c++ - Creating a shared library with a global variable -

this question asked in g++ compiler context under windows.

let's want create shared library, mylibrary.dll.

mylibrary.h:

#ifndef _mylibrary_h #define _mylibrary_h  #ifdef mylibrary_dll_export #define mylibrary_api __declspec(dllexport) #else #define mylibrary_api __declspec(dllimport) #endif  class mylibrary_api xyz{ public:     double x; };  mylibrary_api extern xyz xyz; // correct, want declare, hence "extern"  #endif 

mylibrary.cpp:

#include "mylibrary.h"  xyz xyz; // definition of xyz 

i compile 2 files,

mylibrary.dll libmylibrary.a 

now, lets want use variable xyz in program link against import library (i.e. -lmylibrary), , include header mylibrary.h of course.

i seem getting undefined reference when compile.

undefined reference `__imp__zn2vx6................ 

how define "global" variable in shared library, i.e. variable exists in 1 copy , can shared among threads?

edit:

i turned out reason had declared variable xyz inside 2 namespace in mylibrary.h. when moved variable outside namespaces , b (i.e. global namespace) worked.

i had this:

mylibrary.h:

#ifndef _mylibrary_h #define _mylibrary_h  #ifdef mylibrary_dll_export #define mylibrary_api __declspec(dllexport) #else #define mylibrary_api __declspec(dllimport) #endif  namespace a{  namespace b{  class mylibrary_api xyz{ public:     double x; };  mylibrary_api extern xyz xyz; // correct, want declare, hence "extern"  } // b  } //  #endif 

mylibrary.cpp:

#include "mylibrary.h"  a::b::xyz xyz; // definition of xyz 

i tried use xyz outside mylibrary referring as

a::b::xyz 

question: refer xyz in wrong way or must place global variables in shared libraries in global namespace?