Building an index: Copies or pointers?

Question

I have a data structure that stores ... well, data. Now, I need to access various pieces of data in slightly different manner, so I'm essentially building an in-memory index. But I'm wondering: should the index hold pointers or copies?

To elaborate, say I have

class Widget
{
    // Ways to access the list of gears...
private:
    std::list<Gears> m_gears;
};

Now, I have two Widgets, and there exists between these two a mapping between their Gears. Currently, this is

boost::unordered_map<Gear, Gear>

but Gear is a fairly hefty class, and I feel like making so many copies is poor design. I could store a pointer, but then the mapping is only valid for the lifetime of the corresponding Widgets, and you start getting ->s... (And if that std::list ever changes to a std::vector, it gets more complex...)

Pertaining to the copies, it's actually slightly worse: There's two boost::unordered_maps, one for each direction. So, for each Gear, I'm making up to 2 copies of it.

Alternatively, I could put the index inside the Widget class, but I feel like this violates the responsibilities of the Widget class.

Answer 1

You might try Boost Pointer Container Library: http://www.boost.org/doc/libs/1_43_0/libs/ptr_container/doc/ptr_container.html

I think it addresses exactly the problem you are facing.

Answer 2

Could you store all gears in one place, like statically in the gears class, and then have each mapping AND widget store only the reference/index to it?

You would have to keep track of references to each gear so you know when you can dispose them, but that should be easy enough.