事情是这样的，我有一个c++接口，接收一个类似void(*CB)(Data&)的函数指针，当我尝试将此接口暴露给python的时候，AI一步到位给我写好了转换代码如下：
#include <cstdio> #include <pybind11/pybind11.h> #include <pybind11/stl.h> #include <pybind11/functional.h> namespace py = pybind11; typedef void(*CB)(int&); class Test{ public: void test(CB cb) { printf("printf"); } }; PYBIND11_MODULE(testpy, m) { m.doc() = "pybind11 wrapper for testpy class"; py::class_<Test>(m, "Test") .def(py::init<>(), "Constructor for the Test class") .def("test", [](Test& self, std::function<void(int&)> callback) { //... ... auto cb = [callback](int&d){ callback(d); }; // 下面这行报错，错误信息类似： error: cannot convert ‘pybind11_init_testpy(pybind11::module_&)::<lambda(Test&, std::function<void(int&)>)>::<lambda(int&)>’ to ‘CB’ {aka ‘void (*)(int&)’} return self.test(cb); } }import testpy def test_cb(i): print(f'test_cb i = {i}') t = testpy.Test() t.test()首先，这里有个结论，AI给出的代码是错误的(带捕获参数的lambda不能直接转换为裸指针，可看注释部分)，但是形式上面给出了转换代码的大致框架，这里精简后，整个问题的核心就是：pybind11的std::function怎么转换为函数指针。
下面我们看看怎么解决上面3个问题，并解决最终问题。
问题1：python的函数怎么转换为pybind11中的std::function（底层原理）
要解决这个问题，我们得了解python的函数传递过来，这里的callback到底是什么东西？由于pybind11是大量的宏来生成代码，为了快速得到我们要的内容，我们使用gdb来对堆栈进行分析。
#0 test_cb (i=@0x7fffffffd714: 9999) at testpy.cpp:19 #1 0x00007ffff73bfded in Test::test (this=0xc49820, cb=0x7ffff73ab120 <test_cb(int&)>) at testpy.cpp:13 #2 0x00007ffff73ab3f7 in operator()(Test &, std::function<void(int&)>) const (__closure=0xc49678, self=..., callback=...) at testpy.cpp:33 #3 0x00007ffff73ac08c in pybind11::detail::argument_loader<Test&, std::function<void(int&)> >::call_impl<void, pybind11_init_testpy(pybind11::module_&)::<lambda(Test&, std::function<void(int&)>)>&, 0, 1, pybind11::detail::void_type>(struct {...} &, std::index_sequence, pybind11::detail::void_type &&) (this=0x7fffffffd8a0, f=...) at /usr/include/pybind11/cast.h:1480 #4 0x00007ffff73abd00 in pybind11::detail::argument_loader<Test&, std::function<void(int&)> >::call<void, pybind11::detail::void_type, pybind11_init_testpy(pybind11::module_&)::<lambda(Test&, std::function<void(int&)>)>&>(struct {...} &) (this=0x7fffffffd8a0, f=...) at /usr/include/pybind11/cast.h:1454 #5 0x00007ffff73ab9b7 in operator() (__closure=0x0, call=...) at /usr/include/pybind11/pybind11.h:254 #6 0x00007ffff73aba6c in _FUN () at /usr/include/pybind11/pybind11.h:224 #7 0x00007ffff73bd52c in pybind11::cpp_function::dispatcher (self=0x7ffff75fedc0, args_in=0x7ffff7421f80, kwargs_in=0x0) at /usr/include/pybind11/pybind11.h:946 #8 0x0000000000581ecf in cfunction_call (func=0x7ffff740ba10, args=<optimized out>, kwargs=<optimized out>) at ../Objects/methodobject.c:537 #9 0x0000000000549205 in _PyObject_MakeTpCall (tstate=0xba5748 <_PyRuntime+459656>, callable=0x7ffff740ba10, args=<optimized out>, nargs=2, keywords=0x0) at ../Objects/call.c:240 #10 0x0000000000549c3d in _PyObject_VectorcallTstate (kwnames=<optimized out>, nargsf=<optimized out>, args=<optimized out>, callable=<optimized out>, tstate=<optimized out>) at ../Include/internal/pycore_call.h:90 #11 0x00000000005d7109 in _PyEval_EvalFrameDefault (tstate=tstate@entry=0xba5748 <_PyRuntime+459656>, frame=<optimized out>, frame@entry=0x7ffff7fb2020, throwflag=throwflag@entry=0) at Python/bytecodes.c:2706 #12 0x00000000005d564b in _PyEval_EvalFrame (throwflag=0, frame=0x7ffff7fb2020, tstate=0xba5748 <_PyRuntime+459656>) at ../Include/internal/pycore_ceval.h:89 #13 _PyEval_Vector (kwnames=0x0, argcount=0, args=0x0, locals=0x7ffff75f9a80, func=0x7ffff75da160, tstate=0xba5748 <_PyRuntime+459656>) at ../Python/ceval.c:1683 #14 PyEval_EvalCode (co=co@entry=0x7ffff75604b0, globals=globals@entry=0x7ffff75f9a80, locals=locals@entry=0x7ffff75f9a80) at ../Python/ceval.c:578 #15 0x00000000006087b2 in run_eval_code_obj (locals=0x7ffff75f9a80, globals=0x7ffff75f9a80, co=0x7ffff75604b0, tstate=0xba5748 <_PyRuntime+459656>) at ../Python/pythonrun.c:1722 #16 run_mod (mod=<optimized out>, filename=<optimized out>, globals=0x7ffff75f9a80, locals=0x7ffff75f9a80, flags=<optimized out>, arena=<optimized out>) at ../Python/pythonrun.c:1743 #17 0x00000000006b4853 in pyrun_file (fp=fp@entry=0xbf6480, filename=filename@entry=0x7ffff7409ca0, start=start@entry=257, globals=globals@entry=0x7ffff75f9a80, locals=locals@entry=0x7ffff75f9a80, closeit=closeit@entry=1, flags=0x7fffffffe0a8) at ../Python/pythonrun.c:1643 #18 0x00000000006b45ba in _PyRun_SimpleFileObject (fp=fp@entry=0xbf6480, filename=filename@entry=0x7ffff7409ca0, closeit=closeit@entry=1, flags=flags@entry=0x7fffffffe0a8) at ../Python/pythonrun.c:433 #19 0x00000000006b43ef in _PyRun_AnyFileObject (fp=0xbf6480, filename=filename@entry=0x7ffff7409ca0, closeit=closeit@entry=1, flags=flags@entry=0x7fffffffe0a8) at ../Python/pythonrun.c:78 #20 0x00000000006bc455 in pymain_run_file_obj (skip_source_first_line=0, filename=0x7ffff7409ca0, program_name=0x7ffff75f9bf0) at ../Modules/main.c:360 #21 pymain_run_file (config=0xb48328 <_PyRuntime+77672>) at ../Modules/main.c:379 #22 pymain_run_python (exitcode=0x7fffffffe09c) at ../Modules/main.c:629 #23 Py_RunMain () at ../Modules/main.c:709 #24 0x00000000006bbf3d in Py_BytesMain (argc=<optimized out>, argv=<optimized out>) at ../Modules/main.c:763 #25 0x00007ffff7c2a1ca in __libc_start_call_main (main=main@entry=0x518ac0 <main>, argc=argc@entry=2, argv=argv@entry=0x7fffffffe2e8) at ../sysdeps/nptl/libc_start_call_main.h:58 #26 0x00007ffff7c2a28b in __libc_start_main_impl (main=0x518ac0 <main>, argc=2, argv=0x7fffffffe2e8, init=<optimized out>, fini=<optimized out>, rtld_fini=<optimized out>, stack_end=0x7fffffffe2d8) at ../csu/libc-start.c:360 #27 0x00000000006574f5 in _start ()从上面看，核心就是从python解释器，到了pybind11::cpp_function::dispatcher，然后到了我们的test函数。如果我们对pybind11不熟悉的话，我们还是需要深入去看pybind11才能回答我们上面的问题，但是我这里想到了另外一个办法。
我们都知道，pybind11底层是由cpython实现的，因此我们通过cpython来实现上面的同样的功能是什么样子的呢？直接让AI生成示例如下：
#define PY_SSIZE_T_CLEAN #include <Python.h> #include <cstdio> #include <functional> // --- C++ 逻辑模拟部分 --- typedef void(*CB)(int&); class Test{ public: void test(CB cb) { int i = 9999; cb(i); } }; void test_cb(int& i) { printf("test_cb from cxx i = %d\n", i); } // --- CPython 包装部分 --- // 定义 Python 中的 Test 对象结构 typedef struct { PyObject_HEAD Test* cpp_obj; // 指向实际的 C++ 对象 } PyTestObject; void pybind11_like_func(Test& self, std::function<void(int&)> callback) { //... ... auto cb = [callback](int&d){ callback(d); }; int i = 8888; cb(i); // return self.test(cb); return self.test(test_cb); } // Test.test(callback) 的实现 static PyObject* PyTest_test(PyTestObject* self, PyObject* args) { PyObject* pycallback = NULL; // 1. 解析参数，期望得到一个可调用对象 if (!PyArg_ParseTuple(args, "O", &pycallback)) { return NULL; } if (!PyCallable_Check(pycallback)) { PyErr_SetString(PyExc_TypeError, "Parameter must be callable"); return NULL; } // 2. 核心：模拟 std::function<void(int&)> 的构造 // 我们在这里捕获 py_callback 指针。注意：实际生产中需要处理引用计数 std::function<void(int&)> cpp_callback = [pycallback](int& d) { // A. 必须获取 GIL，因为回调可能由 C++ 触发 PyGILState_STATE gstate = PyGILState_Ensure(); // B. 参数转换：C++ int& -> Python Long PyObject* arg = PyLong_FromLong((long)d); PyObject* arg_tuple = PyTuple_Pack(1, arg); // C. 调用 Python 函数 PyObject* result = PyObject_CallObject(pycallback, arg_tuple); // D. 错误处理与清理 if (!result) { PyErr_Print(); } Py_XDECREF(result); Py_DECREF(arg_tuple); Py_DECREF(arg); // E. 释放 GIL PyGILState_Release(gstate); }; pybind11_like_func(*self->cpp_obj, cpp_callback); Py_RETURN_NONE; } // --- 类型与模块定义 --- static PyMethodDef PyTest_methods[] = { {"test", (PyCFunction)PyTest_test, METH_VARARGS, "Execute test with callback"}, {NULL, NULL, 0, NULL} }; static PyTypeObject PyTestType = { PyVarObject_HEAD_INIT(NULL, 0) .tp_name = "testcpy.Test", .tp_basicsize = sizeof(PyTestObject), .tp_itemsize = 0, .tp_flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_BASETYPE, .tp_methods = PyTest_methods, .tp_new = PyType_GenericNew, }; static struct PyModuleDef testcpymodule = { PyModuleDef_HEAD_INIT, "testcpy", "CPython version of testcpy", -1, NULL }; PyMODINIT_FUNC PyInit_testcpy(void) { PyObject* m; if (PyType_Ready(&PyTestType) < 0) return NULL; m = PyModule_Create(&testcpymodule); if (m == NULL) return NULL; Py_INCREF(&PyTestType); PyModule_AddObject(m, "Test", (PyObject*)&PyTestType); return m; }其实我们已经看到了，我们用cpython来实现的话，调用的std::function一定是一个带状态的callable obj。至此，我们已经解决了问题1。
问题2：pybind11中的std::function 怎么转换为c++层的裸函数
实际的方法就是在pybind11代码层，添加一个全局静态变量进行转换，参考如下代码(重点查看PyCBWrapper相关的内容)：
#include <cstdio> #include <pybind11/pybind11.h> #include <pybind11/stl.h> #include <pybind11/functional.h> #include <functional> namespace py = pybind11; typedef void(*CB)(int&); class Test{ public: void test(CB cb) { int i = 9999; cb(i); } }; struct PyCBWrapper { static std::function<void(int&)> py_cb; static void trampoline(int& i) { if (nullptr != py_cb) PyCBWrapper::py_cb(i); } }; std::function<void(int&)> PyCBWrapper::py_cb = nullptr; void test_cb(int& i) { printf("test_cb from cxx i = %d\n", i); } PYBIND11_MODULE(testpy, m) { m.doc() = "pybind11 wrapper for testpy class"; py::class_<Test>(m, "Test") .def(py::init<>(), "Constructor for the Test class") .def("test", [](Test& self, std::function<void(int&)> callback) { //... ... auto cb = [callback](int&d){ callback(d); }; int i = 8888; cb(i); PyCBWrapper::py_cb = callback; // return self.test(cb); return self.test(PyCBWrapper::trampoline); }); }问题3：pybind11中的std::function对象复制的注意事项
问题2的这段代码会直接运行报错，堆栈如下
#0 __pthread_kill_implementation (no_tid=0, signo=6, threadid=<optimized out>) at ./nptl/pthread_kill.c:44 #1 __pthread_kill_internal (signo=6, threadid=<optimized out>) at ./nptl/pthread_kill.c:78 #2 __GI___pthread_kill (threadid=<optimized out>, signo=signo@entry=6) at ./nptl/pthread_kill.c:89 #3 0x00007ffff7c4527e in __GI_raise (sig=sig@entry=6) at ../sysdeps/posix/raise.c:26 #4 0x00007ffff7c288ff in __GI_abort () at ./stdlib/abort.c:79 #5 0x00000000004b1252 in ?? () #6 0x00000000004b2908 in _Py_FatalErrorFunc () #7 0x00000000004b2cd2 in ?? () #8 0x000000000060861e in ?? () #9 0x00000000006a6e93 in PyEval_AcquireThread () #10 0x00007ffff73b9d20 in pybind11::gil_scoped_acquire::gil_scoped_acquire (this=0x7fffffffd760) at /usr/include/pybind11/gil.h:82 #11 0x00007ffff73dd1b1 in pybind11::detail::type_caster<std::function<void (int&)>, void>::load(pybind11::handle, bool)::func_handle::~func_handle() (this=0xc49870, __in_chrg=<optimized out>) at /usr/include/pybind11/functional.h:97 #12 0x00007ffff73dd324 in pybind11::detail::type_caster<std::function<void (int&)>, void>::load(pybind11::handle, bool)::func_wrapper::~func_wrapper() (this=0xc49870, __in_chrg=<optimized out>) at /usr/include/pybind11/functional.h:103 #13 0x00007ffff73e0ff2 in std::_Function_base::_Base_manager<pybind11::detail::type_caster<std::function<void(int&)>, void>::load(pybind11::handle, bool)::func_wrapper>::_M_destroy (__victim=...) at /usr/include/c++/13/bits/std_function.h:175 --Type <RET> for more, q to quit, c to continue without paging-- #14 0x00007ffff73e0ce4 in std::_Function_base::_Base_manager<pybind11::detail::type_caster<std::function<void(int&)>, void>::load(pybind11::handle, bool)::func_wrapper>::_M_manager (__dest=..., __source=..., __op=std::__destroy_functor) at /usr/include/c++/13/bits/std_function.h:203 #15 0x00007ffff73e02aa in std::_Function_handler<void(int&), pybind11::detail::type_caster<std::function<void(int&)>, void>::load(pybind11::handle, bool)::func_wrapper>::_M_manager (__dest=..., __source=..., __op=std::__destroy_functor) at /usr/include/c++/13/bits/std_function.h:282 #16 0x00007ffff73b62c1 in std::_Function_base::~_Function_base (this=0x7ffff73ff440 <PyCBWrapper::py_cb>, __in_chrg=<optimized out>) at /usr/include/c++/13/bits/std_function.h:244 #17 0x00007ffff73bff78 in std::function<void(int&)>::~function (this=0x7ffff73ff440 <PyCBWrapper::py_cb>, __in_chrg=<optimized out>) at /usr/include/c++/13/bits/std_function.h:334 #18 0x00007ffff7c47a76 in __run_exit_handlers (status=0, listp=<optimized out>, run_list_atexit=run_list_atexit@entry=true, run_dtors=run_dtors@entry=true) at ./stdlib/exit.c:108 #19 0x00007ffff7c47bbe in __GI_exit (status=<optimized out>) at ./stdlib/exit.c:138 #20 0x00007ffff7c2a1d1 in __libc_start_call_main (main=main@entry=0x518c60, argc=argc@entry=2, argv=argv@entry=0x7fffffffda28) at ../sysdeps/nptl/libc_start_call_main.h:74 #21 0x00007ffff7c2a28b in __libc_start_main_impl (main=0x518c60, argc=2, argv=0x7fffffffda28, init=<optimized out>, fini=<optimized out>, rtld_fini=<optimized out>, stack_end=0x7fffffffda18) at ../csu/libc-start.c:360 #22 0x0000000000657b05 in _start ()从堆栈分析可知，问题出在程序退出的时候，PyCBWrapper::py_cb（全局静态变量）析构的时候，这个时候其实python解释器已经退出了，再使用python解释器相关的资源，就会报错。这个其实就是问题3。由于从上面的例子可以知道，这个时候的PyCBWrapper::py_cb是一个捕获了python函数的PyObject的std::function，那么解决方案也很简单，那就是在上面的pybind11代码层中添加如下核心代码：
PYBIND11_MODULE(testpy, m) { m.doc() = "pybind11 wrapper for testpy class"; py::class_<Test>(m, "Test") .def(py::init<>(), "Constructor for the Test class") .def("test", [](Test& self, std::function<void(int&)> callback) { //... ... auto cb = [callback](int&d){ callback(d); }; int i = 8888; cb(i); PyCBWrapper::py_cb = callback; // return self.test(cb); self.test(PyCBWrapper::trampoline);
