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

atlcdlldllexport

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.

Best Answer

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.

Related Solutions

C++ – Pure base class needs to be exported from DLL

It looks like its a compiler warning and not an error, so it should still work. The compiler is just letting you know that you are doing something that makes it easy for you to screw up. It should be perfectly acceptable to do this as long as both DLLs and the core program agree on the definition of the base class.

You should be able to use a pragma to supress the warning:

http://forums.devx.com/archive/index.php/t-84785.html

C++ – how to use an exported class (__declspec(dllexport))in an stl template

You should not be using __declspec(dllexport) into the header files that code using your DLL will be using, as they need to be using __declspec(dllimport).

Therefore you should create a macro that uses dllexport if a particular other define is set, and dllimport if it is not.

In a common header somewhere:

#ifdef EXPORTING_FOO
#define FOO_API __declspec(dllexport)
#else
#define FOO_API __declspec(dllimport)
#endif

You can instantiate a template:

extern template class FOO_API Templ<MyClass, int >;

within an included header. Note the extern here. Within one compilation unit declare the same but without the extern and without the FOO_API, thus:

template class Templ<MyClass, int >;

This will mean that the code using your library will not instantiate the template but will be using the one in your library. This is particularly useful when the template has virtual members.

If the template is one from the standard library or boost, the code using yours will have to be using the same version as yours or there could be a serious problem.

Given that in your own example it appears in the private area suggests you want to refactor it out of the interface for your library. Ideally your library should only expose public methods and members other than forward declarations. Private copy-constructors and assignments in order to make a class non-copyable and non-assignable are fine - they are not really part of the implementation they are part of the interface of your class (you are saying that you cannot copy or assign them).

Best Answer

Related Solutions

C++ – Pure base class needs to be exported from DLL

C++ – how to use an exported class (__declspec(dllexport))in an stl template

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