Get pointer to Petsc matrix?

Question

In the C++ interface of fenics; How do I get the pointer to the underlying Petsc object of a matrix so that I can directly use the functionality of Petsc? (I need direct access to the matrix entries, so that I can change some values.)

Answer 1

// Create PETSc-based matrix
PETScMatrix A;

// Access underlying PETSc object
Mat mat = A.mat();

Answer 2

You can use a dynamic pointer cast to reach the base vector/matrix

For example:

std::dynamic_pointer_cast<PETScVector>(x.vector())

I find this useful if I have a Function which has a vector that was created by an internal factory. I can check if PETSc is in use, and then do the dynamic cast to do anything PETSc specific (like apply("")).