C++类成员能当做变量类型吗？

Question

1. 弱逼问题：S::*xmeth是个啥？ S是个类吧，xmeth是类的一个成员？成员能当做变量类型吗？

2. 贴上相关代码

template<class S, class A1, class A2, class R> void
rpcs::reg(unsigned int proc, S*sob, int (S::*meth)(const A1 a1, const A2 a2, 
            R & r))
{
    class h1 : public handler {
        private:
            S * sob;
            int (S::*meth)(const A1 a1, const A2 a2, R & r);
        public:
            h1(S *xsob, int (S::*xmeth)(const A1 a1, const A2 a2, R & r))
                : sob(xsob), meth(xmeth) { }
            int fn(unmarshall &args, marshall &ret) {
                A1 a1;
                A2 a2;
                R r;
                args >> a1;
                args >> a2;
                if(!args.okdone())
                    return rpc_const::unmarshal_args_failure;
                int b = (sob->*meth)(a1, a2, r);
                ret << r;
                return b;
            }
    };
    reg1(proc, new h1(sob, meth));
}

1. 贴上报错信息

2. 贴上相关截图

3. 已经尝试过哪些方法仍然没解决（附上相关链接）

Answer 1

What is S::*xmeth? S is a class, right? xmeth is a member of the class?

According to template parameters

template<class S, class A1, class A2, class R>

It can be seen that S, A1, A2 and R are all classes. So,

S::*       // 表示指向类 S 的成员的指针
S::*xmeth  // 表示该指针的变量名是 xmeth

Can

members be used as variable types?

No, the type of the third parameter of function reg is member function pointer, not a member.

Specifically, the variable name of the third parameter of function reg is meth and the type is int (S::*)(const A1, const A2, R &). That is, meth is a pointer to a member function of class S. The member function of this class S has three parameters. The types of these three parameters are const A1, const A2 and R & respectively. This The return value type of member functions of type S is int.

might be easier to understand in English: meth is a pointer points to member function of class S whose parameters are const A1, const A2 and R &, return type is int.

If you don’t understand it well, you can learn it first How to read function pointer, you can also use the cdecl website.

Answer 2

int (S::*meth)(const A1 a1, const A2 a2, R & r);
//这是你这个模板中的成员变量的声明
//变量名字是meth
//类型是 int (S::*)(const A1, const A2, R&), 也就是一个函数指针
//不过 比较特别的是一个 类的成员函数 指针，不是普通的全局函数指针
// 

Answer 3

This is a member function pointer. Generally speaking, the length ranges from 4byte to 16byte. Internally, according to different ABI conventions, it describes the calling addressing method of member functions given a class pointer. I look at your class. There is also an S* member. This class should be a wrapper for class member functions

The calling method in your code is like this (sob->*meth) (parameter)