Passing reference from Python to c++ function wrapped with swig for return value

Question

Disclaimer, I am a swig and python noob

I have my own c++ library and I am wrapping it to use in python with swig.

My c++ class is like this:

public MyCppClass()
{
public:
   void MyFunction(char* outCharPtr, string& outStr, int& outInt, long& outLong)
   {
         outCharPtr = new char[2];
         outCharPtr[0] = "o";
         outCharPtr[1] = "k";

         outStr = "This is a result";

         outInt = 1;

         outLong = (long)12345;
    }

}

Now I wrap this class using swig and say the module is called MyClass.

What I want to achieve in python is the following code (OR SAY PSEUDO CODE because if it was the code it would be working) and output:

import module MyClass
from MyClass import MyCppClass

obj = MyCppClass();

outCharPtr = "";
outStr = "";
outInt = 0;
outLong = 0;

obj.MyFunction(outCharPtr, outStr, outInt, outLong);

print(outCharPtr);
print(outStr);
print(outInt);
print(outLong);

The output I want is:

>>>Ok
>>>This is a result
>>>1
>>>12345

I am using python 3.4

I am really apologetic if this is something basic but I have already spent around 8 hours on the resolution and can't come up with anything.

Any help will be very appreciated.

Thanks.

One problem is that Python doesn't have out parameters. You've created immutable str and int Python objects, so that desired syntax won't work. The objects can't change. What SWIG can do for you is convert C/C++ output parameters to a tuple of return values. For example, outCharPtr,outStr,outInt,outLong = obj.MyFunction(). — Mark Tolonen
– Mark Tolonen, Commented May 19, 2015 at 18:22

Community · Accepted Answer · 2020-06-20 09:12:55Z

3

Here's one way to do it. I made some modifications to your non-working example:

example.i

%module example

%include <std_string.i>
%include <cstring.i>

%cstring_bounded_output(char* outCharPtr, 256)
%apply std::string& OUTPUT {std::string& outStr};
%apply int& OUTPUT {int& outInt};
%apply long& OUTPUT {long& outLong};

%inline %{

class MyCppClass
{
public:
   void MyFunction(char* outCharPtr, std::string& outStr, int& outInt, long& outLong)
   {
         outCharPtr[0] = 'o';
         outCharPtr[1] = 'k';
         outCharPtr[2] = '\0';

         outStr = "This is a result";

         outInt = 1;

         outLong = (long)12345;
    }

};

%}

Example use:

>>> import example
>>> c=example.MyCppClass()
>>> c.MyFunction()
['ok', 'This is a result', 1, 12345]

edited Jun 20, 2020 at 9:12

CommunityBot

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answered May 19, 2015 at 18:45

Mark Tolonen

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2 Comments

Taha Khan Over a year ago

Thanks a lot for your answer, I actually got this working this way. But is there any other way to do it? Although its working hundred percent correctly, it looks a bit ugly.

Mark Tolonen Over a year ago

That's the easy way. It's utilizing pre-built helpers to configure the output parameters. Those will apply to any functions with parameters named the same way, or you can be even more generic and say %apply long& OUTPUT {long&}; to make all long& parameters output parameters.

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