Question

Is it valid to develop a DLL in C++ that returns boost shared pointers and uses them as parameters?

So, is it ok to export functions like this?

1.) boost::shared_ptr<Connection> startConnection();
2.) void sendToConnection(boost::shared_ptr<Connection> conn, byte* data, int len);

In special: Does the reference count work across DLL boundaries or would the requirement be that exe and dll use the same runtime?

The intention is to overcome the problems with object ownership. So the object gets deleted when both dll and exe don’t reference it any more.

Answer 1

According to Scott Meyers in Effective C++ (3rd Edition), shared_ptrs are safe across dll boundaries. The shared_ptr object keeps a pointer to the destructor from the dll that created it.

In his book in Item 18 he states, “An especially nice feature of
tr1::shared_ptr is that it automatically uses its per-pointer deleter
to eliminate another potential client error, the “cross-DLL problem.”
This problem crops up when an object is created using new in one
dynamically linked library (DLL) but is deleted in a different DLL. On
many platforms, such cross-DLL new/delete pairs lead to runtime
errors. tr1::shared_ptr avoid the problem, because its default deleter
uses delete from the same DLL where the tr1::shared_ptr is created.”

Tim Lesher has an interesting gotcha to watch for, though, that he mentions here. You need to make sure that the DLL that created the shared_ptr isn’t unloaded before the shared_ptr finally goes out of scope. I would say that in most cases this isn’t something you have to watch for, but if you’re creating dlls that will be loosely coupled then I would recommend against using a shared_ptr.

Another potential downside is making sure both sides are created with compatible versions of the boost library. Boost’s shared_ptr has been stable for a long while. At least since 1.34 it’s been tr1 compatible.