C++:member reference or pointer?

Question

I have a collection (currently boost::ptr_vector) of objects (lets call this vec) that needs to be passed to a few functors. I want all of the functors to have a reference/pointer to the same vec which is essentially a cache so that each functor has the same data cache. There are three ways that I can think of doing this:

1. Passing a boost::ptr_vector<object>& to the constructor of Functor and having a boost::ptr_vector<object>& member in the Functor class

2. Passing a boost::ptr_vector<object>* to the constructor of Functor and having a boost::ptr_vector<object>* member in the Functor class

3. avoid the use of boost::ptr_vector and directly pass an array (object*) to the constructor

I have tried to use method 3, but have been told constantly that I should use a vector instead of a raw pointer. So, I tried method 2 but this added latency to my program due to the extra level of indirection added by the pointer. I am using method 1 at the moment, however I may need to reassign the cache during the lifetime of the functor (as the data cache may change) so this may not be an appropriate alternative.

Which I don't fully understand. I assume somewhere along the way the functor is being copied (although these are all stored in a ptr_vector themselves).

Is method 3 the best for my case? method 2, is too slow (latency is very crucial), and as for method 1, I have been advised time and again to use vectors instead.

Any advice is much appreciated

Answer 1

A reference in C++ can only be initialized ('bound') to a variable.

After that point, a reference can not be "reseated" (made to refer to a different variable) during it's lifetime.

This is why a default copy constructor could conceivably be generated, but never the assignment operator, since that would require the reference to be 'changed'.

My recommended approach here is to use a smart pointer instead of a reference.

• std::unique_ptr (simplest, takes care of allocation/deallocation)
• std::shared_ptr (more involved, allows sharing of the ownership)

In this case:

std::shared_ptr<boost::ptr_vector<object> > m_coll;

would seem to be a good fit