CMake unresolved externals - c++

Question

I am trying to compile a very simple CMake project using Visual Studio 2013, however I am getting the following error upon trying to compile it:
error LNK1120: 1 unresolved externals cmake_issue\build\Debug\cmake_issue.exe 1 1 cmake_issue
error LNK2001: unresolved external symbol "public: static int Other::value" (?value#Other##2HA) cmake_issue\build\test.obj cmake_issue
I have a base directory with the following CMakeLists.txt in:
project(cmake_issue)
add_subdirectory(other)
add_executable(cmake_issue src/test.cc)
target_link_libraries(cmake_issue other)
And the contents of src/test.cc:
#include <cstdio>
#include "other/other.h"
int main(int argc, char *argv[]) {
printf("value = %d\n", Other::value);
return 0;
}
And a subdirectory called other with the following CMakeLists.txt in:
add_library(other SHARED src/other.cc)
target_include_directories(other PUBLIC include)
target_link_libraries(other)
And the contents of other/include/other/other.h:
#ifndef _OTHER_H_
#define _OTHER_H_
class __declspec(dllexport) Other {
public:
static int value;
};
#endif
And the contents of other/src/other.cc:
#include "other/other.h"
int Other::value = 30;
If I build the project with cmake and then open the generated sln in Visual Studio, both of the projects appear in the Solution Explorer.
If I right click and build other, it builds fine. However if I try and build cmake_issue, I get the errors above. It looks like the cmake_issue solution is not using the other.dll (or other.lib) files being generated when compiling the other solution.
I can upload a zip of the source if it is needed.

Answer 1

Okay, the problem is not on CMake side, but with C++. When you used dllexport'ed class in your executable, its definintion should read class __declspec(dllimport) Other. This code works fine, for example:
#include <cstdio>
class __declspec(dllimport) Other {
public:
static int value;
int a();
};
int main(int argc, char *argv[]) {
printf("value = %d\n", Other::value);
return 0;
}
Here's a link with complete solution: https://social.msdn.microsoft.com/Forums/en-US/6c43599d-6d9d-4709-abf5-4d1e3f5e4fc9/exporting-static-class-members