Get parts of a problem from a separated function

Question

I would like to define some of the parts of my problem in a separated function. To demonstrate what I want, I modified the Poisson demo and took the boundary conditions out of main() into construct_bc(). This way I get Segmentation Fault. How can I do it right?

#include <dolfin.h>
#include "Poisson.h"

using namespace dolfin;

// Source term (right-hand side)
class Source : public Expression
{
  void eval(Array<double>& values, const Array<double>& x) const
  {
    double dx = x[0] - 0.5;
    double dy = x[1] - 0.5;
    values[0] = 10*exp(-(dx*dx + dy*dy) / 0.02);
  }
};

// Normal derivative (Neumann boundary condition)
class dUdN : public Expression
{
  void eval(Array<double>& values, const Array<double>& x) const
  {
    values[0] = sin(5*x[0]);
  }
};

// Sub domain for Dirichlet boundary condition
class DirichletBoundary : public SubDomain
{
  bool inside(const Array<double>& x, bool on_boundary) const
  {
    return x[0] < DOLFIN_EPS or x[0] > 1.0 - DOLFIN_EPS;
  }
};

DirichletBC construct_bc(Poisson::FunctionSpace const& V) {
  Constant u0(0.0);
  DirichletBoundary boundary;
  return DirichletBC(V, u0, boundary);
}

int main()
{
  // Create mesh and function space
  UnitSquareMesh mesh(32, 32);
  Poisson::FunctionSpace V(mesh);

  // Define boundary condition
  DirichletBC bc = construct_bc(V); 

  // Define variational forms
  Poisson::BilinearForm a(V, V);
  Poisson::LinearForm L(V);
  Source f;
  dUdN g;
  L.f = f;
  L.g = g;

  // Compute solution
  Function u(V);
  solve(a == L, u, *bc);

  return 0;
}

Answer 1

Which version of dolfin are you using? In the latest version, DirichletBC requires shared_ptrs as arguments.

In older versions of dolfin I believe the arguments to the DirichletBC constructor were passed by reference. A quick glance at your code suggests that the variables u0 and boundary are going out of scope once you return from construct_bc since they are allocated on the stack. Therefore the DirichletBC members are references to memory which is no longer valid.