__declspec (dllexport) custom type variable - c++

Question

I'm working on a Visual Studio solution with multiple projects (Visual Studio 2013). One project is for generating a .dll file, other projects use that .dll file and generate .exe files. When I export a standard type variable everything works fine. But in case if I want to use my custom defined type I get an compilation error. Here is an example
// Dll.cpp
#define DllExport __declspec (dllexport)
DllExport int maxPackSize = 20;
// my custom type
struct DllExport Header
{
int m_data; // some data
};
DllExport Header qHead = { 100 };
// Exe.cpp
#define DllImport __declspec (dllimport)
DllImport extern int packetSize; // OK
struct DllImport Header;
DllImport extern Header qHead; // leads to an error
When I use qHead in my Exe.cpp I get a compilation error on that line. Error is like
error C2027: use of undefined type 'Header'
What am I doing wrong? Any ideas?

Answer 1

The error message you get is a compiler error (not a linker error!).
You need to define Header prior to its first use.
You could move struct DllExport Header { ... }; to a separate header file (e.g. Dll.h) and then #include "Dll.h" in Exe.cpp.

Answer 2

You need to export your custom type in a header, so that your .exe can see the type. For example,
Dll.h
#ifdef EXPORT_SYMBOLS
#define DLL_EXPORT __declspec(dllexport)
#else
#define DLL_EXPORT __declspec(dllimport)
#endif
struct DLL_EXPORT Header
{
int m_data;
};
Exe.cpp
#include "Dll.h"
Header qHead;
You need to add EXPORT_SYMBOLS to the Dll's preprocessor flags so that the correct __declspec macro is switched.