In the Boost Type Traits library there is compile time functionality for querying information about a C++ type. This information is very useful during template metaprogramming and forms the basis, along with the constructs of the Boost MPL library, and some other compile time libraries, for much of the template metaprogramming in Boost.
One area which is mostly missing in the Type Traits library is the ability to determine what C++ inner elements are part of a user-defined type, where the inner element may be a general nested type, a specific nested type such as a class/struct, enumeration, or a union, a function or data member, static function or static data member, or class template.
There has been some of this functionality in Boost, both in already existing libraries and in libraries on which others have worked but which were never submitted for acceptance into Boost. An example with an existing Boost library is Boost MPL, where there is functionality, in the form of macros and metafunctions, to determine whether an enclosing type has a particular nested type or nested class template. An example with a library which was never submitted to Boost is the Concept Traits Library from which much of the functionality of this library, related to type traits, was taken and expanded.
It may also be possible that some other Boost libraries, highly dependent on advanced template metaprogramming techniques, also have internal functionality to introspect a type's elements at compile time. But to the best of my knowledge this sort of functionality has never been incorporated in a single Boost library. This library is an attempt to do so, and to bring a recognizable set of interfaces to compile-time type introspection to Boost so that other metaprogramming libraries can use them for their own needs.