如何在Cython中返回新的c++对象

How to return new C++ objects in Cython?

本文关键字：c++ 对象 返回 Cython      更新时间：2023-10-16

我怀疑有一个简单的答案，但是我需要一些帮助来开始使用Cython。我有一个现有的c++代码库，我想通过Cython暴露给Python。对于我想公开的每个类，我创建了一个Python cpp类_ClassName和Python包装器类ClassName。

一个最小的例子:

Object.h
CythonMinimal.pyx
setup.py

Object.h含量:

class Object {
public:
    Object clone() {
        Object o;
        return o; 
    }
};

CythonMinimal.pyx含量:

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)
这是我在编译时得到的错误: 
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_Object (cppclass类型)，但Objet.clone应该返回Object (Python类型)。但如何?

你在一个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()

这样写:

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)
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