How to handle Method returning nullptr for shared_ptr in SWIG (C++ to Java)

Question

I need to port an existing C++ library to Java. In the C++ code there is a factory method:

std::shared_ptr<Role> createRole(std::string name)

which will return a nullptr if it was not possible to create the Role.

Unfortunately if I try to port this code with SWIG it will fail silently at this point. I guess that is because Java doesn't know about nullptr and SWIG is most likely ignoring it. Is my assumption right? Because I'm originally a Java developer and don't know much about SWIG and C++...

Sadly I can't change the existing C++ code. Is there any workaround for this? Or will I have to write a Wrapper for this special case? This problem occures in 4 or 5 places in the project.

How is `std::shared_ptr` translated? I guess using Java's `null` as equivalent to C++'s `nullptr` could be a possible solution. — Simon Kraemer, Feb 16 '16 at 15:15
I don't know it exactly. SWIG seems to map them to Role (the Java Role class SWIG generated). The shared_ptr is mentioned in the SWIG interface file with %shared_ptr(Role) — little_planet, Feb 16 '16 at 15:18
Would I have to modify the SWIG generated file and add the `null` in case the c++ library method returns a `nullptr`? Which files should I modify, I don't want to blow up the SWIG code. — little_planet, Feb 16 '16 at 15:28
http://www.swig.org/Doc3.0/CPlusPlus11.html#CPlusPlus11_null_pointer_constant — Simon Kraemer, Feb 16 '16 at 15:53
@Alexander Solovets Sry, "hang" was wrong. It fails silently without any exception. — little_planet, Feb 16 '16 at 20:09
@Simon Kraemer Yes I saw the documentation. But does that mean the application shouldn't crash at this point? — little_planet, Feb 16 '16 at 20:09

score 1 · Answer 1 · answered Feb 16 '16 at 21:33

The problem is highly likely in your code. I just checked out the following code with gcc-4.8 and swig-2.0, and it worked perfectly:

test.h

#include <string>
#include <memory>

class Test {
  public:
  std::shared_ptr<std::string> test(bool isNull) {
    return isNull ? nullptr : std::make_shared<std::string>("test");
  }
};

test.i

%module "test"

%include "std_shared_ptr.i"
%include "std_string.i"

%shared_ptr( std::string )

%{
#include "test.h"
%}

%include "test.h"

check.py

import test
t = test.Test()
print t.test(True)
print t.test(False)

The output of python check.py is:

None
<Swig Object of type 'std::shared_ptr< std::string > *' at 0x7f03436501e0>swig/python detected a memory leak of type 'std::shared_ptr< std::string > *', no destructor found.

Guess I will have to check the library and my code again. Many thanks for clarifying this issue from your side. — little_planet, Feb 17 '16 at 06:54

How to handle Method returning nullptr for shared_ptr in SWIG (C++ to Java)

1 Answers1