如何在 Cython 中返回新的 C++ 对象？

Question

我怀疑这个问题有一个简单的答案，但我需要一些帮助才能开始使用 Cython。我有一个现有的 C++ 代码库，我想通过 Cython 向 Python 公开。对于我要公开的每个类，我创建了一个 Cython cppclass _ClassName 和 Python 包装类 ClassName。

一个小例子：

Object.h
CythonMinimal.pyx
setup.py

Object.h的内容：

class Object {

public:

    Object clone() {
        Object o;
        return o; 
    }

};

CythonMinimal.pyx的内容：

cdef extern from "Object.h":
    cdef cppclass _Object "Object":
        _Object() except +
        _Object clone()


cdef class Object:

    cdef _Object *thisptr

    def __cinit__(self):
        self.thisptr = new _Object()

    def __dealloc__(self):
        del self.thisptr

    def clone(self):
        return self.thisptr.clone()

setup.py的内容

from distutils.core import setup
from distutils.extension import Extension
from Cython.Build import cythonize
from Cython.Distutils import build_ext

import os

os.environ["CC"] = "g++-4.7"
os.environ["CXX"] = "g++-4.7"


modules = [Extension("CythonMinimal",
                     ["CythonMinimal.pyx"],
                     language = "c++",
                     extra_compile_args=["-std=c++11"],
                     extra_link_args=["-std=c++11"])]

for e in modules:
    e.cython_directives = {"embedsignature" : True}

setup(name="CythonMinimal",
     cmdclass={"build_ext": build_ext},
     ext_modules=modules)

这是我编译时遇到的错误：

cls ~/workspace/CythonMinimal $ python3 setup.py build
running build
running build_ext
cythoning CythonMinimal.pyx to CythonMinimal.cpp

Error compiling Cython file:
------------------------------------------------------------
...

    def __dealloc__(self):
        del self.thisptr

    def clone(self):
        return self.thisptr.clone()
                          ^
------------------------------------------------------------

    CythonMinimal.pyx:18:27: Cannot convert '_Object' to Python object
    building 'CythonMinimal' extension
    creating build
    creating build/temp.macosx-10.8-x86_64-3.3
    g++-4.7 -Wno-unused-result -fno-common -dynamic -DNDEBUG -g -O3 -Wall -Wstrict-prototypes -I/usr/local/include -I/usr/local/opt/sqlite/include -I/usr/local/Cellar/python3/3.3.0/Frameworks/Python.framework/Versions/3.3/include/python3.3m -c CythonMinimal.cpp -o build/temp.macosx-10.8-x86_64-3.3/CythonMinimal.o -std=c++11
    cc1plus: warning: command line option '-Wstrict-prototypes' is valid for C/ObjC but not for C++ [enabled by default]
    CythonMinimal.cpp:1:2: error: #error Do not use this file, it is the result of a failed Cython compilation.
    error: command 'g++-4.7' failed with exit status 1

我假设_Object.clone 需要返回一个_Object（cppclass 类型），但Objet.clone 应该返回一个Object（Python 类型）。但是怎么做呢？

Answer 1

您正在一个 Python 函数中返回一个 C++ 对象，该函数只允许返回 Python 对象：

def clone(self):
    return self.thisptr.clone()

把它变成这样：

cdef _Object clone(self) except *:
    return self.thisptr.clone()

但这取决于你想要做什么，你可能想要返回 Object 而不是 _Object，所以我会这样修改它：

cdef class Object:
    cdef _Object thisobj
    cdef _Object *thisptr    

    def __cinit__(self, Object obj=None):
        if obj:
            self.thisobj = obj.thisobj.clone()
        self.thisptr = &self.thisobj

    def __dealloc__(self):
        pass

    def clone(self):
        return Object(self)