我试图将指向派生类数据成员的指针强制转换为指向基类数据成员的指针，但以下代码无法编译:

class Base 
{
public:
    virtual void f() {}
};

class Derived : public Base 
{
public:
    void f() override {}
};

class Enclosing
{
public:
    Derived member;
};

int main()
{
    Derived Enclosing::*p = &Enclosing::member;
    auto bp = static_cast<Base Enclosing::*>(p); // compile error
}

所以我改用reinterpret_cast，代码编译:

auto bp = reinterpret_cast<Base Enclosing::*>(p); // passes compile

我尝试直接使用bp:

Enclosing instance;
(instance.*bp).f(); // calls Base::f

这不是我所期望的，因为Enclosing 中的成员实际上是Derived 类型。 然后我试了这个:

(&(instance.*bp))->f(); // calls Derived::f

它适用于我的环境，但这种行为是否得到保证？

最佳答案

你不可能得到你想要的。您只能在这种情况下使用数据成员获得多态性:pointer to member of D of D of type T converted to pointer of member of B of type T where B 是 D 的基类。

如您所愿，将指向类型 T1 的 X 成员的指针 转换为指向类型 T2 的 X 成员的指针 是不允许的或未定义的行为。

要实现多态性，你需要这样的东西:

Enclosing e;
Derived Enclosing::* d = &Enclosing::d;

Base* b = &(e.*d);
return b->foo();    // calls Derived::foo

§4 Standard conversions [conv]

Standard conversions are implicit conversions with built-in meaning.

§4.11 Pointer to member conversions [conv.mem]

1. A null pointer constant (4.10) can be converted to a pointer to member type. [...]

2. A prvalue of type “pointer to member of B of type cv T”, where B is a class type, can be converted to a prvalue of type “pointer to member of D of type cv T”, where D is a derived class (Clause 10) of B. [...]

所以我们看到对于标准转换(隐式转换)你的转换是不允许的

§5.2.9 Static cast [expr.static.cast]

12. A prvalue of type “pointer to member of D of type cv1 T” can be converted to a prvalue 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. [...]

对于static_cast，我们看到基本上只允许对指向成员的指针进行标准转换。这就是您所观察到的，static_cast 是一个编译器错误。

§ 5.2.10 Reinterpret cast [expr.reinterpret.cast]

A prvalue of type “pointer to member of X of type T1” can be explicitly converted to a prvalue of a different type “pointer to member of Y of type T2” if T1 and T2 are both function types or both object types. [...]. The result of this conversion is unspecified, except in the following cases:

(10.1) converting a prvalue of type “pointer to member function” to a different pointer to member function type and back to its original type yields the original pointer to member value.

(10.2) converting a prvalue of type “pointer to data member of X of type T1” to the type “pointer to data member of Y of type T2” (where the alignment requirements of T2 are no stricter than those of T1) and back to its original type yields the original pointer to member value.

对于 reinterpret_cast，我们看到转换是允许的(如您所见，没有编译器错误。但是结果是未指定，除了提到的 2 种情况意味着转换回原始值。它不适用于我们的情况，这意味着您使用 reinterpret_cast 的代码具有未定义的行为。

此外

§5.5 Pointer-to-member operators [expr.mptr.oper]

Abbreviating pm-expression .*cast-expression as E1.*E2, E1 is called the object expression . If the dynamic type of E1 does not contain the member to which E2 refers, the behavior is undefined.

这证明 instance.*bp 是有效的当且仅当 bp 指向的对象必须存在于实例中。这意味着 bp 的类型。

关于c++ - 向上转换指向数据成员的指针及其多态行为，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/48580559/