Unresolved external on static data member from another DLL - c++

Question

Consider the following source of From.dll library defining the From class:
Header:
// From.h
#include <afxwin.h>
#include <afxext.h>
class AFX_EXT_CLASS From
{
public:
static const char staticMember[666];
};
Implementation:
// From.cpp
#include "From.h"
const char From::staticMember[] = "Total Crap";
This library static member staticMember must be used from the following To class, which must also give rise to a DLL (To.dll):
Header:
// To.h
#include <afxwin.h>
#include <afxext.h>
class AFX_EXT_CLASS To
{
public:
static const char* retExtStatic();
};
Implementation:
// To.cpp
#include "To.h"
#include "From.h"
const char* To::retExtStatic()
{
return From::staticMember;
}
But I keep getting this LNK2001: unresolved external symbol "public: static char const * const From::staticMember" (?staticMember#From##2QBDB) when linking To.dll, no matter if it's directly on Visual Studio or if it's a cmake generated nmake:
Linking CXX shared library To.dll
Creating library To.lib and object To.exp
To.cpp.obj : error LNK2001: unresolved external symbol "public: static char const * const From::staticMember" (?staticMember#From##2QBDB)
To.dll : fatal error LNK1120: 1 unresolved externals
The first DLL is compiled and linked allright. The following are the relevant parts of my CMakeLists.txt:
add_definitions (-D_AFXDLL -D_AFXEXT -DPSAPI_VERSION=1)
set (CMAKE_MFC_FLAG 2) # Use MFC in a shared DLL
# FROM
file (GLOB FROM ${CMAKE_CURRENT_SOURCE_DIR}/From.cpp)
add_library (From SHARED ${FROM})
target_link_libraries (From)
# TO
file (GLOB TO ${CMAKE_CURRENT_SOURCE_DIR}/To.cpp)
add_library (To SHARED ${TO})
target_link_libraries (To From)
What am I doing wrong to export this static data member?

Answer 1

As #Niall and #Igor Tnadetnik pointed out on comments on the question, when a DLL needs another DLL, it's not safe to include the header file of the DLL to be used if therein the class is declared with AFX_EXT_CLASS. In this case, as the client code is also a DLL, AFX_EXT_CLASS would be defined as __declspec(dllexport) and not as __declspec(dllimport) as it should.