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?

Neither u nor En are defined in the Python fragment either. Please give a minimal complete verifiable example. — Roland Smith
– Roland Smith, Commented Aug 18, 2015 at 21:34
It isn't. It's another compiler error. En and u are defined in the main and I cannot pass them to the class because f is a virtual method with a fixed signature. — Nicola Ferro
– Nicola Ferro, Commented Aug 18, 2015 at 21:37
Declare them outside a function (outside main) and use them as ::En and ::u. — cdarke
– cdarke, Commented Aug 18, 2015 at 21:41

Borealid · Accepted Answer · 2015-08-18 21:28:11Z

4

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.

answered Aug 18, 2015 at 21:28

Borealid

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Comments

erol yeniaras · Accepted Answer · 2015-08-19 14:48:50Z

0

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!

edited Aug 19, 2015 at 14:48

answered Aug 19, 2015 at 14:23

erol yeniaras

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