Is it possible to use a pointer-to-member to get an actual pointer?

问题

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));
    }

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));
    }
};

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));
                  ^

Now, I thought I could be clever and tried

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

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.

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?

回答1:

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));

来源：https://stackoverflow.com/questions/27724971/is-it-possible-to-use-a-pointer-to-member-to-get-an-actual-pointer

标签

c++

pointers

pointer-to-member