[Python-Dev] Baffled by PyArg_ParseTupleAndKeywords modification

[Thomas Wouters]

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 const_cast<> 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 PyArg_ParseTupleAndKeywords() 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>

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