C++链接器错误Singleton类中的未定义引用

Question

我正在尝试将此java单例类移植到c ++:

public class Singleton {
private static Singleton uniqueInstance;

private Singleton() {} 
public static Singleton getInstance() {
    if (uniqueInstance == null) {
        uniqueInstance = new Singleton();
    }
        return uniqueInstance;
    }
}

我移植到这个C++代码:

class Singleton {
private:
  Singleton() {
    cout << "new Singleton is called" << endl;
  }
  static Singleton* uniqueInstance;
public:
   static Singleton* getInstance() {
    if (!uniqueInstance) {
      uniqueInstance = new Singleton();
    }
    return uniqueInstance;
  }
};

但是我无法编译这个!和gcc链接器发生错误.

Answer 1

确保static在声明之外定义成员:

Singleton* Singleton::uniqueInstance = nullptr;