Boost keywords template

Question

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.

Answer 1

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));