630. arrays of valarray

Section: 29.7.2.2 [valarray.cons] Status: C++11 Submitter: Martin Sebor Opened: 2007-01-28 Last modified: 2016-02-10

Priority: Not Prioritized

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Discussion:

Section 29.3 [numeric.requirements], p1 suggests that a valarray specialization on a type T that satisfies the requirements enumerated in the paragraph is itself a valid type on which valarray may be instantiated (Footnote 269 makes this clear). I.e., valarray<valarray<T> > is valid as long as T is valid. However, since implementations of valarray are permitted to initialize storage allocated by the class by invoking the default ctor of T followed by the copy assignment operator, such implementations of valarray wouldn't work with (perhaps user-defined) specializations of valarray whose assignment operator had undefined behavior when the size of its argument didn't match the size of *this. By "wouldn't work" I mean that it would be impossible to resize such an array of arrays by calling the resize() member function on it if the function used the copy assignment operator after constructing all elements using the default ctor (e.g., by invoking new value_type[N]) to obtain default-initialized storage) as it's permitted to do.

Stated more generally, the problem is that valarray<valarray<T> >::resize(size_t) isn't required or guaranteed to have well-defined semantics for every type T that satisfies all requirements in 29.3 [numeric.requirements].

I believe this problem was introduced by the adoption of the resolution outlined in N0857, Assignment of valarrays, from 1996. The copy assignment operator of the original numerical array classes proposed in N0280, as well as the one proposed in N0308 (both from 1993), had well-defined semantics for arrays of unequal size (the latter explicitly only when *this was empty; assignment of non empty arrays of unequal size was a runtime error).

The justification for the change given in N0857 was the "loss of performance [deemed] only significant for very simple operations on small arrays or for architectures with very few registers."

Since tiny arrays on a limited subset of hardware architectures are likely to be an exceedingly rare case (despite the continued popularity of x86) I propose to revert the resolution and make the behavior of all valarray assignment operators well-defined even for non-conformal arrays (i.e., arrays of unequal size). I have implemented this change and measured no significant degradation in performance in the common case (non-empty arrays of equal size). I have measured a 50% (and in some cases even greater) speedup in the case of assignments to empty arrays versus calling resize() first followed by an invocation of the copy assignment operator.

[ Bellevue: ]

If no proposed wording by June meeting, this issue should be closed NAD.

[ 2009-07 Frankfurt ]

Move resolution 1 to Ready.

Howard: second resolution has been commented out (made invisible). Can be brought back on demand.

Proposed resolution:

Change 29.7.2.3 [valarray.assign], p1 as follows:

valarray<T>& operator=(const valarray<T>& x);

-1- Each element of the *this array is assigned the value of the corresponding element of the argument array. The resulting behavior is undefined if When the length of the argument array is not equal to the length of the *this array. resizes *this to make the two arrays the same length, as if by calling resize(x.size()), before performing the assignment.

And add a new paragraph just below paragraph 1 with the following text:

-2- Postcondition: size() == x.size().

Also add the following paragraph to 29.7.2.3 [valarray.assign], immediately after p4:

-?- When the length, N of the array referred to by the argument is not equal to the length of *this, the operator resizes *this to make the two arrays the same length, as if by calling resize(N), before performing the assignment.

[ pre-Sophia Antipolis, Martin adds the following compromise wording, but prefers the original proposed resolution: ]

[ Kona (2007): Gaby to propose wording for an alternative resolution in which you can assign to a valarray of size 0, but not to any other valarray whose size is unequal to the right hand side of the assignment. ]