Указатель на объект из указателя на член класса

class A
{
int i;
} obj;
 
int main()
{
int A::* ptr_to_member = &A::i;
int* ptr_to_int = &obj.i;
};

class A
{
int i;
public:
void show();
};
void A::show(){
cout<<i;
}
int main()
{
A *ptr_to_int,obj;
ptr_to_int = &obj;
ptr_to_int->show();
}

template<typename Type, typename MemberType, MemberType Type::*member>
inline uintptr_t member_offset()
{
    constexpr uintptr_t ret = (uintptr_t)&(((Type*)0)->*member);
    return ret;
};
 
template<typename Type, typename MemberType, MemberType Type::*member>
inline Type* member_container(MemberType* ptr)
{
    Type* ret = (Type*)((char*)ptr - member_offset<Type,MemberType,member>());
    return ret;
};

container_of(ptr_to_int, class_type, NAME_of_int_as_member);

template<typename Type, typename MemberType, MemberType Type::*member>
struct member_helper
{
    constexpr static uintptr_t offset = (uintptr_t)&(((Type*)0)->*member);
};
 
 
template<typename Type, typename MemberType, MemberType Type::*member>
inline Type* member_container(MemberType* ptr)
{
    Type* ret = (Type*)((char*)ptr - member_helper<Type,MemberType,member>::offset);
    return ret;
};