add new storage with name
    {
        List.insert(make_pair(Name, unique_ptr<Storage>(new T())));
    }

    Storage* Get(string Name)              // get storage by name
    {
        return List[Name].get();
    }
};

Say Position is a special storage type.

struct Position : public Storage
{
    int X;
    int Y;
};

Thanks to the great answers on my last question the Add function already works. What I want to improve is the Get function. It reasonable returns a pointer Storage* what I can use like the following.

int main()
{
    StorageManager Manager;
    Manager.Add<Position>("pos");    // add a new storage of type position

    auto Strge = Manager.Get("pos"); // get pointer to base class storage
    auto Pstn = (Position*)Strge;    // convert pointer to derived class position

    Pstn->X = 5;
    Pstn->Y = 42;
}

It there a way to get rid of this pointer casting by automatically returning a pointer to the derived class? Maybe using templates?

Answer 1

use:

template< class T >
T* Get(std::string const& name)
{
    auto i = List.find(name);
    return i == List.end() ? nullptr : static_cast<T*>(i->second.get());
}

And then in your code:

Position* p = Manager.Get<Position>("pos");