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From: David Abrahams (dave_at_[hidden])
Date: 2004-03-12 14:19:08

"Greg Clayton" <Greg.Clayton_at_[hidden]> writes:

>> Greg Clayton wrote:
>> > I am trying to do explicitly instantiate specific shared_ptr class
>> > for export from a Win32 DLL (currently with MSVC6, and eventually
> in
>> > VS2003).
>> First things first. Why do you need to do that? Have you
>> tried not exporting it, have you encountered any problems?
> Yes, shared pointers can't be created and handed off from DLL to DLL
> the way they are currently implemented.

I don't understand why people keep making this claim. As long as you
don't unload DLL B, you're golden. The only other alternative
prohibits unloading DLL A. I don't think that dynamically generating
a fake DLL to hold the destructors would be appropriate, so IMO
shared_ptr is as well-behaved as it can be wrt DLLs.

> If DLL A has an STL container of shared pointers, and DLL B gets
> _dynamically_ loaded and adds an item to DLL A's container (creates
> a shared pointer to an object, and DLL B adds that shared pointer
> through an interface it to DLL A's STL container of shared
> pointers), DLL B can not be unloaded. If DLL B does get unloaded you
> will crash when your STL container goes out of scope (when it tries
> to delete the owned pointer). The virtual function table for the
> shared pointer lives in the DLL that creates it if you just use
> something like:
> typedef boost::shared_ptr<SomeClass> SomeClassSP;
> in a header file that comes from DLL A (for other DLLs to link to). So
> I need to instantiate and export everything for that shared pointer
> from DLL A (since it is the DLL that owns the STL containers of
> objects) so that the vtable stays valid for the lifetime of the shared
> pointer object.

Solution: pass the objects from B to A via std::auto_ptr.

Dave Abrahams
Boost Consulting

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