Remove from a std::set<shared_ptr<T>> by T*

Question

I have a set of shared pointers:

std::set<boost::shared_ptr<T>> set;

And a pointer:

T* p;

I would like to efficiently remove the element of set equal to p, but I can't do this with any of the members of set, or any of the standard algorithms, since T* is a completely different type to boost::shared_ptr<T>.

A few approaches I can think of are:

• somehow constructing a new shared_ptr from the pointer that won't take ownership of the pointed to memory (ideal solution, but I can't see how to do this)
• wrapping / re-implementing shared_ptr so that I can do the above
• just doing my own binary search over the set

Help!

Answer 1

If the reason for using the set is that you need to efficiently find pointers of type T, then the obvious answer is not to make it a set of shared pointers! Instead, wrap the set in a class which manages the lifetimes of the pointers that the set contains.

Answer 2

You can use boost::ptr_set if you want the set to have ownership of the objects, or boost::reference_wrapper if you just want the set to store references to them. If you use shared_ptr in one place in your code, you will have to use it in all places, or risk terrible crashes (dangling pointers, already deleted objects etc.). The exception is weak_ptr, a pointer that points to an object held by a shared_ptr but does not share ownership.

Answer 3

Construct a shared_ptr<T> from T with a null_deleter (see boost:::shared_ptr FAQ).

That way the types are compatible and you don't have to worry about your temporary shared_ptr deleting any object.

Or, as one of the comments say, if you can change T to inherit from enable_shared_from_this you could get a correct shared ptr from your object.