CMake unresolved externals

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