How to apply SWIG OUTPUT typemaps for class types in Python?

Question

I am having some trouble generating a Python wrapper around a C++ library using SWIG (version 3.0.6).

My issue relates to applying the OUTPUT typemap, specifically i

Answer 1

I think you need to use pointers. I am also not sure what happens, when mixing out typemaps and return statements. A minimal example file tst.i:

%module tst

%{

  // declaration:
  void add(long *resultLong, const long arg1,const long arg2);
  long mul(const long a, const long b);

  // the code:
  void add(long *resultLong, const long arg1,const long arg2) {
    *resultLong = arg1 + arg2;
  }
  long mul(const long a, const long b) {
    return a*b;
  }

%}

// The wrapper:
%apply (long* OUTPUT) { long* resultLong }; 
void add(long* resultLong, const long arg1,const long arg2);
long mul(const long a, const long b);

After translating (I always use CMake), the usage in python would be:

import tst
x = tst.add(3, 4)  # results in 7L    
y = tst.mul(3, 4)  # results in 12L

I think it is better using return statements instead of typemaps for scalar datatypes. When interfacing arrays, I recommend using the predefined typemaps of numpy.i.