是否可以使用指向成员的指针来获取实际的指针？ [英] Is it possible to use a pointer-to-member to get an actual pointer?

问题描述

我有一些结构或类 example ，我需要发送一个指针到其任何一个成员。

I have some struct or class example, and I need to send a pointer to any of its members somewhere.

void send_ptr(void *ptr, std::size_t size);

struct obj {};

struct example {
    int a;
    obj b;

private:
    static void send_ptr(void *ptr, std::size_t size) {}

public:
    template <typename M>
    void send_member(M *member) {
        send_ptr(member, sizeof(M));
    }

这很好，但我想我可以使用指针到成员介绍一些小静态检查。 （ send_member 的上述实现实际上可以发送任意指针。）

This works fine but I thought I could use pointer-to-member here to introduce a little static checking. (The above implementation of send_member can actually send an arbitrary pointer.)

    template <typename M>
    void send_member(M example::*member) {
        send_ptr(&this->*member, sizeof(M));
    }
};

这会失败，因为指针成员取消引用（ this-> *

This fails because pointer-to-member dereference (this->*member) apparently evaluates to an rvalue.

error: lvalue required as unary ‘&’ operand
        send_ptr(&this->*member, sizeof(M));
                  ^

现在，我以为我可以聪明和尝试

Now, I thought I could be clever and tried

&static_cast<M &>(this->*member)

这似乎工作，但老实说，我不完全明白如何。也许是不安全的。我知道 static_cast 可以这种方式使用（有时像完美转发），但我不完全理解这里的语义。

This appears to work but to be honest, I don't fully understand how. Maybe it's unsafe. I know that static_cast can be used this way sometimes (like perfect forwarding) but I don't fully understand the semantics here.

那么是否可以使用指针到成员可靠地获取指向成员的实际指针？ static_cast 是否有效（如果是这样，你能解释一下）？

So is it possible to use a pointer-to-member to reliably get an actual pointer to the member? Does the static_cast work (if so, could you explain how)? Or is there a simpler way?

推荐答案

运算符& 比 operator-> * 运算符优先级 $ c>和运算符。。 & 将绑定到 this ，这是一个 prvalue ，而不是 glvalue 其中你可以取地址。使用括号来排序评估：

operator& has greater operator precedence than operator->* and operator.*. The & will bind to this first which is a prvalue, not a glvalue of which you can take the address. Use parentheses to sequence the evaluation:

send_ptr(&(this->*member), sizeof(M));

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