[SOLVED] Undefined refrence to static member variables in C++ 20 shared library only when using CMake

Issue

I've been migrating my project to use CMake but I've stumbled upon an issue with static member variables. Even when there is a definition for it in the .cpp file.

I've created a minimal reproducable example.

Directory structure:

- dll
  - dll.cpp
  - dll.h
- main.cpp
- CMakeLists.txt

CMakeLists.txt

cmake_minimum_required(VERSION 3.20)

add_compile_definitions(BUILD)
add_library(sharedLib SHARED dll/dll.cpp dll/dll.h)

link_libraries(sharedLib)
add_executable(exec main.cpp)

main.cpp

#include "dll/dll.h"
#include <iostream>

int main( )
{
    std::cout << A::member << std::endl;

    return 0;
}

dll.cpp

#include "dll.h"

int A::member = 10;

dll.h

#pragma once

#if !defined(BUILD)
#define API __declspec(dllimport)
#else
#define API __declspec(dllexport)
#endif

class API A
{
public:
    static int member;
};

Solution

The add_compile_definitions command add macros globally. For all targets. In your example it will be set for both the DLL and the application.

Use target_compile_definitions instead, to set macros for a single target:

# add_compile_definitions(BUILD)
add_library(sharedLib SHARED dll/dll.cpp dll/dll.h)
target_compile_definitions(sharedLib PRIVATE BUILD)

---

It's similar with link_libraries, it's a global command. Use target_link_libraries instead, to set only for a specific target.

Answered By - Some programmer dude

Answer Checked By - Gilberto Lyons (WPSolving Admin)