Re: casting from char* to const char*
joshuamaurice@gmail.com () wrote (abridged):
On Mar 21, 7:17 am, Falk Tannh?user <tannhauser86549s...@free.fr>
wrote:
A cast is only required for function overload resolution, and in
this case, both const_cast and static_cast will do, although
the former probably better conveys the intention:
I disagree. const_cast is usually signs of bad code / bad design /
hacks. I would use static_cast, although if possible I would make a
temp variable to just do an implicit cast with a nice comment I'm
doing this to select the correct overload.
Finally, if you need to assign const-ness to pick the right
overload, I think there's something wrong with the library
you're using.
I imagine one use is to implement a non-const version in terms of a const
version, in order to preserve constness. Eg:
const char strchr( const char *str, char ch ) {
while (true) {
if (*str == ch)
return str;
if (!*str)
return NULL;
++str;
}
}
char *strchr( char *str, char ch ) {
return const_cast<char *>(
strchr( const_cast<const char *>(str), ch ) );
}
Here the result of the function is based on one of its arguments, so to
preserve const correctness we have to have two overloads. It would be a
maintenance headache to duplicate the body of the function. We could
avoid one cast by introducing a helper function with a different name,
but this is simpler and more direct (and the helper function would still
need a cast on its result). We could use a template, but that becomes way
more complicated (eg it would match arguments we don't want).
I agree with Falk Tannh?user that static_cast would be less clear here.
What we are doing involves const and const_cast is the correct tool. I
wouldn't avoid const_cast just because it is sometimes a sign of bad code.
Both these casts are quite safe.
I'm merely noting the library has a poor interface if there
are overloads differing by arguments of pointer to const and
just pointer with different behavior.
In this case the overloads have the same behaviour. Indeed, we define one
in terms of the other to help ensure that.
-- Dave Harris, Nottingham, UK.
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