In functional programming languages, combinator functions take functions as arguments, and produce new functions with related capabilities. For example in the following pseudo code:

f2 = twice(sin)

twice returns a function that applies the argument function twice. So for all x

f2(x) == sin(sin(x))

In C++ a combinator typically takes some callable type(s) as arguments, and returns a function object providing some new functionality.

For the purposes of the Traversal Library we define hierarchical data structures as types built out of other types. For example a std::pair<int,char> is a hierarchical data structure built from the types int and char. A more complex structure could then be built from this type, for example std::vector<std::pair<int, char> >, and so on and so on. Arbitrarily complex types can be built up in this way, forming into a natural hierarchy.

C++, the STL and Boost provide a rich collection of tools for building up such types, such as structs, pairs, Boost.Variant, Boost.Optional, pointer and smart pointer types, etc. etc.