I very frequently need to get a pointer to a specific row of a matrix, to pass to a low-level function. For speed reasons, I do not want to copy the row data. I have defined my matrices to be row-major so that copying should not be necessary. I expected to be able to call .row().data() without incurring any copying, but I can see from profiling that indeed a copy is made. Do I need to do my own pointer/size arithmetic? Of not, what is the nice way to do it?

Here are the relevant code fragments.

typedef Matrix< complex<float>, Dynamic, Dynamic, RowMajor > RowMajorComplexMat;

RowMajorComplexMat bigMat;

for( int band =0; band < mnBands; ++ band )
{
const RowMajorComplexMat & thisRow = bigMat.row(band); // takes time, but shouldn't

doSomethingWith( thisRow.data() );
}

bigMat.row(band).data() does not perform any copy. However, the returned type of .row(.) is not a Matrix<> object. It is a Block<....> expression whose precise type is given by RowMajorComplexMat::RowXpr.

ggael wrote:bigMat.row(band).data() does not perform any copy. However, the returned type of .row(.) is not a Matrix<> object. It is a Block<....> expression whose precise type is given by RowMajorComplexMat::RowXpr.

Thank you, that is interesting to learn. So, if I write the following, should the problem go away?

for( int band =0; band < nBands; ++ band )
{
const RowMajorComplexMat::RowXpr & thisRow = bigMat.row(band);

doSomethingWith( thisRow.data() );
}

Do not use a reference:

RowMajorComplexMat::ConstRowXpr thisRow = bigMat.row(band);

Do you ever feel like someone should write a book about Eigen?