I'm not sure if this is a stupid question, but is there a runtime equivalent of boost.mpl's identity
metafunction?

Something that just returns it's single argument, eg.,

template <typename T> T identity( T t ) { return t; }

I think it's trivially simple to write, but I wonder if it already exists in Boost or
indeed the STL?

(I need this because I need to pass a function object which simply returns an
object I already have - maybe I've missed a more obvious method?)

Thx,

Rob.