C++ valarray vs. vector

During the standardization of C++98, valarray was designed to allow some sort of fast mathematical computations. However, around that time Todd Veldhuizen invented expression templates and created blitz++, and similar template-meta techniques were invented, which made valarrays pretty much obsolete before the standard was even released. IIRC, the original proposer(s) of valarray abandoned it halfway into the standardization, which (if true) didn't help it either.

ISTR that the main reason it wasn't removed from the standard is that nobody took the time to evaluate the issue thoroughly and write a proposal to remove it.

Please keep in mind, however, that all this is vaguely remembered hearsay. Take this with a grain of salt and hope someone corrects or confirms this.

I know valarrays have some syntactic sugar

I have to say that I don't think std::valarrays have much in way of syntactic sugar. The syntax is different, but I wouldn't call the difference "sugar." The API is weird. The section on std::valarrays in The C++ Programming Language mentions this unusual API and the fact that, since std::valarrays are expected to be highly optimized, any error messages you get while using them will probably be non-intuitive.

Out of curiosity, about a year ago I pitted std::valarray against std::vector. I no longer have the code or the precise results (although it shouldn't be hard to write your own). Using GCC I did get a little performance benefit when using std::valarray for simple math, but not for my implementations to calculate standard deviation (and, of course, standard deviation isn't that complex, as far as math goes). I suspect that operations on each item in a large std::vector play better with caches than operations on std::valarrays. (NOTE, following advice from musiphil, I've managed to get almost identical performance from vector and valarray).

In the end, I decided to use std::vector while paying close attention to things like memory allocation and temporary object creation.

Both std::vector and std::valarray store the data in a contiguous block. However, they access that data using different patterns, and more importantly, the API for std::valarray encourages different access patterns than the API for std::vector.

For the standard deviation example, at a particular step I needed to find the collection's mean and the dif