Cython如何将char **转换为const char **？

Question

I am trying to use Cython to write a wrapper around a C++ library. However, I am running into an issue now, as one of the functions in the library takes the parameter const char** . Apparently, C++ is unable to do this conversion, ( Why am I getting an error converting a 'float**' to 'const float**'? ) which leaves me in a dilemma, as I am passing in a list of strings, let's call it x into the function, and I am trying to generate the corresponding char** object, let's call it a , using malloc and a for loop:

def f(x):
 cdef char** a = <char**> malloc(len(x) * sizeof(char*))
 for index, item in enumerate(x):
  a[index] = item
 ......

Is there a workaround here? The only thing I can think of is using const_cast , and I can't find any details of whether or not that is implemented in Cython....

Answer 1

The following code compiles in cPython V20.0. Does that solve your problem?

# distutils: language = c++

from libc.stdlib cimport malloc

def f(x):
    cdef const char** a = <const char**> malloc(len(x) * sizeof(char*))
    for index, item in x:
        a[index] = item

Answer 2

There is this old answer but I would implement to_cstring_array little bit differently (use of strdup , no PyString_AsString )

from libc.stdlib cimport malloc, free
from libc.string cimport strdup

cdef char ** to_cstring_array(list strings):
    cdef const char * s
    cdef size_t l = len(strings)

    cdef char ** ret = <char **>malloc(l* sizeof(char *))
    # for NULL terminated array
    # cdef char ** ret = <char **>malloc((l + 1) * sizeof(char *))
    # ret[l] = NULL

    for i in range(l):
        s = strings[i]
        ret[i] = strdup(s)
    return ret