RE: [boost] Explicit instantiation of boost::shared_ptr<SomeClasss>

From: Greg Clayton (Greg.Clayton_at_[hidden])
Date: 2004-03-12 12:07:02

> -----Original Message-----
> From: Peter Dimov [mailto:pdimov_at_[hidden]]
> Sent: Thursday, March 11, 2004 4:48 PM
> To: boost_at_[hidden]
> Subject: Re: [boost] Explicit instantiation of
> boost::shared_ptr<SomeClasss>
> 
> 
> 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. 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.

> [...]
> 
> > When I look through the exports, some needed class functions seem to
> > be missing from the DLL, and linking fails. Most notable are:
> >
> > void boost::shared_ptr<SomeClass>::reset(SomeClass*)
> >
> > This function is declared in the shared_ptr header file as:
> > template<class Y> void reset(Y * p);
> 
> This is a function template. A template can't be exported, only an
> instantiation.
> 
>     template void boost::shared_ptr<SomeClass>::reset(SomeClass*);
>
> should instantiate reset with Y=SomeClass.

After I sent the original email to this list I tried something similar, but it didn't seem to work (didn't see it in my export list). I will try it with VS2003 and see if I have better luck.