Re: A solution to warning C4251 - class needs to have dll-interface...?

On Tue, 16 Oct 2007 20:37:14 +0200, "Niels Dekker - no return address"
<unknown@xxxxxxxxxxxxxxx> wrote:

The following piece of code usually triggers warning C4251:
'MyClass::m_object' : class 'ThirdPartyTemplate<T>' needs to have
dll-interface to be used by clients of class 'MyClass':

template <typename T>
class ThirdPartyTemplate
{
public:
// Constructor.
ThirdPartyTemplate() {}
};

class __declspec(dllexport) MyClass
{
ThirdPartyTemplate<int> m_object; // warning C4251!
};

Is there really anything wrong about it?

It's nothing #pragma warning(disable) can't cure. :)

Here at LKEB (www.lkeb.nl), we
often get this kind of warning, when using Boost or STL. Now it appears
that one can get rid of the warning, by wrapping the ThirdPartyTemplate<int>
object inside a simple struct:

struct WrapperStruct
{
ThirdPartyTemplate<int> wrappedObject;
};

class __declspec(dllexport) MyClassWithoutWarning
{
WrapperStruct m_wrapper; // No warning! :-)
};

Does this approach really solve the problem, or does it only hide the
message?

It merely hides it. Think about the motivation for this warning. It's
emitted when you use a non-dllexported class X in a dllexported class Y.
What's so bad about that? Well, suppose Y has an inline function y_f that
calls a function x_f belonging to X that is not also inline. If y_f is
inlined inside some client that doesn't statically link X, the link will
fail, because x_f won't be found. Concerning your code, ThirdPartyTemplate
is a class template, and template code is generated wherever it's needed,
so this is not an issue. You may end up with some code bloat due to
duplicated template instantiations, but you can usually ignore this. There
are two workarounds (and I use that term lightly):

1. You can explicitly instantiate and dllexport the specializations you use
from some DLL, or more generally,

2. You can define a wrapper class that has no inline functions and defines
all the potentially compiler-generated functions (default ctor, copy ctor,
dtor, and assignment operator) and dllexport that.

It looks like you're sort of on your way to (2) with MyClass.

--
Doug Harrison
Visual C++ MVP
.

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