是dynamic_cast<>限制在继承层次结构中的直接转换？ [英] Is a dynamic_cast<> limited to direct casts along the inheritance hierarchy?

问题描述

CODE

struct A { };           // virtual details there, but left out
struct B { };           // virtual details there, but left out
struct C : A, B { };    // virtual details there, but left out

C c;
B& b = c;
A& a = dynamic_cast<A&>( b );  // will this cast succeed at run-time?

请注意，我已省略了虚拟详细信息以保持代码简单。

Note that I have left out the virtual details to keep the code simple.

如果dynamic_cast<>限于沿着继承层次结构的直接转换，那么我期望上述代码在运行时失败（因为B&与A&不相关）。

If dynamic_cast<> is limited to direct casts along the inheritance hierarchy, then I expect the code above to fail at run-time (because B& is unrelated to A&).

然而，如果它更通用/灵活，代码应该工作，因为被引用对象的真实类型是C类型（和C可以被称为B&或A&）。

However, if it is more generic/flexible, the code should work, because the true-type of the referred to object is of C type (and C can be referred to as B& or A&).

C ++规范对这种情况有什么看法？

推荐答案

现在，代码不会编译，因为没有任何类有任何虚函数 - 需要使用 dynamic_cast 。如果你添加至少一个虚函数（例如，虚拟dtor到 B ，那么是的，因为 C 。

Right now, the code won't compile because none of the classes has any virtual functions -- a requirement to use dynamic_cast. If you add at least one virtual function (e.g., a virtual dtor to B, then yes, since a C is derived publicly from B, the cast will succeed.

这是一个简单的演示代码：

Here's a bit of quick demo code:

#include <iostream>

struct A {
    virtual void junk() { std::cout << "A"; }
    void trash() { std::cout << "A"; }
    virtual ~A() {}
};

struct B { 
    virtual ~B(){} 
    void trash() { std::cout << "B"; }
    void junk() { std::cout << "B"; }
};

struct C : virtual A, virtual B { 
    void trash()  { std::cout << "C"; }
    void junk() { std::cout << "C"; }
};    

int main() {
    C c;
    B& b = c;
    A& a = dynamic_cast<A&>(b); 
    a.trash();
    std::cout << "\n";
    a.junk();
}

输出（使用VC ++和g ++，这不是最前沿的，所以除了一个真正的古老的编译器，我希望它是正确的）：

The output (with both VC++ and g++, and this isn't cutting edge, so I'd expect anything but a truly ancient compiler to get it right) is:

A
C

显示 a 的静态类型为 A ，但动态类型为 C 。

Showing that the a has a static type of A, but a dynamic type of C.

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