c++ - 基类静态引用数据成员上 C++ 对象切片行为的奇怪案例

Question

我正在经历一个奇怪的对象切片案例。我正在从事需要单例类的项目，因此我的基类和派生类都是单例。以下示例案例描述了我的情况。

这是我的基类

// Base.h
class Base
{
    public:

        static Base& base;

        virtual void doSomething(){ cout<<"Base Do Something"<<endl; }

    protected:

        Base();
        virtual ~Base();
        static Base& getBaseInstance();

    private:
};

//Base.cpp
Base::Base()
{
    //ctor
}

Base::~Base()
{
    //dtor
}

Base& Base::getBaseInstance()
{
    static Base object;
    return object;
}
Base& Base::base=Base::getBaseInstance();

这是我的派生类

class Derived: public Base
{
    public:

    static Derived& derived;
    virtual void doSomething(){ cout<<"Derive Do Something"<<endl; }

  static Derived& getDerivedInstance();
    protected:
        Derived();
        virtual ~Derived();

    private:
};

Derived::Derived()
{
    //ctor
}

Derived::~Derived()
{
    //dtor
}

Derived& Derived::derived=Derived::getDerivedInstance();

Derived& Derived::getDerivedInstance()
{
    static Derived object;
    return object;
}

最后这是我的主要功能

int main()
{
    cout << "Hello world!" << endl;
    Base::base.doSomething();
    Derived::derived.doSomething();

    Base::base=Derived::derived;

    Base::base.doSomething();

    Base::base=Derived::getDerivedInstance();

    Base::base.doSomething();

    Base& r = Derived::derived;

    r.doSomething();

    Base::base=Derived::getDerivedInstance();
    Base::base.doSomething();
    return 0;
}

我得到以下输出

Hello world!
Base Do Something
Derive Do Something
Base Do Something
Base Do Something
Derive Do Something
Base Do Something

所以我的问题是，既然对象切片不应该对引用起作用，那为什么我不能覆盖 Base::base 引用我创建为 Base 类的静态数据成员与 Derived 对象？虽然这在 Base& r = Derived::derived;

上运行良好

我的意思是，当我调用 do something with r.doSomething() 时，我得到派生类的 doSomething。但是

却不是这样

    Base::base=Derived::derived;    
    Base::base.doSomething();

或

    Base::base=Derived::getDerivedInstance();    
    Base::base.doSomething();

任何形式的澄清将不胜感激。谢谢。

Answer 1

当你做的时候

Base::base=Derived::derived;

您没有设置基引用来引用派生类。这是赋值运算符，它所做的只是将 derived 的 Base 部分赋给 base。

base 仍然是 Base 类型，并且永远不会更改，因为引用只能初始化一次，您可以使用

Base& Base::base=Base::getBaseInstance();

如果您想要这种重新分配行为，您将需要使用指针类型。