[Python-Dev] Baffled by PyArg_ParseTupleAndKeywords modification (original) (raw)

Thomas Wouters thomas at xs4all.net
Fri Feb 10 17:53:39 CET 2006

On Fri, Feb 10, 2006 at 11:30:30AM -0500, Jeremy Hylton wrote:

On 2/10/06, Guido van Rossum <guido at python.org> wrote: > OMG. Are we now adding 'const' modifiers to random places? I thought > "const propagation hell" was a place we were happily avoiding by not > falling for that meme. What changed?

I added some const to several API functions that take char* but typically called by passing string literals. In C++, a string literal is a const char* so you need to add a constcast<> to every call site, which is incredibly cumbersome. After some discussion on python-dev, I made changes to a small set of API functions and chased the const-ness the rest of the way, as you would expect. There was nothing random about the places const was added. I admit that I'm also puzzled by Jack's specific question. I don't understand why an array passed to PyArgParseTupleAndKeywords() would need to be declared as const. I observed the problem in my initial changes but didn't think very hard about the cause of the problem. Perhaps someone with better C/C++ standards chops can explain.

Well, it's counter-intuitive, but a direct result of how pointer equivalence is defined in C. I'm rusty in this part, so I will get some terminology wrong, but IIRC, a variable A is of an equivalent type of variable B if they hold the same type of data. So, a 'const char ' is equivalent to a 'char ' because they both hold the memory of a 'char'. But a 'const char' (or 'const *char[]') is not equivalent to a 'char **' (or 'char *[]') because the first holds the address of a 'const char *', and the second the address of a 'char *'. A 'char * const *' is equivalent to a 'char **' though.

As I said, I got some of the terminology wrong, but the end result is exactly that: a 'const char **' is not equivalent to a 'char **', even though a 'const char *' is equivalent to a 'char *'. Equivalence, in this case, means 'can be automatically downcasted'. Peter v/d Linden explains this quite well in "Expert C Programming" (aka 'Deep C Secrets'), but unfortunately I'm working from home and I left my copy at a coworkers' desk.

-- Thomas Wouters <thomas at xs4all.net>

Hi! I'm a .signature virus! copy me into your .signature file to help me spread!

More information about the Python-Dev mailing list