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](http://stackoverflow.com/help/mcve). — Roland Smith, Aug 18 '15 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, Aug 18 '15 at 21:37
Declare them outside a function (outside main) and use them as `::En` and `::u`. — cdarke, Aug 18 '15 at 21:41
Possible duplicate of [Convert Python program to C/C++ code?](https://stackoverflow.com/q/4650243/608639) — jww, Nov 21 '19 at 13:35

score 4 · Accepted Answer · answered Aug 18 '15 at 21:28

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.

erol yeniaras · Answer 2 · 2015-08-19T14:48:50.580

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!

How to convert Python to C++?

2 Answers2