Python to C/C++ const char question

Question

I am extending Python with some C++ code.

One of the functions I'm using has the following signature:

int PyArg_ParseTupleAndKeywords(PyObject *arg, PyObject *kwdict,
                                char *format, char **kwlist, ...);

(link: http://docs.python.org/release/1.5.2p2/ext/parseTupleAndKeywords.html)

The parameter of interest is kwlist. In the link above, examples on how to use this function are given. In the examples, kwlist looks like:

  static char *kwlist[] = {"voltage", "state", "action", "type", NULL};

When I compile this using g++, I get the warning:

warning: deprecated conversion from string constant to ‘char*’

So, I can change the static char* to a static const char*. Unfortunately, I can't change the Python code. So with this change, I get a different compilation error (can't convert char** to const char**). Based on what I've read here, I can turn on compiler flags to ignore the warning or I can cast each of the constant strings in the definition of kwlist to char *. Currently, I'm doing the latter. What are other solutions?

Sorry if this question has been asked before. I'm new.

Dyche

Answer 1

Does PyArg_ParseTupleAndKeywords() expect to modify the data you are passing in? Normally, in idiomatic C++, a const <something> * points to an object that the callee will only read from, whereas <something> * points to an object that the callee can write to.

If PyArg_ParseTupleAndKeywords() expects to be able to write to the char * you are passing in, you've got an entirely different problem over and above what you mention in your question.

Assuming that PyArg_ParseTupleAndKeywords does not want to modify its parameters, the idiomatically correct way of dealing with this problem would be to declare kwlist as const char *kwlist[] and use const_cast to remove its const-ness when calling PyArg_ParseTupleAndKeywords() which would make it look like this:

PyArg_ParseTupleAndKeywords(..., ..., ..., const_cast<char **>(kwlist), ...);

Tramline

I'm pretty sure PyArg_ParseTupleAndKeywords() does not expect to modify kwlist. Your suggestion is helpful. Thanks. – Dyche

Answer 2

There is an accepted answer from seven years ago, but I'd like to add an alternative solution, since this topic seems to be still relevant.

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If you don't like the const_cast solution, you can also create a write-able version of the string array.

char s_voltage[] = "voltage";
char s_state[] = "state";
char s_action[] = "action";
char s_type[] = "type";
char *kwlist[] = {s_voltage, s_state, s_action, s_type, NULL};

The char name[] = ".." copies the your string to a writable location.

Meneau