Function write_function

cpp/dolfinx/io/VTKFile.cpp:315–703

//----------------------------------------------------------------------------
template <dolfinx::scalar T, std::floating_point U>
void write_function(
    const std::vector<std::reference_wrapper<const fem::Function<T, U>>>& u,
    double time, pugi::xml_document* xml_doc,
    const std::filesystem::path& filename)
{
  if (!xml_doc)
    throw std::runtime_error("VTKFile has been closed");
  if (u.empty())
    return;

  // Extract the first function space with pointwise data. If no
  // pointwise functions, take first FunctionSpace.
  auto V0 = u.front().get().function_space();
  assert(V0);
  for (auto& v : u)
  {
    auto V = v.get().function_space();
    assert(V);
    if (!impl::is_cellwise(*V->element()))
    {
      V0 = V;
      break;
    }
  }

  // Check compatibility for all functions
  auto mesh0 = V0->mesh();
  assert(mesh0);
  auto element0 = V0->element();
  for (auto& v : u)
  {
    auto V = v.get().function_space();
    assert(V);

    // Check that functions share common mesh
    assert(V->mesh());
    if (V->mesh() != mesh0)
    {
      throw std::runtime_error(
          "All Functions written to VTK file must share the same Mesh.");
    }

    // Check that v isn't a sub-function
    if (!V->component().empty())
      throw std::runtime_error("Cannot write sub-Functions to VTK file.");

    auto e = V->element();
    assert(e);

    // Check that element uses point evaluations
    if (!e->interpolation_ident())
    {
      throw std::runtime_error("Only Lagrange functions are supported. "
                               "Interpolate Functions before output.");
    }

    // Check that pointwise elements are the same (up to the block size)
    if (!impl::is_cellwise(*e))