This page is a snapshot from the LWG issues list, see the Library Active Issues List for more information and the meaning of CD1 status.

### 543. valarray slice default constructor

Section: 26.7.4 [class.slice] Status: CD1 Submitter: Howard Hinnant Opened: 2005-11-03 Last modified: 2016-01-28

Priority: Not Prioritized

View all issues with CD1 status.

Discussion:

If one explicitly constructs a slice or glice with the default constructor, does the standard require this slice to have any usable state? It says "creates a slice which specifies no elements", which could be interpreted two ways:

1. There are no elements to which the slice refers (i.e. undefined).
2. The slice specifies an array with no elements in it (i.e. defined).

Here is a bit of code to illustrate:

>     #include <iostream>
>     #include <valarray>

>     int main()
>     {
>         std::valarray<int> v(10);
>         std::valarray<int> v2 = v[std::slice()];
>         std::cout << "v[slice()].size() = " << v2.size() << '\n';
>     }

Is the behavior undefined? Or should the output be:

>     v[slice()].size() = 0

There is a similar question and wording for gslice at 26.3.6.1p1.

Proposed resolution:

[Martin suggests removing the second sentence in 26.7.4.2 [cons.slice] as well.]

Change 26.7.4.2 [cons.slice]:

> 1 - The default constructor for `slice` creates a `slice` which specifies no elements. The default constructor is equivalent to `slice(0, 0, 0)`. A default constructor is provided only to permit the declaration of arrays of slices. The constructor with arguments for a slice takes a start, length, and stride parameter.

Change 26.7.6.2 [gslice.cons]:

> 1 - The default constructor creates a `gslice` which specifies no elements. The default constructor is equivalent to `gslice(0, valarray<size_t>(), valarray<size_t>())`. The constructor with arguments builds a `gslice` based on a specification of start, lengths, and strides, as explained in the previous section.