Is there a C++ Standard Template Library class that provides efficient string concatenation functionality, similar to C#'s StringBuilder or Java's StringBuffer?
The C++ way would be to use std::stringstream or just plain string concatenations. C++ strings are mutable so the performance considerations of concatenation are less of a concern.
with regards to formatting, you can do all the same formatting on a stream, but in a different way, similar to cout. or you can use a strongly typed functor which encapsulates this and provides a String.Format like interface e.g. boost::format