我最近在C ++中编写了对Python 3的扩展，但是当我在python中调用C ++时遇到了一些麻烦，而且我不打算使用第三方库。
我曾经使用过不能调用Python绑定C ++虚拟成员函数的方法，但是可以删除virtual关键字。
它运行到return PyObject_CallObject(pFunction, args);时崩溃了，但我没有找到原因。
这是我的代码：
class A
{
PyObject_HEAD
public:
A()
{
std::cout << "A::A()" << std::endl;
}
~A()
{
std::cout << "A::~A()" << std::endl;
}
virtual void test()
{
std::cout << "A::test()" << std::endl;
}
};
class B : public A
{
public:
B()
{
std::cout << "B::B()" << std::endl;
}
~B()
{
std::cout << "B::~B()" << std::endl;
}
static PyObject *py(B *self) {
self->test();
return PyLong_FromLong((long)123456);
}
};
static void B_dealloc(B *self)
{
self->~B();
Py_TYPE(self)->tp_free((PyObject *)self);
}
static PyObject *B_new(PyTypeObject *type, PyObject *args, PyObject *kwds)
{
B *self = (B*)type->tp_alloc(type, 0);
new (self)B;
return (PyObject*)self;
}
static PyMethodDef B_methods[] = {
{"test", (PyCFunction)(B::py), METH_NOARGS, nullptr},
{nullptr}
};
static struct PyModuleDef example_definition = {
PyModuleDef_HEAD_INIT,
"example",
"example",
-1,
B_methods
};
static PyTypeObject ClassyType = {
PyVarObject_HEAD_INIT(NULL, 0) "example.B", /* tp_name */
sizeof(B), /* tp_basicsize */
0, /* tp_itemsize */
(destructor)B_dealloc, /* tp_dealloc */
0, /* tp_print */
0, /* tp_getattr */
0, /* tp_setattr */
0, /* tp_reserved */
0, /* tp_repr */
0, /* tp_as_number */
0, /* tp_as_sequence */
0, /* tp_as_mapping */
0, /* tp_hash */
0, /* tp_call */
0, /* tp_str */
0, /* tp_getattro */
0, /* tp_setattro */
0, /* tp_as_buffer */
Py_TPFLAGS_DEFAULT | Py_TPFLAGS_BASETYPE, /* tp_flags */
"B objects", /* tp_doc */
0, /* tp_traverse */
0, /* tp_clear */
0, /* tp_richcompare */
0, /* tp_weaklistoffset */
0, /* tp_iter */
0, /* tp_iternext */
B_methods, /* tp_methods */
nullptr, /* tp_members */
0, /* tp_getset */
0, /* tp_base */
0, /* tp_dict */
0, /* tp_descr_get */
0, /* tp_descr_set */
0, /* tp_dictoffset */
nullptr, /* tp_init */
0, /* tp_alloc */
B_new, /* tp_new */
};
PyMODINIT_FUNC PyInit_example(void)
{
PyObject *m = PyModule_Create(&example_definition);
if (PyType_Ready(&ClassyType) < 0)
return NULL;
Py_INCREF(&ClassyType);
PyModule_AddObject(m, "B", (PyObject*)&ClassyType);
return m;
}
PyObject* importModule(std::string name)
{
PyObject* pModule = PyImport_ImportModule(name.c_str()); // module name
if (pModule == nullptr)
{
std::cout << "load module error!" << std::endl;
return nullptr;
}
return pModule;
}
PyObject* callFunction(PyObject* pModule, std::string name, PyObject* args = nullptr)
{
PyObject* pFunction = PyObject_GetAttrString(pModule, name.c_str()); // function name
if (pFunction == nullptr)
{
std::cout << "call function error!" << std::endl;
return nullptr;
}
return PyObject_CallObject(pFunction, args);
}
int main()
{
// add module
PyImport_AppendInittab("example", PyInit_example);
// init python
Py_Initialize();
{
PyRun_SimpleString("import sys");
PyRun_SimpleString("import os");
PyRun_SimpleString("sys.path.append(os.getcwd() + '\\script')"); // add script path
}
// import module
PyImport_ImportModule("example");
PyObject* pModule = importModule("Test");
if (pModule != nullptr)
{
PyObject* pReturn = callFunction(pModule, "main");
}
PyErr_Print();
Py_Finalize();
system("pause");
return 0;
}
答案 0 :(得分：2)
我认为OP正在使用CPython API。 （我们使用CPython，部分代码看起来相似/熟悉。）
顾名思义，它是用C语言编写的。
因此，在使用它编写C ++类的Python绑定时，开发人员必须意识到CPython及其C API对C ++并不了解。必须仔细考虑这一点（类似于为C ++类库编写C绑定）。
当我编写Python Wrapper类时，我总是使用struct s来做（以记住这一点）。可以在CPython的包装器中使用C ++继承来类似于包装的C ++类的继承（但这是我上面的规则中的唯一例外）。
struct和class在C ++中是一回事，唯一的例外是，public中的所有内容默认为struct，但private在class中。 SO: Class vs Struct for data only?顺便说一句。 CPython将访问它。 C指针强制转换（重新解释强制转换）的成员变量结构组件（例如ob_base），甚至不会识别private-安全性尝试。
恕我直言，值得一提的是POD（普通旧数据，也称为被动数据结构），因为这就是C ++包装器类的原因与C兼容。SO: What are Aggregates and PODs and how/why are they special?为此提供了全面的概述。
在CPython包装器类中引入至少一个virtual成员函数会带来致命的后果。仔细阅读以上链接可以使这一点变得清楚。但是，我决定通过一些示例代码来说明这一点：
#include <iomanip>
#include <iostream>
// a little experimentation framework:
struct _typeobject { }; // replacement (to keep it simple)
typedef size_t Py_ssize_t; // replacement (to keep it simple)
// copied from object.h of CPython:
/* Define pointers to support a doubly-linked list of all live heap objects. */
#define _PyObject_HEAD_EXTRA \
struct _object *_ob_next; \
struct _object *_ob_prev;
// copied from object.h of CPython:
/* Nothing is actually declared to be a PyObject, but every pointer to
* a Python object can be cast to a PyObject*. This is inheritance built
* by hand. Similarly every pointer to a variable-size Python object can,
* in addition, be cast to PyVarObject*.
*/
typedef struct _object {
_PyObject_HEAD_EXTRA
Py_ssize_t ob_refcnt;
struct _typeobject *ob_type;
} PyObject;
/* PyObject_HEAD defines the initial segment of every PyObject. */
#define PyObject_HEAD PyObject ob_base;
void dump(std::ostream &out, const char *p, size_t size)
{
const size_t n = 16;
for (size_t i = 0; i < size; ++p) {
if (i % n == 0) {
out << std::hex << std::setw(2 * sizeof p) << std::setfill('0')
<< (size_t)p << ": ";
}
out << ' '
<< std::hex << std::setw(2) << std::setfill('0')
<< (unsigned)*(unsigned char*)p;
if (++i % n == 0) out << '\n';
}
if (size % n != 0) out << '\n';
}
// the experiment:
static PyObject pyObj;
// This is correct:
struct Wrapper1 {
PyObject_HEAD
int myExt;
};
static Wrapper1 wrap1;
// This is possible:
struct Wrapper1Derived: Wrapper1 {
double myExtD;
};
static Wrapper1Derived wrap1D;
// This is effectively not different from struct Wrapper1
// but things are private in Wrapper2
// ...and Python will just ignore this (using C pointer casts).
class Wrapper2 {
PyObject_HEAD
int myExt;
};
static Wrapper2 wrap2;
// This is FATAL - introduces a virtual method table.
class Wrapper3 {
private:
PyObject_HEAD
int myExt;
public:
Wrapper3(int value): myExt(value) { }
virtual ~Wrapper3() { myExt = 0; }
};
static Wrapper3 wrap3{123};
int main()
{
std::cout << "Dump of PyObject pyObj:\n";
dump(std::cout, (const char*)&pyObj, sizeof pyObj);
std::cout << "Dump of Wrapper1 wrap1:\n";
dump(std::cout, (const char*)&wrap1, sizeof wrap1);
std::cout << "Dump of Wrapper1Derived wrap1D:\n";
dump(std::cout, (const char*)&wrap1D, sizeof wrap1D);
std::cout << "Dump of Wrapper2 wrap2:\n";
dump(std::cout, (const char*)&wrap2, sizeof wrap2);
std::cout << "Dump of Wrapper3 wrap3:\n";
dump(std::cout, (const char*)&wrap3, sizeof wrap3);
return 0;
}
编译并运行：
Dump of PyObject pyObj:
0000000000601640: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
0000000000601650: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
Dump of Wrapper1 wrap1:
0000000000601600: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
0000000000601610: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
0000000000601620: 00 00 00 00 00 00 00 00
Dump of Wrapper1Derived wrap1D:
00000000006015c0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00000000006015d0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00000000006015e0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
Dump of Wrapper2 wrap2:
0000000000601580: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
0000000000601590: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00000000006015a0: 00 00 00 00 00 00 00 00
Dump of Wrapper3 wrap3:
0000000000601540: d8 0e 40 00 00 00 00 00 00 00 00 00 00 00 00 00
0000000000601550: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
0000000000601560: 00 00 00 00 00 00 00 00 7b 00 00 00 00 00 00 00
pyObj，wrap1，wrap1D，wrap2的转储仅包含00个–难怪我将它们做成static 。 wrap3看起来有些不同，部分原因是构造函数（7b == 123），部分原因是C ++编译器将VMT ponter放入了d8 0e 40很可能属于的类实例中。 （我假设VMT指针具有任何函数指针的大小，但我真的不知道编译器如何在内部组织事物。）
想象一下，当CPython获取wrap3的地址，将其强制转换为PyObject*，并写入_ob_next指针，该指针的偏移量为0，该指针用于将Python对象链接成double时，会发生什么情况？链接列表。 （希望发生崩溃或其他会使情况变得更糟的事情。）
再想象一下OP的创建功能会发生什么
static PyObject *B_new(PyTypeObject *type, PyObject *args, PyObject *kwds)
{
B *self = (B*)type->tp_alloc(type, 0);
new (self)B;
return (PyObject*)self;
}
当B的放置构造函数覆盖PyObject中可能发生的tp_alloc()内部结构的初始化时。