无法在其他.cpp文件中使用单例静态函数：“未定义的引用”

Question

我有一个ResourceManager类，我在这里尝试将其用作单例。我在下面列出了文件resources.hpp和resources.cpp。最后的代码清单是ResourceManager::instance()的用法和下面的错误消息。我尝试了很多诸如前向声明等操作，但是找不到让编译器成功的方法。

resources.hpp

namespace X11 {
      class ResourceManager {
      private:
        static ResourceManager* _instance;
        ResourceManager() {
          //logic
        };
        ResourceManager (const ResourceManager&);
        ~ResourceManager() = default;
      public:
        static ResourceManager* instance();
        std::vector<sf::Font> fonts;
      };
    }

resources.cpp

 #include "resources.hpp"

    namespace X11 {
      ResourceManager* ResourceManager::_instance = 0;
      ResourceManager* ResourceManager::instance() {
        if (!ResourceManager::_instance)
          ResourceManager::_instance = new ResourceManager();
        return ResourceManager::_instance;
      }
    }

用法（initialized.cpp）

 #include "initializer.hpp"
  // initializer.hpp includes "resources.hpp"

  void Initializer::init_right_bar(Menu& menu) {
    // logic
    text.setFont(ResourceManager::instance()->fonts[0]);
    // logic
  }

错误消息

[ 30%] Building CXX object CMakeFiles/vilsoc.dir/src/initializer.cpp.o
[ 40%] Building CXX object CMakeFiles/vilsoc.dir/src/vilsoc.cpp.o
[ 50%] Linking CXX executable vilsoc
/usr/bin/ld: CMakeFiles/vilsoc.dir/src/initializer.cpp.o: in function `X11::Initializer::init_right_bar(X11::Menu&)':
initializer.cpp:(.text+0x1715): undefined reference to `X11::ResourceManager::instance()'
collect2: error: ld returned 1 exit status

已解决 这些评论得出的结论是，我不是通过CMake更新我的makefile，因此链接器找不到源文件。