python - PyArg_ParseTupleAndKeywords 抛出警告 : ISO C++ forbids converting a string constant to ‘char*’ [-Wwrite-strings]

Question

搜索有关如何使用 PyArg_ParseTupleAndKeywords 的示例我发现了这些问题:

1. How can one use PyArg_ParseTupleAndKeywords to parse a tuple with optional arguments as well as keywords?
2. How does PyArg_ParseTupleAndKeywords work?

他们都使用类似static char* kwlist[] = {"a", "b", NULL}

static int PyClass_init(PyClass* self, PyObject* args, PyObject* kwargs) {
    char* path;
    char* regex;
    static char* kwlist[] = {"", "", NULL};

    if( !PyArg_ParseTupleAndKeywords( args, kwargs, "s|s", kwlist, &path, &regex ) ) {
        return -1;
    }

    // other code ...
    return 0;
}

在 setup.py 上使用 language = "c++" 编译它并使用 -std=c++11 构建它会抛出此警告:

x86_64-linux-gnu-gcc -pthread -DNDEBUG -g -fwrapv -O0 -Wall -g -fstack-protector-strong -Wformat -Werror=format-security -Wdate-time -D_FORTIFY_SOURCE=2 -fPIC -Isource -I/usr/include/python3.6m -c source/test.cpp -o build/temp.linux-x86_64-3.6/source/test.o -O0 -g -ggdb -std=c++11 -fstack-protector-all
source/test.cpp: In function ‘int PyClass_init(PyClass*, PyObject*, PyObject*)’:
source/test.cpp:41:42: warning: ISO C++ forbids converting a string constant to ‘char*’ [-Wwrite-strings]
   static char* kwlist[] = {"a", "b", NULL};
                                          ^

搜索此错误我发现了这个问题 Why is conversion from string constant to 'char*' valid in C but invalid in C++进行修复，但将修复应用为 static char* const kwlist[] = {"a", "b", NULL}; 保留/升级引入错误的警告:

source/test.cpp:41:50: warning: ISO C++ forbids converting a string constant to ‘char*’ [-Wwrite-strings]
     static char* const kwlist[] = {"a", "b", NULL};
                                                  ^
source/test.cpp:41:50: warning: ISO C++ forbids converting a string constant to ‘char*’ [-Wwrite-strings]
source/test.cpp:43:89: error: invalid conversion from ‘char* const*’ to ‘char**’ [-fpermissive]
     if( !PyArg_ParseTupleAndKeywords( args, kwargs, "s|s", kwlist, &path, &regex ) ) {
                                                                                         ^
In file included from /usr/include/python3.6m/Python.h:117:0,
                 from source/test.cpp:3:
/usr/include/python3.6m/modsupport.h:17:41: note:   initializing argument 4 of ‘int _PyArg_ParseTupleAndKeywords_SizeT(PyObject*, PyObject*, const char*, char**, ...)’
 #define PyArg_ParseTupleAndKeywords     _PyArg_ParseTupleAndKeywords_SizeT
                                         ^
/usr/include/python3.6m/modsupport.h:17:41: note: in definition of macro ‘PyArg_ParseTupleAndKeywords’
 #define PyArg_ParseTupleAndKeywords     _PyArg_ParseTupleAndKeywords_SizeT
                                         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
error: command 'x86_64-linux-gnu-gcc' failed with exit status 1

如何使用 static char* kwlist[] = {"a", "b", NULL} 和 C++ 11 的等效构造来消除警告> 同时与 Python C API 要求兼容？

---

更新

建议我尝试 static const char* kwlist[] = {"a", "b", NULL} 但 PyArg_ParseTupleAndKeywords 不接受它:

source/test.cpp:43:89: error: invalid conversion from ‘const char**’ to ‘char**’ [-fpermissive]
     if( !PyArg_ParseTupleAndKeywords( args, kwargs, "s|s", kwlist, &filepath, &rawregex ) ) {
                                                                                         ^
In file included from /usr/include/python3.6m/Python.h:117:0,
                 from source/test.cpp:3:
/usr/include/python3.6m/modsupport.h:17:41: note:   initializing argument 4 of ‘int _PyArg_ParseTupleAndKeywords_SizeT(PyObject*, PyObject*, const char*, char**, ...)’
 #define PyArg_ParseTupleAndKeywords     _PyArg_ParseTupleAndKeywords_SizeT
                                         ^
/usr/include/python3.6m/modsupport.h:17:41: note: in definition of macro ‘PyArg_ParseTupleAndKeywords’
 #define PyArg_ParseTupleAndKeywords     _PyArg_ParseTupleAndKeywords_SizeT
                                         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
error: command 'x86_64-linux-gnu-gcc' failed with exit status 1

Answer 1

您从中复制的示例代码可能是 C，其中警告没有被认真考虑(gcc 报告此类代码没有错误，其中 g++ 弹出一个即使没有警告选项也会发出警告)。

这里的解决方法是像这样添加 const 关键字:

static const char* kwlist[] = {"a", "b", NULL};

您尝试的 const 关键字没有应用于值，而是应用于指针。

然后让函数接受一个 char ** 使用 const_cast:

const_cast<char **>(kwlist)

像这样:

PyArg_ParseTupleAndKeywords( args, kwargs, "s|s", const_cast<char **>(kwlist), &path, &regex ) ) 

该函数不会通过隐式“契约(Contract)”更改值(因为它是 python API，不会骗你)，所以在这种情况下，转换为 const 就可以了)

PS:this other answer给出的备选方案也有效。请注意，使用空的可变字符串并不比转换为常量更安全，因为如果被调用的函数决定写入空字符串，它也会调用未定义的行为。