Is valarray<valarray<built-in type> > safe?

Question

After reading this http://gcc.gnu.org/ml/libstdc++/2000-q1/msg00123.html, it seems that using a valarray of valarrays is not guaranteed by the standard to be safe. But when I read the list of requirements (http://en.cppreference.com/w/cpp/numeric/valarray) for T in valarray, it seems that T=valarray<built-in type> satisfy all of them. Is valarray<valarray<built-in type> > safe?

Answer 1

No, it's not safe.

In particular, valarray can throw¹ under some circumstances (e.g., see §26.6.1/6), but if the type you put into a valarray throws (ever), you get undefined behavior (§26.2/2).

It's mostly unnecessary anyway. valarray supports 2D addressing via slices.

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_{1. Note: it's allowed to throw, but never directly required to do so. At the same time, many operations on non-const valarrays are required to return entire an entire new valarray, which involves dynamic allocation. If that fails, there seems to be little choice but to throw an exception.}