Problem understanding 'boost::intrusive::list_member_hook<>'

From: John Emmas (john_at_[hidden])
Date: 2021-09-07 11:11:43

I'm in the early stages of trying to understand boost::intrusive::list' 
so forgive me if I'm just doing something stupid!!  Using VS2019 I wrote 
a small demo app which uses 'boost::intrusive::list_base_hook<>':-

     #include <boost/intrusive/list.hpp>
     #include <string>
     #include <iostream>

     using namespace std;

     class animal : public boost::intrusive::list_base_hook<>
     {
     public:
         animal (string n, int l) : name{move(n)}, legs{l} {}

         string name;
         int legs;
     };

     using animal_list = boost::intrusive::list<animal>;

     template<typename T>
     void print_list (const boost::intrusive::list<T>& elem)
     {
         for (const T& a : elem)
             cout << "A " << a.name << " has " << a.legs << " legs" << '\n';
     }

     int main()
     {
         animal a1{"dog", 4};
         animal a2{"spider", 6};
         animal a3{"budgie", 2};
         animal_list animals;

         animals.push_back(a1);
         animals.push_back(a2);
         animals.push_back(a3);
         print_list (animals);

         return 0;
     }

Everything runs fine and I see the expected output but I've noticed that 
some other developers use something called a 'list_member_hook' rather 
than my 'list_base_hook'  - so class animal would look like this:-

     class animal
     {
      public:
         animal (string n, int l) : name{move(n)}, legs{l} {}

         string name;
         int legs;
         boost::intrusive::list_member_hook<> _animal_hook;
     };

and then later on in the program there'd be a couple of lines looking 
something like this:-

     typedef boost::intrusive::member_hook<animal, 
boost::intrusive::list_member_hook<>, &animal::_animal_hook> 
AnimalHookOption;
     typedef boost::intrusive::list<animal, AnimalHookOption> Animals;

But when I try that here, VS2019 throws up this compiler error:-

     error C2039: 'default_list_hook': is not a member of 'animal'

Can anyone explain what I'm doing wrong??

John