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

Here is a thread with a good discussion of this.

In short, the compiler is warning you that, in effect, your exported class does not seperate the interface from the implementation. If the members in question are not accessible to the clients, make them private and #pragma away the warning for that member/class. If the members are accessible and used by clients, then you will need to provide indirect access to the members through accessors and mutators.

Answer 2

I usually get this warning when I make the silly mistake of building the DLL with runtime library Single/Multithreaded instead of Single/MultithreadedDLL. You might want to check that in your project settings.