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Python is one of my favourite languages
:-)
I don't see why I should have to cast any variable to be const at
any point to pass it to a const parameter. It's the use of the
parameter inside the function that makes no changes; this should
not affect the value being passed in.
The benefits of const should be:
1. To give errors to the person writing the function trying to
modify the const data; not the person wishing to pass a variable
to a const parameter.
2. You should be able to pass a constant, like a string literal,
as a const parameter.
To add const to the C API it seems to me I would have to a) be
inconsistent in its use, b) make the API harder to read, c) break
previous C programs by requiring them to add casts d) describe the
inconsistent use of const in the docs to fend off future
re-discussions.
char** is not cryptic for C!
That's why I would add const to a C++ layer, even though my first
objection above still seems to occur.
Ian
On 17/05/13 05:54, Frank Pagliughi wrote:
Ugh. This is why people program in Python.
On 05/16/2013 07:18 PM, Ian Craggs wrote:
1) I can't pass a char** to a const char** or a
const char * const * without a cast. Adding casts requires
changing programs, apart from the principle of adding what seem
unnecessary casts. See:
int f(const char * const * c)
{
c = 0;
c[1] = 0;
c[0][0] = 1;
}
int main(int argc, char** argv)
{
f(argv); /* cast needed - why? */
return 0;
}
OK. I see this, as a warning, when using C with GCC 4.6 and clang
3.0 (but not with Visual C 2010, which is apparently incorrect).
This really surprised me.
C++ accepts this without warning. It will apply the const-ness all
the way down. I found an explanation, but it wanders off into the
world of "who would actually do that?"
http://c-faq.com/ansi/constmismatch.html
In C you can declare an array of topics like this (which gives the
same warning as argv):
char* filts[] = { "topic1", "topic2", "topic3" };
In C++ this declaration used to be allowed (perhaps with a mild
complaint from the compiler) but I believe is now prohibited in
C++11. You must specify the const:
const char* filts[] = { "topic1", "topic2", "topic3"
};
You can also do this in C, and then you don't get the warning when
calling f()...
2) For char** topic, do you want const char** or const char*
const *? Presumably the latter, which I think is verging on the
hard to understand.
Yes, unfortunately the second is more proper. The first would
protect the contents of the strings themselves, whereas the second
also protects the array of pointers to the strings. Perhaps using
array [] notation would make it a little less cryptic?
void f(const char* const c[])
{
c = 0; // This is fine, normal pass-by value
semantics
c[1] = 0; // compiler error!
c[0][0] = 1; // compiler error!
}
But, honestly, I'd settle for the first. C++ is moving to enforce
the const-ness of string literals, but it seems to accept the
"filts" array as-is, even if you leave out the second const.
void f(const char** c)
{
c = 0; // This is fine
c[1] = 0; // This is probably a bug, but allowed
by the compiler
c[0][0] = 1; // compiler error!
}
f(filts); // No complaints.
I'd still vote for the 1st, though. I'd rather get the compiler
error than have to look for that bug.
Frank
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