Re: valarray_copy implementation question

[Christian BRUEL <christian dot bruel at st dot com>]

Hello,

Thanks a lot for this clarification, I understand that the destination 
must contain the 'n' selected elements assigned ("compacted", not at 
their original slot). Re-reading the 26.3.8.2 chapter with this in mind 
become suddenly perfectly clear :-).

But regarding my original out-of-bound src report, I'm still convinced 
that another test is needed, or a change in the iterated is needed.
Couldn't we just iterate on the mask and simplify the valarray_copy 
loops as something like :

      _Tp* __p (__a._M_data);
      _Tp* __q (__b._M_data);

      for (__ok = __m.M_data; __ok < __ok + __n; ++__ok, ++__p)
	{
	  if (*__ok )
            *__q++ = *__p;
	}

many thanks,

Christian


Gabriel Dos Reis wrote:
> On Mon, Nov 16, 2009 at 9:26 AM, Christian BRUEL <christian.bruel@st.com> wrote:
> > Hello,
> >
> > The standard interpretation that lead to the implementation of
> > __valarray_copy used for the mask_array assignment in the valarray looks
> > strange.
> >
> > From what I see in the comments and implementation (xsee
> > valarray_array.tcc), there is a big possibility to go out of bounds if there
> > is at least a false value in the boolean mask before the first true ones.
> >
> > The comments are indeed explicit:
> >
> >  // Copy n consecutive elements from a into elements of b.  Elements
> >  // of b are skipped if the corresponding element of m is false.  m
> >  // must contain at least n true elements.  b must have at least as
> >  // many elements as the index of the nth true element of m.  I.e. if
> >  // m has 15 elements with 5 false followed by 10 true, b must have
> >  // at least 15 elements.
> >
> > But in the 14882:1998 standard section 26.3.8.2 all I see is that the mask
> > acts as a selector on the source arguments, so their is a constraint that
> > the size of the bool array is smaller or equal to the size of the
> > destination array, regardless of mask values.
> >
> > I think the words "selected elements" means that the elements are selectable
> > and so that me must stop after n elements, not after n true elements. But
> > I'm quite not sure how to read this standard.
> >
> > Thanks for any clarification. if this is a possible bug I'll post to
> > bugzilla.
> >
> > Christian
>
> Christian,
>
>   Before we get deep into implementation details (and worry about
> whatever valgrind might say), I believe we should first concentrate
> on the semantics.
>
> The slice_array, gslice_array, mask_array and friends have no other
> purpose but to help implement array expressions.  In the specific
> case at hand, the idea is that given an `dst_ary' and a `mask', we
> can select slots to assign to with values from a `src_ary', as in
>
>     dst_ary[mask] = src_ary;
>
> In this statement, `dst_ary' and `mask' must have the same length
> (and the valarray component is free to assume they are, without
> having to check).  Then the expression `dst_ary[mask]' conceptually
> constructs an array of some length `n'.  That `n' is the number
> of selected elements (number of 'true' slots in the mask).  The source
> array `src_ary' must be of length `n' -- but the valarray component
> is free to assume that is the case, without having to check.
>
> Does that help?
>
> -- Gaby