Derive from C++ base class in SWIGged Python

Question

Note: The corresponding gist is here.

I have an abstract base class and a method that accepts a pointer to the base class, e.g.,

#ifndef MYTEST_HPP
#define MYTEST_HPP

#include <iostream>
#include <memory>

class MyBaseClass {
  public:
    virtual
    double
    eval(const double x) const = 0;
};

class Square: public MyBaseClass {
  public:
    virtual
    double
    eval(const double x) const
    {
      return x*x;
    }
};


void
mytest(const std::shared_ptr<MyBaseClass> & a) {
  std::cout << a->eval(1.0) << std::endl;
  std::cout << a->eval(2.0) << std::endl;
  std::cout << a->eval(3.0) << std::endl;
}

#endif // MYTEST_HPP

After SWIGging this with

%module mytest

%{
#define SWIG_FILE_WITH_INIT
#include "mytest.hpp"
%}

%include <std_shared_ptr.i>
%shared_ptr(MyBaseClass);
%shared_ptr(Square);

%include "mytest.hpp"

I can create Square instances and feed them into mytest from within Python, e.g.,

import mytest
a = mytest.Square()
mytest.mytest(a)

As expected, this will print

1.0
4.0
9.0

I'd now like to derive more classes from MyBaseClass, but from Python. Unfortunately, simply doing

class Cube(mytest.MyBaseClass):
    def __init__(self):
        return

    def eval(self, x):
        return x*x*x

c = Cube()
mytest.mytest(c)

results in the error

Traceback (most recent call last):
  File "../source/test.py", line 14, in <module>
    mytest.mytest(c)
TypeError: in method 'mytest', argument 1 of type 'std::shared_ptr< MyBaseClass > const &'

Any hints?

score 1 · Accepted Answer · edited May 23 '17 at 12:32

Got it (via https://stackoverflow.com/a/9042139/353337):

Add the director feature to MyBaseClass

%module(directors="1") mytest

%{
#define SWIG_FILE_WITH_INIT
#include "mytest.hpp"
%}

%include <std_shared_ptr.i>
%shared_ptr(MyBaseClass);
%shared_ptr(Square);

%feature("director") MyBaseClass;

%include "mytest.hpp"

and properly initialize the class in Python

class Cube(mytest.MyBaseClass):
    def __init__(self):
        mytest.MyBaseClass.__init__(self)
        return

    def eval(self, x):
        return x*x*x

Derive from C++ base class in SWIGged Python

1 Answers1