template function lookup second type based on first type

Question

In the below code the caller needs to supply two types to the function template. This works fine however there is a correlation between the types 'RT' and 'Type'. A simple example; if RT is 'Maritime' then Type is always 'NM'. So I am thinking the 'Type' is a bit redundant and maybe it could be removed? How can I accomplish this?

template<typename RT, typename Type>
RT make_obj(Object obj)
{
  return RT(
    obj.value1(),
    make_another_obj<Type>(obj.value2()));
}

Answer 1

A simple example; if RT is 'Maritime' then Type is always 'NM'

If there is a one to one relation then you can use a trait and remove the redundant argument:

template <typename RT> struct Type_from_RT;
template <> struct Type_from_RT<Maritime> { using type = NM; };
// more specializations for other RTs ...

// for convenience:
template <typename RT> using Type_from_RT_t = Type_from_RT<RT>::type;

template<typename RT>
RT make_obj(Object obj)
{
  return RT(
    obj.value1(),
    make_another_obj<Type_from_RT_t<RT>>(obj.value2()));
}

Alternatively the RT types could supply a type member alias directly.