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

From: Joaquin M López Muñoz (joaquinlopezmunoz_at_[hidden])
Date: 2021-09-09 11:16:45

El 09/09/2021 a las 11:28, John Emmas via Boost-users escribió:
> On 08/09/2021 15:56, John Emmas wrote:
>>
>> I realise there are two types of hook available for 
>> 'boost::intrusive::list' ( the list_member_hook<> and the 
>> list_base_hook<>) - so are there certain situations where one would 
>> preferable over the other?
>>
>
> Overnight I got told (off-list) that when 2 x classes are both 
> declared using the 'list_base_hook<>' option, I'll get a runtime crash 
> if I try to add the same object to both lists.  In other words, this 
> will crash at the specified line:-
>
> Â Â Â  class animal : public boost::intrusive::list_base_hook<>
> Â Â Â  {
> Â  Â  Â Â  [...]
> Â Â Â  };
>
> [...]
>
> Â Â Â Â Â Â  animals.push_back (a2);Â  // <--- 'bulldog' works okay
> [...]
> Â Â Â Â Â Â  dogs.push_back (a2);Â Â Â Â  // <--- 'bulldog' crashes !!! 

You're trying to insert a2 into dogs when the element is alredy inserted 
into animals.
You can either remove a2 from animals beforehand:

 Â Â  animals.erase(Animals::s_iterator_to(a2));

or, if you mean for the elements to be allowed in both lists at the same 
time, use tags:

 Â Â Â  #include <boost/intrusive/list.hpp>
 Â Â Â  #include <string>
 Â Â Â  #include <utility>

 Â Â Â  typedef 
boost::intrusive::list_base_hook<boost::intrusive::tag<struct 
animal_tag>> animals_hook;
 Â Â Â  typedef 
boost::intrusive::list_base_hook<boost::intrusive::tag<struct dog_tag>> 
dogs_hook;

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

 Â Â Â Â Â Â Â  std::string name;
 Â Â Â Â Â Â Â  int legs;
 Â Â Â  };

 Â Â Â  typedef 
boost::intrusive::list<animal,boost::intrusive::base_hook<animals_hook>> 
Animals;
 Â Â Â  typedef 
boost::intrusive::list<animal,boost::intrusive::base_hook<dogs_hook>> Dogs;

 Â Â Â  int main()
 Â Â Â  {
 Â Â Â Â Â Â Â  animal a1{"labrador", 4};
 Â Â Â Â Â Â Â  animal a2{"bulldog", 4};

 Â Â Â Â Â Â Â  Animals animals;
 Â Â Â Â Â Â Â  animals.push_back (a2);Â  // <--- 'bulldog' works okay

 Â Â Â Â Â Â Â  Dogs dogs;
 Â Â Â Â Â Â Â  dogs.push_back (a1);Â Â Â Â  // <--- 'labrador' works okay
 Â Â Â Â Â Â Â  dogs.push_back (a2);Â Â Â Â  // <--- 'bulldog' works okay

 Â Â Â Â Â Â Â  return 0;
 Â Â Â  }

Joaquín M López Muñoz