Re: [BUG] valarray_copy, with mask_array, libstdc++-v3

[Luke Kenneth Casson Leighton <lkcl at samba-tng dot org>]

On Thu, Jul 04, 2002 at 01:19:27PM -0400, Ben Elliston wrote:
> >>>>> "Luke" == Luke Kenneth Casson Leighton <lkcl@samba-tng.org> writes:
> 
>   Luke> valarray<bool> b(4);
>   Luke> b[0] = false;
>   Luke> b[1] = true;
>   Luke> b[2] = true;
>   Luke> b[0] = false;
> 
>   Luke> valarray<int> i(4);
>   Luke> valarray<int> j;

i should point out that, in order to stop you getting confused,
this example is best modified to:

   valarray<int> j(4);

however, whether the valarray is initially declared or not
has absolutely nothing to do with the bug.

to make the example even clearer to you, please try to compile
the example like this, instead:

   valarray<int> j(i[b]);

and please, this time, actually _do_ bother to compile example
code that i send you, rather than wasting my time.

again.

l.

>   Luke> i = 9;
> 
>   Luke> j = i[b];
> 
>   Luke> should create an array (j) of two items.
> 
>   Luke> instead, it overwrites memory and corrupts things
>   Luke> because... well... here's a patch :)
> 
> In Section 22.4.2 of The C++ Programming Language (3rd edition):
> 
> ``A valarray can be assigned to another of the same size.  As one
>   would expect, v1 = v2 copies every element of v2 into its
>   corresponding position in v1.  If valarrays have different sizes,
>   the result of assignment is undefined.  Because valarray is designed
>   to be optimised for speed, it would be unwise to assume that
>   assigning with a valarray of the wrong size would cause an easily
>   comprehensible error (such as an exception) or other "reasonable"
>   behaviour.''
> 
> Ben

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