Warning C4251 when building a DLL that exports a class containing an ATL::CString member

Question

I am converting an ATL-based static library to a DLL and am getting the following warning on any exported classes that use the ATL CString class (found in atlstr.h):

warning C4251: 'Foo::str_' : class 'ATL::CStringT' needs to have dll-interface to be used by clients of class 'Foo'

I am correctly declaring the Foo class as exported via __declspec(dllexport). Is this a warning I can safely ignore or am I doing something wrong? The DLL project settings are set to dynamically link with ATL, but this doesn't seem to make any difference.

For example:

#ifdef DLLTEST_EXPORTS
#define DLLTEST_API __declspec(dllexport)
#else
#define DLLTEST_API __declspec(dllimport)
#endif

// This class is exported from the DLLTest.dll
class DLLTEST_API Foo
{
public:
 Foo();
 CString str_; // WARNING C4251 HERE
};

All clients of this DLL will also be using ATL.

Answer 1

This thread gives what I consider a better answer, by Doug Harrison (VC++ MVP):

[This warning is] 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.

Answer 2

This Microsoft page helped me with it.

How to export an instantiation of a Standard Template Library (STL) class and a class that contains a data member that is an STL object