Example: Interfacing MTL 4

ViennaCL objects can directly initialized with data of objects from the MTL 4 library. The necessary wrappers are provided with ViennaCL, but must be enabled explicitly via the preprocessor constant VIENNACL_HAVE_MTL4. The following code snippet shows how transfer from MTL 4 to ViennaCL and vice versa is triggered:

Transfer from MTL 4 to ViennaCL and back
// other ViennaCL and MTL 4 includes here
// main code:
mtl::dense2D<double>         mtl4_densematrix(dim, dim);
mtl::compressed2D<double>    mtl4_sparsematrix;
mtl::dense_vector<double>    mtl4_vector(5, 0.0);
// fill MTL 4 objects in a very sophisticated way with numbers here
viennacl::matrix<double>             viennacl_densematrix(dim, dim);
viennacl::compressed_matrix<double>  viennacl_sparsematrix(dim, dim);
viennacl::vector<double>             viennacl_vector(dim);
// copy data from MTL4 objects to ViennaCL objects
viennacl::copy(mtl4_densematrix,  viennacl_densematrix);
viennacl::copy(mtl4_sparsematrix, viennacl_sparsematrix);
viennacl::copy(mtl4_vector,       viennacl_vector);
// do some heavy linear algebra with ViennaCL here 
// (iterative solvers, BLAS 3 routines, etc.)
// copy back to MTL 4:
viennacl::copy(viennacl_densematrix,  mtl4_densematrix);
viennacl::copy(viennacl_sparsematrix, mtl4_sparsematrix);
viennacl::copy(viennacl_vector,       mtl4_vector);

Similar to Eigen, the iterative solvers from ViennaCL can directly be called for MTL 4 objects. However, it should be noted that MTL 4 already provides its own set of iterative solvers.

Iterative Solvers from ViennaCL with MTL 4 objects
//other ViennaCL and Eigen includes here
//main code:
using namespace viennacl::linalg;
mtl::compressed2D<double> mtl4_matrix;
mtl::dense_vector<double> mtl4_rhs(dim, 0.0);
mtl::dense_vector<double> mtl4_result(dim, 0.0);
// fill mtl4_matrix and mtl4_rhs here (e.g. by Finite Element Methods)
// Solve system using Conjugate Gradients from ViennaCL
mtl4_result = solve(mtl4_matrix, mtl4_rhs, cg_tag());
// Solve system using Stabilized BiConjugate Gradients from ViennaCL
mtl4_result = solve(mtl4_matrix, mtl4_rhs, bicgstab_tag());
// Solve system using Generalized Minimum Residual Method from ViennaCL
mtl4_result = solve(mtl4_matrix, mtl4_rhs, gmres_tag());