How to convert Python to C++?

Question

I'm trying to run a code written in C++ which should be the same as one I have in Python. They both use Fenics.

My problem is that I cannot translate a Python class into a C++ class.

For instance, my Python code is:

V = VectorFunctionSpace(mesh, "Lagrange", 1)
u = Function(V)
En = 5*dx - dot(3, u)*ds(2)

def f(self, x):
    u.vector()[:] = x
    return assemble(En)

The problem here is that the compiler cannot find En as it is not defined inside the class.

My C++ code:

double f(const GenericVector& x)
{
     u.vector() = x;
     return assemble(F);
}

int main()
{
   //...things that apparently work...
   Function u(Vh);
   MyClass::BilinearForm F;

}

How can I solve this?

Answer 1

C++ has globals in the same way Python has globals.

If you want access to something defined outside your class, either prefix it with its class name and make it static ( MyClass::Thingy ), make it a global, give it an accessor function ( MyInstance.GetThingy() ), or pass it in directly as an argument to functions that use it.

Answer 2

As one of many methods, you can simply try defining F as global variable as follows:

...

MyClass::BilinearForm F; // The global scope

double f(const GenericVector& x)
{
     u.vector() = x;
     return assemble(F);
}

int main()
{
   //...things that apparently work...
   Function u(Vh);
   // you can use F here or call the function f
    ...

}

Here F is a global variable and you can use it in any method in this scope. I do not know how your class is defined or whether you are using a custom namespace but you may get rid of the scope resolution (MyClass::) part based on your implementation if BilinearForm is an accessible class here. You can also define F as a static variable so if you could provide more details, I am sure you might get more precise feedback.

Hope that helps!