Hi Joerg, Hi Fres! Thanks for the explanations!

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This is the old problem of binding references to temporaries: parameters of `swap()' are *non-const* references, and they cannot be bound to temporaries returned by `row()'.

I only came up with this idea, because a similar (but not identical) syntax is valid in the MTL. I will perhaps just for fun try to dig into their sourcecode to see how they do trick, how they avoid the temporaries and what price they pay for being able to do it. Thanks for the help, ublas is great! Stephan ----------------------------------------------------------- Stephan Puchegger Email: Stephan.Puchegger@ap.univie.ac.at

Hi Kresimir, you wrote:

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question is the following: Is it intentional, that it is not possible to compile:

#include #include

int main () { using namespace boost::numeric::ublas; matrix<double> m (4, 4); for (unsigned int i = 0; i < m.size1 (); ++ i) for (unsigned int j = 0; j < m.size2 (); ++ j) m(i, j) = 4*i+j;

std::cout << m << std::endl;

swap(row(m,0), row(m,2));

std::cout << m << std::endl; }

No. This one compiles with ICC 7.0 and fails with GCC 3.2.1.

It also fails with como. As it should.

Thanks for checking that!

This is the old problem of binding references to temporaries: parameters of `swap()' are *non-const* references, and they cannot be bound to temporaries returned by `row()'.

It seems that `project()' raises its ugly head again :o(

Not sure about that. Maybe swap()'s signatures are inadequate for the view classes. If I change matrix_row<>'s swap() member signatures from void swap (matrix_row &mr); friend void swap (matrix_row &mr1, matrix_row &mr2); to void swap (matrix_row mr); friend void swap (matrix_row mr1, matrix_row mr2); the following test program compiles on GCC 3.2.1 and Intel 7.0: ---------- #include #include int main () { using namespace boost::numeric::ublas; matrix<double> m (4, 4); for (unsigned int i = 0; i < m.size1 (); ++ i) for (unsigned int j = 0; j < m.size2 (); ++ j) m(i, j) = 4*i+j; std::cout << m << std::endl; swap(row(m,0), row(m,2)); row(m,0).swap(row(m,2)); swap(m.row(0), m.row(2)); m.row(0).swap(m.row(2)); matrix_row r1(m, 0), r2(m, 2); swap(r1, r2); r1.swap(r2); std::cout << m << std::endl; } ---------- Interestingly enough, the same doesn't hold for 'const matrix_row &' instead ;-) I've already corrected swap() members for all view classes and intend to upload my current version after some regression testing. Thanks again, Joerg

Hi Stephan, you wrote:

Your changes are very welcome! The new code does require far less local variables and is much more compact on the user side.

Correct me if I am wrong (I did not dig into the internals of ublas), but does not swap() rather change the matrix itself and not the matrix_row representations?

Exactly.

If this is the case, then the whole topic of "not changing temporaries in not-const references" is of course true, but does not reflect the situation, since not the temporaries, but the matrix, on which the temporaries rely, is changed. Then the new signatures do reflect the situation much better.

But nevertheless it's pretty surprising, that one needs to swap these 'by value' ;-) Best, Joerg P.S.: New download is available at groups.yahoo.com/group/ublas-dev/files/ublas_2003_04_08.zip P.P.S.: Basically regression tested.