Re: [ublas] pass by reference for vector(_proxy)

Subject: Re: [ublas] pass by reference for vector(_proxy)
From: David Bellot (david.bellot_at_[hidden])
Date: 2010-03-26 11:53:34

nice little example. Will be in the tutorial. :-)

On Fri, Mar 26, 2010 at 13:30, Nasos Iliopoulos <nasos_i_at_[hidden]> wrote:

>  Hello Klass,
>
> you should use expressions for that (i.e. vector_expression or
> matrix_expression). Here is a sort example:
>
>
> #include <boost/numeric/ublas/vector.hpp>
>
> #include <boost/numeric/ublas/vector_expression.hpp>
>
> #include <boost/numeric/ublas/vector_proxy.hpp>
>
> #include <boost/numeric/ublas/io.hpp>
>
> using namespace boost::numeric::ublas;
>
> template <class AE>
>
> void test(const vector_expression<AE> &ve) {
>
> std::cout << ve()(10) << std::endl;
>
> }
>
> int main()
>
> {
>
> vector<double> v1(20);
>
> v1(10) = -10; v1(11)=-9;
>
> scalar_vector<double> v2(20,2);
>
>  test(v1);
>
> test(project(v1, range(1,20)));
>
> test(v1+v2);
>
>  return 0;
>
> }
>
>
> Take note of the following:
> - It is very important that in the function you have to reach the
> underlying object by calling operator() (due to the Curriously Reccuring
> Pattern ublas is using). After v() the rest are he same as when normally
> calling a member function of the class.
>
> - The last call (test(v1+v2)) also does not copy the objects. The power of
> expression templates allows for just passing references to them and only the
> required operations will be executed  (i.e. when in this function v()(10) is
> called, only the operation v1(10)+v2(10) will be executed). This will hold
> no matter how complicated the expression will be (as long as the elementary
> operations are supported by the expression template mechanism).
>
> Take a look at this for other tips and techniques with ublas:
>
> http://www.crystalclearsoftware.com/cgi-bin/boost_wiki/wiki.pl?Effective_UBLAS
> http://www.guwi17.de/ublas/index.html
>
> Best
> Nasos
>
>
> > Date: Thu, 25 Mar 2010 17:17:36 +0100
> > From: K.Vantournhout_at_[hidden]
> > To: ublas_at_[hidden]
> > Subject: [ublas] pass by reference for vector(_proxy)
>
> >
> > Hi all,
> >
> > I recently started to use uBlas and I could not figure the following out.
> >
> > If you want to pass a vector or vector_proxy as a reference to a
> function,
> > how do you do this so that the data is not coppied?
> >
> > The function
> >
> > void foo(const vector<double> &v) {
> > }
> >
> > makes a pure reference to any pure vector<double> passed to the function,
>
> > however when passing a vector_proxy to it, it starts to copy the data.
> >
> > Is there a way to avoid this?
> >
> > Regards
> >
> > --
> > ZAPHOD Hey, this rock...
> > FORD Marble...
> > ZAPHOD Marble...
> > FORD Ice-covered marble...
> > ZAPHOD Right... it's as slippery as... as... What's the slipperiest
> > thing you can think of?
> > FORD At the moment? This marble.
> > ZAPHOD Right. This marble is as slippery as this marble.
> >
> > - Zaphod and Ford trying to get a grip on things in Brontitall, Fit the
> Tenth.
> > _______________________________________________
> > ublas mailing list
> > ublas_at_[hidden]
> > http://lists.boost.org/mailman/listinfo.cgi/ublas
> > Sent to: nasos_i_at_[hidden]
>
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-- 
David Bellot, PhD
david.bellot_at_[hidden]
http://david.bellot.free.fr