boost::multi_index using custom key extractor syntax question?

Question

I am applying example 6 of boost multi_index examples. https://www.boost.org/doc/libs/1_71_0/libs/multi_index/doc/examples.html#example6

I am still learning the syntax of key extraction.i find that member key extractor require for its third argument a pointer to class member of type type where class is first arg and type is second arg .

template<class Class,typename Type,Type Class::*PtrToMember>
struct member;  

But in example 6 the author uses 3rd arg which is not pointer .....

key_from_key< 
  member<car_manufacturer,
         const std::string, 
         &car_manufacturer::name>, 
  member<car_model, 
         const car_manufacturer *, 
         car_model::manufacturer> >

this should be & to make pointer which points to the member of class car_model of type const car_manufacturer * so giving us pointer to pointer .....but he just use the member identifiercar_model::manufacturer which is only pointer to member.
So why did the author omits the & from third argument??

if any more code required i will put it.

Answer 1

Congratulations, you found an error in Boost.MultiIndex docs :-)

It should be &car_model::manufacturer rather than car_model::manufacturer, as you correctly point out. What's more, car_model::manufacturer is not even legal C++, as, if anything, it would declare a reference to a member, which does not exist in the language.

Additionally, the docs say:

struct car_model
{
  std::string       model;
  car_manufacturer* manufacturer;
  int               price;
};

where it should be (as correctly written in the actual code):

struct car_model
{
  std::string             model;
  const car_manufacturer* manufacturer;
  int                     price;
};

Otherwise, member<car_model,const car_manufacturer *,&car_model::manufacturer> wouldn't match the type of the manufacturer member.

I'll fix all of this in the docs. Thanks for spotting the mess.