Calling C++ Classes from Python, with ctypes…
I recently found myself needing to call a C++ class from within Python. I didn’t want to call a separate process (as I had previously when using Thrift – see Using Apache Thrift for Python & C++); but rather to call a C++ library directly.
Before I go on I should say that there are a lot of different ways to do this is Python – and I’ve picked one that worked for me. Other techniques are available – and opinion seems quite divided as to which (if any) technique is ‘best’.
To start with we have our C++ class, written in the usual way.
#include <iostream>
// A simple class with a constuctor and some methods...
class Foo
{
public:
Foo(int);
void bar();
int foobar(int);
private:
int val;
};
Foo::Foo(int n)
{
val = n;
}
void Foo::bar()
{
std::cout << "Value is " << val << std::endl;
}
int Foo::foobar(int n)
{
return val + n;
}
Next we need to place a C wrapper around the C++ code – since the ctypes system cannot use C++ code… To do this we add the following to the bottom of the file.
// Define C functions for the C++ class - as ctypes can only talk to C...
extern "C"
{
Foo* Foo_new(int n) {return new Foo(n);}
void Foo_bar(Foo* foo) {foo->bar();}
int Foo_foobar(Foo* foo, int n) {return foo->foobar(n);}
}
Note that we need to provide a non-class-based name for each Method we want to call.
We now need to build a lib.so file from our code.
We could do this by hand:
$ g++ -c -fPIC foo.cpp -o foo.o $ g++ -shared -W1,-soname,libfoo.so -o libfoo.so foo.o
Or we could use CMake.
The following is the CMakeLists.txtto build foo.cpp.
cmake_minimum_required(VERSION 2.8.9)
set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++11 -Wall")
set(CMAKE_RUNTIME_OUTPUT_DIRECTORY "${CMAKE_SOURCE_DIR}")
set(CMAKE_MACOSX_RPATH 1)
project (foo)
set (SOURCE foo.cpp)
add_library(foo MODULE ${SOURCE})
Note that I was building on my Mac – hence the MacOS specific line 4. This will work okay on Linux; but it’s not needed.
Now that we’ve written and compiled our C++ – we now need to build a Python wrapper for the Class…
import ctypes
lib = ctypes.cdll.LoadLibrary('./libfoo.so')
class Foo(object):
def __init__(self, val):
lib.Foo_new.argtypes = [ctypes.c_int]
lib.Foo_new.restype = ctypes.c_void_p
lib.Foo_bar.argtypes = [ctypes.c_void_p]
lib.Foo_bar.restype = ctypes.c_void_p
lib.Foo_foobar.argtypes = [ctypes.c_void_p, ctypes.c_int]
lib.Foo_foobar.restype = ctypes.c_int
self.obj = lib.Foo_new(val)
def bar(self):
lib.Foo_bar(self.obj)
def foobar(self, val):
return lib.Foo_foobar(self.obj, val)
Note the requirement to define the argument types, and the type of the return value (even if there isn’t one – e.g. you’re returning void). Without this you’ll get a segmentation fault (etc.).
Now that we’ve done everything we need to build the module, we can simply import it from within Python.
For example:
from foo import Foo # We'll create a Foo object with a value of 5... f=Foo(5) # Calling f.bar() will print a message including the value... f.bar() # Now we'll use foobar to add a value to that stored in our Foo object, f print (f.foobar(7)) # Now we'll do the same thing - but this time demonstrate that it's a normal # Python integer... x = f.foobar(2) print (type(x))
The full source code for this simple demo can be found here:
https://github.com/Auctoris/ctypes_demo
20 thoughts on “Calling C++ Classes from Python, with ctypes…”
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What did you mean by “or we could use CMake”, where does CMake store the .so file?
CMake is a simple way to manage building complex code on Linux / Mac systems. Have a look here (http://derekmolloy.ie/hello-world-introductions-to-cmake/) for a good tutorial.
Essentially you create the CMakeLists.txt file as described above, then run cmake to create a standard UNIX Makefile. Then you just run make to do the building. In this case the libfoo.so file will be build in the current working directory.
I hope that helps – and sorry for the delay in replying.
Hi
Why int foobar(int); need to set
lib.Foo_foobar.argtypes = [ctypes.c_void_p, ctypes.c_int]
what is void type mean?
thx.
The c_void_p argument is the Foo* pointer to the object needed by the C wrapper function. In C, void* means ‘pointer to something’ where something is not specified. In python you the Foo.foobar() function calls lib.foobar(foo_obj,n) where foo_obj is the object and n is where the int argument goes.
Thanks for this description, it worked well until I changed the argument und return value from int to double. Now Python gives me a ‘None’ back. I defined the argument types as ctypes.c_double. Any idea whats wrong? Am I missing something?
Not sure I’m afraid… That should work.
Here’s one that runs for me.
foo.cpp
#include <iostream>
#include <cstring>
#include <cstdlib>
// A simple class with a constuctor and some methods...
class Foo
{
public:
Foo(double);
const char* bar();
double foobar(double);
private:
double val;
};
Foo::Foo(double n)
{
val = n;
}
const char* Foo::bar()
{
int i;
std::string s;
std::string msg;
s = std::to_string(val);
msg = "The value is " + s;
i = msg.length();
const char* rv = msg.c_str();
return rv;
}
double Foo::foobar(double n)
{
return val + n;
}
// Define C functions for the C++ class - as ctypes can only talk to C...
extern "C"
{
Foo* Foo_new(double n) {return new Foo(n);}
const char* Foo_bar(Foo* foo) {return foo->bar();}
double Foo_foobar(Foo* foo, double n) {return foo->foobar(n);}
}
foo.py
"""foo.py – a simple demo of importing a calss from C++"""import ctypes
lib = ctypes.cdll.LoadLibrary('./libfoo.so')
class Foo(object):
"""The Foo class supports two methods, bar, and foobar..."""
def __init__(self, val):
lib.Foo_new.argtypes = [ctypes.c_double]
lib.Foo_new.restype = ctypes.c_void_p
lib.Foo_bar.argtypes = [ctypes.c_void_p]
lib.Foo_bar.restype = ctypes.c_char_p
lib.Foo_foobar.argtypes = [ctypes.c_void_p, ctypes.c_double]
lib.Foo_foobar.restype = ctypes.c_double
self.obj = lib.Foo_new(val)
def bar(self):
"""bar returns a string continaing the value"""
return (lib.Foo_bar(self.obj)).decode()
def foobar(self, val):
"""foobar takes an integer, and adds it to the value in the Foo class
- returning the result"""
return lib.Foo_foobar(self.obj, val)
Hi AJP, thank you very much for posting this, its very interesting and helpful!!
Could you please help me build the python wrapper for a specific case? I am using Windows and I already have the compiled “LA_CTYPE.dll” and only the “LA_CTYPE.h” file that gives me clues of what happens in the original “.cpp”.
The “LA_CTYPE.h” has the structure as follows:
typedef struct LA LA;
extern “C” __declspec(dllexport) LA* CreateLala( );
extern “C” __declspec(dllexport) uint16 DestroyLala( LA* LVREF );
extern “C” __declspec(dllexport) int Init( LA* LVREF, bool playBackEna = false, int cameraNum = -1);
extern “C” __declspec(dllexport) void setDepthRange( LA* LVREF, uint16 value, uint16 level=0 );
extern “C” __declspec(dllexport) void setPulseCount( LA* LVREF, uint16 value );
extern “C” __declspec(dllexport) uint16 getPulseCount( LA* LVREF );
.
.
.
How can I write the python program to wrap this? The structure seems very different from your example, so I am confused.
Thanks once again
Hello, thanks for sharing!
One question: Does it work in Windows?
It *should* – but the build process will likely be a bit different. I’m afraid that I don’t use Windows though, so I can’t help much more than this.
Hello, thanks very much for this example, I found it easy to follow, and it got me up and running pretty quickly wrapping a DLL on Windows (the wall-o’-text ctypes docs, while thorough, are a bit dense for a noob trying to understand the steps of the process at a high level)
Great; I’m glad it was helpful!
How the newly created Foo objects gets deleted?
I guess you would need to create Foo_delete() function.
void Foo_delete(Foo* p) { delete(p); }
Then perhaps using the the __del__ method or just use the “with” statement by providing
def __enter__(self):
def __exit__(self):
methods to the python version of Foo.
I’m on Windows. Any idea on how to make this work if I just have the DLL file and not the source code? As I’m using an API? Thanks.
I don’t use Windows so I’m not certain how to do it; but it should be the same (after the steps about building the library code) – as long as you know the correct names for the function entry points.
Ok, thank you!
Hi , How can I implement the array parameters
if c++ export function as
extern “C”
{
void Foo_getIntArr(Foo* foo,int a, int* intArray) {for(int i=0;i<a;i++){intArray[i]= i*10;} }
}
then in def __init__(self, val): section
lib.Foo_getIntArr.argtypes = [ctypes.c_void_p, POINTER(ctypes.c_int) ]
lib.Foo_getIntArr.restype = none
and how do I do for following def code ?
def getIntArr(self, a, ???? ):
return lib.Foo_getIntArr(self.obj, a, ????)
and how do I call this in python test demo ?
Thank you.
My g++ made error like this:
g++: error: unrecognized command line option ‘-W1,-soname,libfoo.so’
What exactly is that option -W1 and -soname, and how can I solve this error?
Try -Wl,-soname=libfoo.so
-Wl (that is a lowercase L) passes an option to the linker
-soname=libfoo.so sets the shared object name to be libfoo.so
https://stackoverflow.com/questions/13547750/g-error-unrecognized-option-soname
https://en.wikipedia.org/wiki/Soname