SWIG: objects of a custom class as output argument (with Python)

Question

(This is a question I asked yesterday, but I simplified it)

I've created a class, of which I want two objects as output arguments of a function (called Test below). But when I run the swig command swig -c++ -python swigtest.i I'm getting the error "Warning 453: Can't apply (MyClass &OUTPUT). No typemaps are defined." I tried adding typemaps, but that doesn't help. I also tried using pointers, pointers to pointers and references to pointers, that doesn't help either.

I feel like I've overlooked something simple, because this should be quite a common thing to do. Or do I need to write a complex typemap, like I've seen around but don't understand (yet)?

Below is my code:

MyClass.h (simplified to make it understandable, so switching to just int doesn't help):

class MyClass
{
    int x;
public:
    int get() const
    {
        return x;
    }
};

void Test(MyClass &obj1, MyClass &obj2);

swigtest.i:

%module swigtest

%include typemaps.i

%{
#define SWIG_FILE_WITH_INIT
%}

%{
#include "MyClass.h"
%}

%include "MyClass.h"

%apply (MyClass& OUTPUT)  { MyClass &obj1 }
%apply (MyClass& OUTPUT)  { MyClass &obj2 }

You should put your answer in a separate answer, not in your question. Also just a note that you can use "%ignore Test" so that you name the python insertion "Test". — Oliver
– Oliver, Commented Feb 20, 2014 at 12:49
I'm sorry, I'm new to posting on Stack Overflow. I just did what you suggested. — Mark VP
– Mark VP, Commented Feb 20, 2014 at 13:53

Jeremy Cowles · Accepted Answer · 2016-10-02 21:57:20Z

2

As noted in my previous comment, the %apply OUTPUT trick only works for a limited set of POD types.

For future Swiggers, this solution worked for me (in C# bindings):

%typemap(cstype) CustomType* "out CustomType"
%typemap(csin,
   pre="    $csclassname temp$csinput = new $csclassname();",
  post="    $csinput = temp$csinput;"
) CustomType* "$csclassname.getCPtr(temp$csinput)"

This generates a public interface with an "out" param for CustomType passed by pointer. The internal P/Invoke interface (csim) is left as raw pointers. The "csin" typemap creates a temp variable and assigns to the output parameter.

Also worth noting that in C#, if MyCustomType is already a reference type, you may not need this, however it's strange to have an API that modifies the parameter value without declaring it as "out" (this actually works for my type, but I prefer the explicit out param).

edited Oct 2, 2016 at 21:57

answered Oct 2, 2016 at 19:41

Jeremy Cowles

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Comments

Oliver · Accepted Answer · 2014-02-19 22:12:43Z

0

Try:

%module swigtest

%{
#define SWIG_FILE_WITH_INIT
#include "MyClass.h"
%}

%include "typemaps.i"
%apply MyClass *OUTPUT { MyClass &obj1, MyClass &obj2 };

%include "MyClass.h"

You could also create a wrapper that returns a std::list:

%include "std_list.i"
%ignore Test;
%rename(Test) TestWrap;
%inline %{
     std::list<MyClass> TestWrap() {
           MyClass obj1, obj2;
           Test(obj1, obj2);
           std::list<MyClass> tempList;
           tempList.push_back(obj1);
           tempList.push_back(obj2);
           return tempList;
     }
%}

edited Feb 19, 2014 at 22:12

answered Feb 18, 2014 at 20:41

Oliver

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6 Comments

Mark VP Over a year ago

The whole point of %apply is so that I can do obj1, obj2 = Test() instead of obj1 = MyClass() obj2 = MyClass() Test(obj1, obj2) Your solution only allows me to do the latter.

Oliver Over a year ago

I'm looking at your question and I don't see where you state this really important piece of info!! Could you update your question with this? I've edited my answer, perhaps it'll be relevant this time :)

Mark VP Over a year ago

I'm sorry, I thought that using %apply MyClass &OUTPUT would have made it clear that I wanted to do that, I should have been clearer. I tried your solution, but unfortunately I still get the same error.

Oliver Over a year ago

Just noticed the argument names were mc instead of obj, I can't remember if that makes a difference, also I believe you have to use pointers for the return. I've just edited, can you try again.

Mark VP Over a year ago

This works, thanks. But finally I used a different method, see my edited question.

|

Mark VP · Accepted Answer · 2014-02-20 13:52:22Z

0

I settled for adding extra Python code in swigtest.i that wraps the test function, so that I can write obj1, obj2 = Test2(). I still think there must be an easier solution,

// swigtest.i:

%module swigtest

%{
#define SWIG_FILE_WITH_INIT
#include "MyClass.h"
%}

%include "MyClass.h"

%insert("python") %{
    def Test2():
        obj1 = swigtest.MyClass()
        obj2 = swigtest.MyClass()
        swigtest.Test(obj1, obj2)
        return obj1, obj2
%}

answered Feb 20, 2014 at 13:52

Mark VP

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