Blitz++ arrays offer functionality and efficiency competitive with Fortran 90, but without any language extensions. The Blitz++ library is able to parse and analyze array expressions at compile time, and performs loop transformations which have until now been the responsibility of optimizing compilers.
This library is not currently under development. BoostuBlas offers an alternative and there is also a discussion there of the rationale for a choice.
See ExpressionTemplate, MatrixTemplateLibrary, TemplatesForNumerics, StdValarray.