[multiarray] problems passing views by reference
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/829ebea7fef28... [2] http://groups.google.com/group/boost-list/browse_thread/thread/4531a7c35a586...
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/829ebea7fef28... [2] http://groups.google.com/group/boost-list/browse_thread/thread/4531a7c35a586...
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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); }
Ron's suggestion did fix my problem. Thanks Ron. Just in case anyone's ever searching for a general MultiArray fill implementation, I've attached the now working code/test case. This might not be a bad example to add to the MultiArray tutorial, both because it touches on an easy mistake and because people ask about fill with some frequency. - Rhys
Thanks for the contribution Rhys. I think it will make a fine addition to MultiArray. Best, Ron On Jan 13, 2010, at 11:48 AM, Rhys Ulerich wrote:
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); }
Ron's suggestion did fix my problem. Thanks Ron.
Just in case anyone's ever searching for a general MultiArray fill implementation, I've attached the now working code/test case.
This might not be a bad example to add to the MultiArray tutorial, both because it touches on an easy mistake and because people ask about fill with some frequency.
- Rhys
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participants (2)
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Rhys Ulerich -
Ronald Garcia