Valarray is something provided in the CeePlusPlus Standard Library
The C++ standard library provides a class valarray for the processing of arrays of numeric values.
quoted from TheCppStandardLibrary p.547 where there is also a comment that it may be better to use BlitzPlusPlus.
These are one dimensional templated arrays which can be manipulated as a whole or accessed element by element or sliced.
CategoryCpp CategoryCppTemplates