Question

It seems to be a hardcoded template with 10 things, but there is also an implementation for a cons template class very similar to Typelist from Lokki. So is the boost tuple template just a wrapper around a Typelist/cons implementation? Or what is the gist of it?

Answer 1

In C++03 (because otherwise we would be talking about std::tuple) there is no variadic template feature, therefore it is emulated with a sufficient number of parameters (*) with a specific default.

You can get a hint of the actual implementation reading the Design decisions rationale:

The end mark of the cons list (nil, null_type, …)

Tuples are internally represented as cons lists:

tuple<int, int>

inherits from

cons<int, cons<int, null_type> >

null_type is the end mark of the list. Original proposition was nil,
but the name is used in MacOS, and might have caused problems, so
null_type was chosen instead. Other names considered were null_t and
unit (the empty tuple type in SML).

Note that null_type is the internal representation of an empty tuple:
tuple<> inherits from null_type.

(*) Usually in Boost the code is generated with the use of macros so that the upper limit is configurable.