Re: [Boost-users] [multiarray] problems passing views by reference

Subject: Re: [Boost-users] [multiarray] problems passing views by reference
From: Ronald Garcia (rxg_at_[hidden])
Date: 2010-01-11 16:11:30

Hi Rhys,

The problem might be the body of fill_functor for the general case.   
"*i" will produce a temporary (a subarray), which you are then trying  
to pass immediately by reference to the smaller fill_functor case.  C+ 
+ doesn't let you pass temporaries by reference, which is a real  
problem when you need proxy references like MultiArray does.  The  
solution should be as follows:

         for (typename Array::iterator i = x.begin(); i != x.end(); + 
+i) {
           typename Array::iterator::reference ri = *i;
             f(ri,v);
         }

That is, assign the temporary to a variable before you call f.

Cheers,
Ron

On Jan 5, 2010, at 11:23 PM, Rhys Ulerich wrote:

> Recently, Hanan Sadar asked about a multi_array fill implementation
> [1].  The standard std::fill_n(x.data(),x.num_elements(),v) answer
> only works when x is a boost::multi_array or boost::multi_array_ref
> but does not handle the general MultiArray concept.
>
> I decided to implement an arbitrary dimensional MultiArray fill
> capable of handling such views correctly.  It's been absolutely
> miserable for reasons I don't understand but which resemble comments
> from James Amundson's recent thread [2].  Attached is my attempt with
> two exasperating lines marked by "Cannot use reference, why?".
>
> For a non-multi_array, non-multi_array_ref instance (i.e. a view)
> using a signature like
>    template< class Array, class V > void operator()(Array x, const V  
> &v);
> allows the code to compile and operate correctly.  However, it incurs
> pass-by-value overhead.  Using
> a signature like
>    template< class Array, class V > void operator()(Array &x, const  
> V &v);
> using pass-by-reference dies at compilation time.  All of my attempts
> to use MultiArray's associated typedefs die, including hours of
> mucking with MultiArray::reference.
>
> What am I missing here?  Why can't I pass a general MultiArray view
> type by reference?
>
> - Rhys
>
> [1] http://groups.google.com/group/boost-list/browse_thread/thread/829ebea7fef28db0/57485403b4e809dc
> [2] http://groups.google.com/group/boost-list/browse_thread/thread/4531a7c35a5865fc/6fdec1189d01acde
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