My question is about this code:

auto& lookup = container.template get<1>();
container.relocate(container.begin(),container.template project<0>(it));

1st: .template

From what I've found, this template keyword specified on my container is using a template, correct me if I'm wrong.

src: (Where and why do I have to put the "template" and "typename" keywords?)

2nd: project<0>(it)

Looking for the definition in the lib, I saw that it needed an iterator as a parameter, but I don't understand the project<0> (same for get<1>).

I found some information like this: https://theboostcpplibraries.com/boost.variant, and posts on Stack Overflow too, but I'm a little bit confused.

CodePudding user response：

As explained here, in both cases you need the template keywords because container::project and container::get are (presumably-- we do not have the code here) template member functions. To instantiate them you need to specify the template parameter. But, as detailed in the linked answer, the compiler would not understand a code like

container.project<0>(it)

and needs the template keyword as in your code.

CodePudding user response：

You are right about the reason why template can be required. Note it is only required in dependent context (where the actual specialization of a template may depend on template parameters).

The documentation is here:

• Indices are accessed via get<N>()

  where N ranges between 0 and the number of comparison predicates minus one. The functionality of index #0 can be accessed directly from a multi_index_container object without using get<0>(): for instance, es.begin() is equivalent to es.get<0>().begin().

  Note that get returns a reference to the index, and not an index object. Indices cannot be constructed as separate objects from the container they belong to, so the following

  // Wrong: we forgot the & after employee_set::nth_index<1>::type
  const employee_set::nth_index<1>::type name_index=es.get<1>();
  

  does not compile, since it is trying to construct the index object name_index. This is a common source of errors in user code.

• Projection Of Iterators

  Given indices i1 and i2 on the same multi_index_container, project can be used to retrieve an i2-iterator from an i1-iterator, both of them pointing to the same element of the container. This functionality allows the programmer to move between different indices of the same multi_index_container when performing elaborate operations:

  typedef employee_set::index<name>::type employee_set_by_name; employee_set_by_name& name_index=es.get<name>();
  
  // list employees by ID starting from Robert Brown's ID
  
  employee_set_by_name::iterator it1=name_index.find("Robert Brown");
  
  // obtain an iterator of index #0 from it1 employee_set::iterator it2=es.project<0>(it1); 
  
  std::copy(it2,es.end(),std::ostream_iterator<employee>(std::cout));