Question

I’m currently designing a object structure for a game, and the most natural organization in my case became a tree. Being a great fan of smart pointers I use shared_ptr‘s exclusively. However, in this case, the children in the tree will need access to it’s parent (example — beings on map need to be able to access map data — ergo the data of their parents.

The direction of owning is of course that a map owns it’s beings, so holds shared pointers to them. To access the map data from within a being we however need a pointer to the parent — the smart pointer way is to use a reference, ergo a weak_ptr.

However, I once read that locking a weak_ptr is a expensive operation — maybe that’s not true anymore — but considering that the weak_ptr will be locked very often, I’m concerned that this design is doomed with poor performance.

Hence the question:

What is the performance penalty of locking a weak_ptr? How significant is it?

Answer 1

From the Boost 1.42 source code (<boost/shared_ptr/weak_ptr.hpp> line 155):

shared_ptr<T> lock() const // never throws
{
    return shared_ptr<element_type>( *this, boost::detail::sp_nothrow_tag() );
}

ergo, James McNellis’s comment is correct; it’s the cost of copy-constructing a shared_ptr.