c++ - 单例类和多重继承

Question

假设我有以下单例基类:

template <class Derived>
class Singleton
{
  public:

    inline static Derived* get();                       

  protected:

    Singleton();                                  
    ~Singleton();                                     
    static std::unique_ptr<Derived> _uptr;                

  private:

    Singleton(const Singleton&);                          
    Singleton & operator = (const Singleton&);             
};

// Initialize the singleton object.
template <class Derived>
std::unique_ptr<Derived> Singleton<Derived>::_uptr = nullptr;

// Constructor
template <class Derived>
Singleton<Derived>::Singleton() {
}

// Constructor
template <class Derived>
Singleton<Derived>::~Singleton() {
}

// Function: get
template <class Derived>
inline Derived* Singleton<Derived>::get() {
  if(_uptr != nullptr) return _uptr.get();
  std::unique_ptr<Derived> tmp(new Derived());
  _uptr = std::move(tmp);
  return _uptr.get();
}

我有以下派生类:

class Foo : public Singleton <Foo> {
  public:
    int value;     
}

int main() {
  Foo A;
  Foo B;
  A.value = 1;
  B.value = 2;
  A.get()->value = 3;
  cout << A.value << endl;  // we get 1
  cout << B.value << endl;  // we get 2
  cout << A.get()->value() << " " << B.get()->value() << endl;  // we get 3
}

我想知道为什么这三种方法在 value 上给出完全不同的输出。 A 和 B 不应该返回相同的值，因为它们继承了相同的单例基础吗？更具体地说，我想实现一个具有全局范围并且只会启动一次的类。

Answer 1

您仍然需要明确删除派生单例类中的(自动创建的)默认构造函数和赋值运算符，以禁止实例化多个实例:

class Foo : public Singleton <Foo> {
public:
    int value;
    delete Foo();
    delete Foo(const Foo&);
    delete Foo& operator(const Foo&);
}