Swig: How to wrap double& (double passed by reference)?

Question

I am using SWIG to access C++ code from Python. How do I elegantly wrap a function that returns values in variables passed by reference like

void set(double&a) {
  a = 42.;
}

I could not find out how to do this. In the best case I'd be able to use the function in Python with Python floats:

>>> b = 2.
>>> set(b)
>>> print b
42.0

At the moment it gives me a TypeError: in method 'TestDouble_set', argument 2 of type 'double &'.

Answer 1

Do it this way:

Your swig interface file:

  %include <typemaps.i>

  %apply double& INOUT { double& a };
  void set(double& a);

Usage in python script:

  a = 0.0
  a = set(a)
  print a

If your function returns something (instead of being a void), do the below in python

  ret, a = set(a)

Checkout the documentation for typemaps in swig. You can do INPUT, OUTPUT & INOUT for arguments. HTH

Note that this solution relies on the SWIG-provided OUTPUT typemaps defined in typemaps.i library, which pre-defines the typemaps being used by the %apply command above.

typemaps.i defines input/output typemaps for C++ primitive types (see the above link to the SWIG documentation for more info); however, you have into include the typemaps.i library in your interface file for SWIG to use them. (Hence why some commenters likely found the original solution wasn't working for them.)

Answer 2

Note that the accepted (correct) solution relies on the SWIG-provided OUTPUT typemaps defined in typemaps.i library, which pre-defines the typemaps being used by the %apply command above.

typemaps.i defines input/output typemaps for C++ primitive types (see the above link to the SWIG documentation for more info); however, you have to include the typemaps.i library in your interface file for SWIG to use them. (Hence why some commenters likely found the original solution wasn't working for them.)

Answer 3

hmm - are you using the latest version of SWIG? The documentation seems to indicate that this works -- from the manual:

C++ references are supported, but SWIG will treat them as pointers. For example, a declaration like this :

class Foo {
public:
    double bar(double &a);
}

will be accessed using a function like this :

double Foo_bar(Foo *obj, double *a) {
    obj->bar(*a);
}

Functions returning a reference will be mapped into functions returning pointers.

I don't know how you map that on the python side...does python have something like perl references?