根据类成员获取类指针

根据类成员地址的偏移获取类指针

template <class P, class M>
size_t my_offsetof(const M P::*member) {
    return (size_t) & (reinterpret_cast<P *>(0)->*member);
}

template <class P, class M>
P *my_container_of_impl(M *ptr, const M P::*member) {
    return reinterpret_cast<P *>(reinterpret_cast<char *>(ptr) -
                                 my_offsetof(member));
}

//这里的意思是根据成员变量，获取类对象的地址，从而转成当前类对象
#define my_container_of(ptr, type, member)                                     
    my_container_of_impl(ptr, &type::member)


class SpeechListener {

    client m_endpoint;
}



void OnOpen(client *c, websocketpp::connection_hdl hdl) {
    SpeechListener *listener = my_container_of(c, SpeechListener, m_endpoint);

}

这里主要使用了 零地址转换

#define OFFSET(structure, member) ((int)&((structure*)0)->member);

第二种方式

struct A
{
    int i;
    char a;
    int j;
    string str1;
    int m;
    int n;
};


    std::cout << "offset:"<< (size_t) &((A*)0)->str1<<endl;

    
    std::cout << "offset:"<< (size_t) &((A*)0)->m<<endl;
    std::cout << "offset:"<< (size_t) &((A*)0)->n<<endl;
    

    string A::*jOffset = &A::str1;
    int nJOffSet =  (size_t)(*(void**)(&jOffset));