I'm attempting to learn SWIG (http://www.swig.org/) in order to extend/expose a C++ library that I have to Python & Java languages.
Before starting this endeavor, I started with a simple SWIG example (http://www.swig.org/tutorial.html) & got that working.
See below (you can see swig woks on my system & that I can load "example" module in Python shell):
linux{me}% swig -python example.i
linux{me}% gcc -c -fPIC example.c example_wrap.c / -I/usr/include/python2.7
gcc: warning: /: linker input file unused because linking not done
linux{me}% gcc -c -fPIC example.c example_wrap.c -I/usr/include/python2.7
linux{me}% ld -shared example.o example_wrap.o -o _example.so
linux{me}% python
Python 2.7.5 (default, Aug 2 2016, 04:20:16)
[GCC 4.8.5 20150623 (Red Hat 4.8.5-4)] on linux2
Type "help", "copyright", "credits" or "license" for more information.
import example
example.fact(5)
120
example.my_mod(5)
Traceback (most recent call last):
File "", line 1, in
TypeError: my_mod() takes exactly 2 arguments (1 given)
example.my_mod(7,3)
1
example.get_time()
'Fri Feb 28 14:54:08 2020\n'
All is good, now the next step is to expose a C++ class, so I followed this tutorial initially (https://realmike.org/blog/2010/07/18/python-extensions-in-cpp-using-swig/). When compared to official SWIG docs this tutorial had a few steps missing, like load the .so shared libray so that Python can call "import" on the library, we will get to that later...
I made a few changes to the SomeClass.h to add constants "#define TypeBool 0" etc...
Here is the relevant dummy C++ header:
#ifndef _SOME_CLASS_H_
#define _SOME_CLASS_H_
class SomeClass {
public:
SomeClass();
SomeClass(int a, int b);
virtual ~SomeClass();
void MethodA(int a = -1);
void MethodB(int b = -1);
void setType(int type);
int GetValA();
int GetValB();
int GetType();
private:
int mValA;
int mValB;
int type;
};
#endif // _SOME_CLASS_H_
Here is the relevant Class file that implements the SomeClass.h:
#include "SomeClass.h"
//refactor to use enum
#define TypeBool 0
#define TypeChar 1
#define TypeByte 2
#define TypeInt 3
#define TypeShort 4
#define TypeFloat 5
#define TypeDouble 6
#define TypeString 7
#define TypeComposite 8
SomeClass::SomeClass() {
mValA = -1;
mValB = -1;
}
SomeClass::~SomeClass() {
}
void SomeClass::MethodA(int a)
{
mValA = mValA * -1;
}
void setType(int type)
{
switch (type)
{
case 0:
type = TypeBool;
break;
case 1:
type = TypeChar;
break;
case 2:
type = TypeByte;
break;
case 3:
type = TypeInt;
break;
case 4:
type = TypeShort;
break;
case 5:
type = TypeFloat;
break;
case 6:
type = TypeDouble;
break;
case 7:
type = TypeString;
break;
case 8:
type = TypeComposite;
break;
}
}
void SomeClass::MethodB(int b)
{
mValB = b;
}
int SomeClass::GetValA()
{
return mValA;
}
int SomeClass::GetValB()
{
return mValB;
}
int SomeClass::GetType()
{
return type;
}
Attempt #1 Here is the SWIG required "interface" file, based on (https://realmike.org/blog/2010/07/18/python-extensions-in-cpp-using-swig/) or http://www.swig.org/Doc4.0/SWIGDocumentation.pdf pages 25-32.
%module mymodule
%{
#include "SomeClass.h"
%}
%include "SomeClass.h"
1) Then I issue the command:
swig -c++ -python -I/home/me/NetBeansProjects/example mymodule.i
This commnand runs and seems to create python & cpp autogenerated wrappers:
-rw-r--r-- 1 me linuxlusers 350 Mar 3 15:53 mymodule.i -rw-r--r-- 1 me linuxlusers 2864 Mar 3 15:55 mymodule.py -rw-r--r-- 1 me linuxlusers 121327 Mar 3 15:55 mymodule_wrap.cxx
2) Then I compile:
g++ -c -fPIC SomeClass.cpp mymodule_wrap.cxx -I/usr/include/python2.7
Which creates object files:
-rw-r--r-- 1 me linuxlusers 60136 Mar 3 15:56 mymodule_wrap.o -rw-r--r-- 1 me linuxlusers 4384 Mar 3 15:56 SomeClass.o
3)Then, I bundle my object files into shared library file ".so" & make that shared library available system-wide so I can load it from Python:
linux{me}% ld -shared mymodule_wrap.o -o _mymodule.so
4) Finally, I call python shell from my command line & attempt to import mymodule:
linux{me}% python
Python 2.7.5 (default, Aug 2 2016, 04:20:16)
[GCC 4.8.5 20150623 (Red Hat 4.8.5-4)] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import mymodule
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "mymodule.py", line 15, in <module>
import _mymodule
ImportError: ./_mymodule.so: undefined symbol: __gxx_personality_v0
>>>
You can see the error: ImportError: ./_mymodule.so: undefined symbol: __gxx_personality_v0.
Attempt $2 (I rewrote the interface file with .cpp implememntation in the %inline tag):
%module mymodule
%{
#include "SomeClass.h"
%}
%include "SomeClass.h"
%constant TypeBool 0
%constant TypeChar 1
%constant TypeByte 2
%constant TypeInt 3
%constant TypeShort 4
%constant TypeFloat 5
%constant TypeDouble 6
%constant TypeString 7
%constant TypeComposite 8
%inline %{
SomeClass::SomeClass() {
mValA = -1;
mValB = -1;
}
SomeClass::~SomeClass() {
}
void SomeClass::MethodA(int a)
{
mValA = mValA * -1;
}
void setType(int type)
{
switch (type)
{
case 0:
type = TypeBool;
break;
case 1:
type = TypeChar;
break;
case 2:
type = TypeByte;
break;
case 3:
type = TypeInt;
break;
case 4:
type = TypeShort;
break;
case 5:
type = TypeFloat;
break;
case 6:
type = TypeDouble;
break;
case 7:
type = TypeString;
break;
case 8:
type = TypeComposite;
break;
}
}
void SomeClass::MethodB(int b)
{
mValB = b;
}
int SomeClass::GetValA()
{
return mValA;
}
int SomeClass::GetValB()
{
return mValB;
}
int SomeClass::GetType()
{
return type;
}
%}
I ran the swig commands & compilation again, however I got the same error:
linux{me}% swig -c++ -python -I/home/me/NetBeansProjects/example mymodule.i
linux{me}% g++ -c -fPIC SomeClass.cpp mymodule_wrap.cxx -I/usr/include/python2.7
linux{me}% ld -shared mymodule_wrap.o -o _mymodule.so
linux{me}% python
Python 2.7.5 (default, Aug 2 2016, 04:20:16)
[GCC 4.8.5 20150623 (Red Hat 4.8.5-4)] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import mymodule
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "mymodule.py", line 15, in <module>
import _mymodule
ImportError: ./_mymodule.so: undefined symbol: __gxx_personality_v0
>>>
KeyboardInterrupt
>>>
Attempt 3 (I rewrote the interface file with header .h in the %inline tag):
%module mymodule
%{
#include "SomeClass.h"
%}
%include "SomeClass.h"
%constant TypeBool 0
%constant TypeChar 1
%constant TypeByte 2
%constant TypeInt 3
%constant TypeShort 4
%constant TypeFloat 5
%constant TypeDouble 6
%constant TypeString 7
%constant TypeComposite 8
%inline %{
class SomeClass {
public:
SomeClass();
SomeClass(int a, int b);
virtual ~SomeClass();
void MethodA(int a = -1);
void MethodB(int b = -1);
void setType(int type);
int GetValA();
int GetValB();
int GetType();
private:
int mValA;
int mValB;
int type;
};
%}
Again, I got the same error (ImportError: ./_mymodule.so: undefined symbol: __gxx_personality_v0.) as the other 2 attempts above.
I attempted compiler flags as recommended in "undefined symbol: __cxa_pure_virtual" error when loading library from java
However, I got the same error.
1) How can I resolve this error?
Attempt #4 Somewhat of a solution:
I wrote whole new simpler class based on another online guide, but without overloaded constructors & functions.
I imported SWIG wrapped C++ class (Word.cpp), by removing multiple constructors & only have single default const.
See below:
#ifndef WORD_H
#define WORD_H
#include <stdio.h>
#include <iostream>
#include <string.h>
using namespace std;
class Word {
public:
Word();
// REMOVED Word(std::string the_word);
//REMOVED Word(const Word& orig);
virtual ~Word();
virtual void updateWord(std::string word);
virtual std::string getWord();
private:
std::string _the_word;
};
#endif /* WORD_H */
SWIG interface for Word.h:
%{
/* Put header files here or function declarations like below */
/*#include "example.h"*/
#include "Word.h"
%}
%include "std_string.i"
/* %include "example.h"*/
%include "Word.h"
And used these compile options:
swig -python example.i
swig -python -c++ example.i
python setup.py build_ext --inplace
Python:
python
Python 2.7.5 (default, Aug 2 2016, 04:20:16)
[GCC 4.8.5 20150623 (Red Hat 4.8.5-4)] on linux2
Type "help", "copyright", "credits" or "license" for more information.
import _example
_example.new_Word()
<Swig Object of type 'Word *' at 0x7f4debcceb70>
_example.new_Word()
swig/python detected a memory leak of type 'Word *', no destructor found.
<Swig Object of type 'Word *' at 0x7f4debcced50>
w = _example.new_Word()
_example.Word_updateWord(w,"beef")
_example.Word_getWord(w)
swig/python detected a memory leak of type 'Word *', no destructor found.
'beef'
_example.Word_updateWord(w,"chicken")
_example.Word_getWord(w)
'chicken'
So without the same function, or constructor signature we are able to export C++ Class as a Python module.
However, this is very limiting, since Object Orientation design patterns use overloading/overriding significantly.
Thanks,
A million!