继承自两个多态类 [英] Inherit from two polymorphic classes

问题描述

给定以下代码

class T {
    public:
        virtual ~T () {}
        virtual void foo () = 0;
};

class U {
    public:
        U() {}
        ~U() {}
        void bar () { std::cout << "bar" << std::endl; }
};

class A : public U, public T {
    public:
        void foo () { std::cout << "foo" << std::endl; }
};

int main () {
    A * a = new A;

    std::vector<U*> u;
    std::vector<T*> t;

    u.push_back(a);

    t.push_back(reinterpret_cast<T*>(u[0]));

    u[0]->bar ();
    t[0]->foo ();

    delete a;
    return 0;
}

我得到我想要的输出

bar
foo

但是，如果我将 U 的定义更改为

However, if I change the definition of U to

class U {
    public:
        U() {}
        virtual ~U() {}
        virtual void bar () { std::cout << "bar" << std::endl; }
};

我仍然编译正常，没有警告/错误，但输出现在

I still compile fine and without warnings/errors but the output is now

bar
bar

关于虚拟声明阻止我调用 foo ？

What is it about the virtual declaration that prevents me from calling into the foo?

推荐答案

首先，在您的示例中没有虚拟基类。包含虚函数的类称为多态。 （在C ++中有虚拟基类这样的东西，但它与你的例子没有关系。）

Firstly, there are no virtual base classes in your example. Classes that contain virtual functions are called polymorphic. (There is such thing as "virtual base classes" in C++ but it has nothing to do with your example.)

其次，你的代码的行为不依赖任何虚拟声明。您通过使用 reinterpret_cast 故意销毁基本指针的完整性。因此，代码的行为是 undefined 。

Secondly, the behavior of your code does not depend on any virtual declarations. You have deliberately destroyed the integrity of the base pointer by using reinterpret_cast. For this reason the behavior of the code is undefined.

从一个基指针直接转换到另一个在您的代码中）调用 cross-cast 。在C ++中可以执行交叉转换的唯一转型是 dynamic_cast 。

A direct cast from one base pointer to another (which is what you are trying to do in your code) is called cross-cast. The only cast in C++ that can carry out a cross-cast is dynamic_cast.

t.push_back(dynamic_cast<T *>(u[0])); 

您可以执行不带 dynamic_cast ，但是你必须使用 static_cast 来将指针下转到派生类型（ A * 然后将其上变换到另一个基本指针类型

You can perform an indirect cross-cast without dynamic_cast, but for that you have to downcast the pointer to the derived type first (A *) using static_cast and then upconvert it to another base pointer type

t.push_back(static_cast<A *>(u[0])); // upconversion to `T *` is implicit

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