CODEWITHSUNDEEP
c++pythonctypes
lelloman
lelloman

Reputation: 14173

how do I return objects from a C++ function with ctypes?

I have two C++ classes, Container and Item, that look like:

class Item{
public:
    Item(std::string* name,int id);
    std::string* getName();
private:
    std::string* name;
    int id;
};

class Container {
public:
    Container();
    Item* getItem(int id);
private:
    std::vector<Item*> items;
};

I want to create and use Container in Python, so I wrote a C interface to compile a shared library:

extern "C" {
    Container* Container_init(){return new Container();}
    Item* Container_getItem(Container* container,int id){return container->getItem(id);}
    std::string* Item_getName(Item* item){return item->getName();}
}

and a Python wrapper:

from ctypes import *

lib = cdll.LoadLibrary(myLibPath)

class Item(object):
    def getName(self):
        return lib.Item_getName(self.obj)

class Container(object):
    def __init__(self):
        self.obj = lib.Container_init()

    def getItem(self,id):
        return lib.Container_getItem(self.obj,id)


lib.Container_getItem.restype = Item
lib.Container_getItem.argtypes = [c_void_p,c_int]

c = Container()
print c.getItem(5).getName()

When this code runs, it raises a TypeError "object() takes no parameters" at line

return lib.Container_getItem(self.obj,id)

I read about restype and argtype in the documentation but I'm obviously missing something, how can I make Container.getItem return an Item in Python?

Upvotes: 5

Views: 2688

Answers (2)

falsetru
falsetru

Reputation: 368894

You can also use cffi.

Here's an cffi alternative:

import cffi

ffi = cffi.FFI()
ffi.cdef('''
    typedef struct _Item Item;
    typedef struct _Container Container;
    Container* Container_init();
    Item* Container_getItem(Container* container,int id);
    const char* Item_getName(Item* item);
''')

lib = ffi.dlopen(myLibPath)

class Item(object):
    def __init__(self, obj):
        self.obj = obj
    def getName(self):
        return lib.Item_getName(self.obj)

class Container(object):
    def __init__(self):
        self.obj = lib.Container_init()
    def getItem(self, id):
        return Item(lib.Container_getItem(self.obj, id))

c = Container()
print ffi.string(c.getItem(5).getName())

Upvotes: 3

falsetru
falsetru

Reputation: 368894

Replace Item_getName as follow to return char * instead of string *:

const char* Item_getName(Item* item) { return item->getName()->c_str(); }

Item class is missing __init__. Change as follow (This is the cause of the TypeError):

class Item(object):
    def __init__(self, obj):
        self.obj = obj
    def getName(self):
        return lib.Item_getName(self.obj)

And add following to Python script (before call getName method) to correctly get name:

lib.Item_getName.restype = c_char_p
lib.Item_getName.argtypes = ()

Then, you will get what you want.

Upvotes: 6

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