“向上转型"是否安全?方法指针并将其与基类指针一起使用?

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问题描述

假设我有一个指针类型，它可以保存基类方法的地址.我可以将子类方法的地址分配给它并期望它正常工作吗?在我的例子中，我将它与基类指针一起使用，对象的动态类型是派生类.

Let's say I have a pointer type that can hold the address of a base class method. Can I assign the address of a subclass method to it and expect it to work correctly? In my case I'm using it with a base class pointer and the dynamic type of the object is the derived class.

struct B
{
    typedef void (B::*MethodPtr)();
};

struct D: public B
{
    void foo() { cout<<"foo"<<endl; }
};

int main(int argc, char* argv[])
{
    D d;
    B* pb = &d;

    //is the following ok, or undefined behavior?
    B::MethodPtr mp = static_cast<B::MethodPtr>(&D::foo);
    (pb->*mp)();
}

标准在谈到 static_cast 时这样说:

The standard says this when talking about static_cast:

5.2.9.9 指向 cv1 T 类型 D 成员的指针"类型的右值可以转换为指向 cv2 T 类型 B 成员的指针"类型的右值，其中 B是 D 的基类(第 10 条)，如果存在从指向 T 类型 B 成员的指针"到指向 T 类型 D 成员的指针"的有效标准转换(4.11)，并且 cv2 相同cv-qualification 与 cv1 相同或更高的 cv-qualification.63)将空成员指针值(4.11)转换为目标类型的空成员指针值.如果类 B 包含原始成员，或者是包含原始成员的类的基类或派生类，则指向成员的结果指针指向原始成员.否则，强制转换的结果是未定义的.[注:虽然B类需要不包含原始成员，取消引用指向成员的指针的对象的动态类型必须包含原始成员；见 5.5.]

5.2.9.9 An rvalue of type "pointer to member of D of type cv1 T" can be converted to an rvalue of type "pointer to member of B of type cv2 T", where B is a base class (clause 10) of D, if a valid standard conversion from "pointer to member of B of type T" to "pointer to member of D of type T" exists (4.11), and cv2 is the same cv-qualification as, or greater cv-qualification than, cv1. 63) The null member pointer value (4.11) is converted to the null member pointer value of the destination type. If class B contains the original member, or is a base or derived class of the class containing the original member, the resulting pointer to member points to the original member. Otherwise, the result of the cast is undefined. [Note: although class B need not contain the original member, the dynamic type of the object on which the pointer to member is dereferenced must contain the original member; see 5.5.]

与往常一样，我很难破译标准.它有点说没关系，但我不能 100% 确定上述文本是否真的适用于我的示例代码中的情况.

As always, I'm having such a hard time deciphering the standard. It kinda says that it is ok, but I'm not 100% sure if the above text really applies to the situation in my example code.

“向上转型"是否安全?方法指针并将其与基类指针一起使用?

问题描述

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