Packing and unpacking of FullMatrix into std::vector

[rezarastak]

I wanted to pack a FullMatrix to std::vector and unpack it afterward in order to use ContinuousQuadratureDataTransfer. I tried to use STL iterators and STL algorithms to perform these operations, but the FullMatrix<Number>::const_iterator class does not work similar to STL iterators. The code

FullMatrix<double> A(3,3);
std::vector<double> v(9);
std::copy(A.begin(), A.end(), v.begin());

produces the error (on GCC7.4)

In file included from /usr/include/c++/7/bits/stl_tree.h:63:0,
                 from /usr/include/c++/7/map:60,
                 from /usr/lib/x86_64-linux-gnu/openmpi/include/openmpi/ompi/mpi/cxx/mpicxx.h:40,
                 from /usr/lib/x86_64-linux-gnu/openmpi/include/mpi.h:2704,
                 from /mnt/scratch/repos/dealii/build/include/deal.II/base/config.h:439,
                 from /mnt/scratch/repos/dealii/include/deal.II/lac/full_matrix.h:20,
                 from /mnt/scratch/repos/dealsims/dealii-fullmatrix-iterator/fullmatrix-iterator.cc:1:
/usr/include/c++/7/bits/stl_algobase.h: In instantiation of ‘static _OI std::__copy_move<false, false, std::random_access_iterator_tag>::__copy_m(_II, _II, _OI) [with _II = dealii::MatrixTableIterators::Iterator<dealii::Table<2, double>, false, (dealii::MatrixTableIterators::Storage)0>; _OI = double*]’:
/usr/include/c++/7/bits/stl_algobase.h:386:44:   required from ‘_OI std::__copy_move_a(_II, _II, _OI) [with bool _IsMove = false; _II = dealii::MatrixTableIterators::Iterator<dealii::Table<2, double>, false, (dealii::MatrixTableIterators::Storage)0>; _OI = double*]’
/usr/include/c++/7/bits/stl_algobase.h:422:45:   required from ‘_OI std::__copy_move_a2(_II, _II, _OI) [with bool _IsMove = false; _II = dealii::MatrixTableIterators::Iterator<dealii::Table<2, double>, false, (dealii::MatrixTableIterators::Storage)0>; _OI = __gnu_cxx::__normal_iterator<double*, std::vector<double, std::allocator<double> > >]’
/usr/include/c++/7/bits/stl_algobase.h:455:8:   required from ‘_OI std::copy(_II, _II, _OI) [with _II = dealii::MatrixTableIterators::Iterator<dealii::Table<2, double>, false, (dealii::MatrixTableIterators::Storage)0>; _OI = __gnu_cxx::__normal_iterator<double*, std::vector<double, std::allocator<double> > >]’
/mnt/scratch/repos/dealsims/dealii-fullmatrix-iterator/fullmatrix-iterator.cc:10:42:   required from here
/usr/include/c++/7/bits/stl_algobase.h:324:18: error: cannot convert ‘const value_type {aka const dealii::MatrixTableIterators::Accessor<dealii::Table<2, double>, false, (dealii::MatrixTableIterators::Storage)0>}’ to ‘double’ in assignment
        *__result = *__first;
        ~~~~~~~~~~^~~~~~~~~~

and the code

FullMatrix<double> A(3,3);
std::vector<double> v(9);
A.fill(v.cbegin());

does not compile either, with the error

/mnt/scratch/repos/dealsims/dealii-fullmatrix-iterator/fullmatrix-iterator.cc: In function ‘void vector_to_matrix()’:
/mnt/scratch/repos/dealsims/dealii-fullmatrix-iterator/fullmatrix-iterator.cc:16:20: error: no matching function for call to ‘dealii::FullMatrix<double>::fill(std::vector<double, std::allocator<double> >::const_iterator)’
   A.fill(v.cbegin());
                    ^
In file included from /mnt/scratch/repos/dealsims/dealii-fullmatrix-iterator/fullmatrix-iterator.cc:1:0:
/mnt/scratch/repos/dealii/include/deal.II/lac/full_matrix.h:304:3: note: candidate: template<class number2> void dealii::FullMatrix<number>::fill(const dealii::FullMatrix<number2>&, dealii::FullMatrix<number>::size_type, dealii::FullMatrix<number>::size_type, dealii::FullMatrix<number>::size_type, dealii::FullMatrix<number>::size_type) [with number2 = number2; number = double]
   fill(const FullMatrix<number2> &src,
   ^~~~
/mnt/scratch/repos/dealii/include/deal.II/lac/full_matrix.h:304:3: note:   template argument deduction/substitution failed:
/mnt/scratch/repos/dealsims/dealii-fullmatrix-iterator/fullmatrix-iterator.cc:16:20: note:   ‘std::vector<double, std::allocator<double> >::const_iterator {aka __gnu_cxx::__normal_iterator<const double*, std::vector<double, std::allocator<double> > >}’ is not derived from ‘const dealii::FullMatrix<number>’
   A.fill(v.cbegin());
                    ^
In file included from /mnt/scratch/repos/dealsims/dealii-fullmatrix-iterator/fullmatrix-iterator.cc:1:0:
/mnt/scratch/repos/dealii/include/deal.II/lac/full_matrix.h:316:3: note: candidate: template<class number2> void dealii::FullMatrix<number>::fill(const number2*) [with number2 = number2; number = double]
   fill(const number2 *);
   ^~~~
/mnt/scratch/repos/dealii/include/deal.II/lac/full_matrix.h:316:3: note:   template argument deduction/substitution failed:
/mnt/scratch/repos/dealsims/dealii-fullmatrix-iterator/fullmatrix-iterator.cc:16:20: note:   mismatched types ‘const number2*’ and ‘__gnu_cxx::__normal_iterator<const double*, std::vector<double, std::allocator<double> > >’
   A.fill(v.cbegin());
                    ^

I can write a patch to fix these. We probabely need to add a conversion operator to FullMatrix<Number>::Accessor and change the interface of the fill() method to accept any iterator.

[drwells]

Yup: I think adding conversion operators to Accessor<TableType, true, storage_order> and Accessor<TableType, false, storage_order> will fix it.

[masterleinad]

As a workaround

FullMatrix<double> A(3,3);
std::vector<double> v(A.m()*A.n());

const double* const start_it = &(A.begin()->value());
const double* const end_it = start_it+v.size();
std::copy(start_it, end_it, v.begin());

A.fill(v.data());

would work (as long as A is not empty).

[masterleinad]

Alternatively, you could also use an intermediate ArrayView object:

FullMatrix<double> A(3,3);
std::vector<double> v(A.m()*A.n());

const auto view = make_array_view(A);
std::copy(view.begin(), view.end(), v.end());

A.fill(v.data());

[drwells]

If you want to do this, I have another recommendation: put the const conversion operator in AccessorBase (like const value_type& value()) instead of what I suggested previously to avoid a little bit of duplicated code.

[rezarastak]

@masterleinad thank you for showing the workarounds. I can incorporate them into my code quickly.

[rezarastak]

I am satisfied with #8278 and I am not planning to submit any code for changing the interface of the fill() member function. I think we can close this issue.