c++ - 派生类的单例模式

Question

在普通的单例模式中，单例类实际上是“密封”的(无法派生):

class A
{
private:
    static A m_instance; // instance of this class, not a subclass

    // private ctor/dtor, so class can't be derived
    A();
    ~A();

public:
    static A& GetInstance() { return m_instance; }

    ...
};

您将如何编写一个本应派生的类，但其派生类只能实例化一次？

Answer 1

How would you write a class that is meant to be derived, but whose derived class should only be instantiated once?

您可以使用 CRTP意识到这一点:

template<typename Derived>
class A
{
protected: // Allow to call the constructor and destructor from a derived class
    A(); 
    ~A();

public:
    static T& GetInstance() { 
        static T theInstance; // Better way than using a static member variable
        return theInstance;
    }

    ...
};

然后像这样使用

class B : public A<B> {
    // Specific stuff for derived class
};

---

请注意，如果基类提供了一些基于派生类提供的接口(interface)的通用实现(除了 GetInstance() 函数)，这种方式最有意义。

每当需要调用基类中的派生类时，您都可以安全地使用 static_cast<Derived*>(this)访问它(不需要 virtual 或纯 virtual 函数):

 template<typename Derived> 
 void A<Derived>::doSomething {
      // Execute the functions from Derived that actually implement the 
      // warranted behavior.
      static_cast<Derived*>(this)->foo();
      static_cast<Derived*>(this)->bar();
 }