c equivalence of const_cast?

[Ben Pfaff]

There are two other semantic differences I would consider
significant enough to mention:

- C++'s "const" declares actual constants by default, and
- the default linkage of C++ "const" identifiers is different.

Oh, yes, I temporarily forgot about those. Thank you for
reminding me.

 

[Old Wolf]

In C doesn't exsist a const keyword

If you mean "the const keyword doesn't exist in C" then you're
wrong (otherwise, I don't know what you were trying to say).
See N869 6.4.1#1 (the list of keywords). I don't have a copy
of C89 so I can't give you a reference to that, but read
K&R2 if you have it.

normally, some compiler support const variables, like gcc,

Compilers must support it

but IMHO it is the best way to remove the const from the source,
and replace it with defines.

Many people argue the opposite (defines circumvent scope and
linkage rules and type-checking). See the thread "What Value
is Offered by Const-Correctness" in c.l.c++ at the moment,
much of it applies to both languages.

Anyway, how would you use a define to avoid the OP's problem,
which was something like:

int some_func(char *ptr); /* does not modify ptr */
char const *other_func(void);

void foo()
{
char *ptr = other_func();
some_func(ptr);
}

 

[Chris Barts]

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Chris Torek wrote:
|>
|>>[const] just isn't the same keyword as in C++, which probably
|>>confuses some people.
|
|
| In article <|
|>It isn't? `const' has largely the same meaning in C and in C++
|>as far as I know. There are some differences in which
|>conversions that change const-ness are allowed implicitly.
|
|
| There are two other semantic differences I would consider
| significant enough to mention:
|
| - C++'s "const" declares actual constants by default, and
| - the default linkage of C++ "const" identifiers is different.
|
| (These two are related, unsurprisingly -- "extern const" in C++
| means what "const" does in C if the variable is not initialized.
| If the variable *is* initialized, though, the C++ "const" is
| still a constant.)
|
| In particular, the following is invalid in C:

Is it still invalid under C99, with VLAs? (I understand that `C'
defaults to C89/C90, but this could possibly confuse someone who
actually had a C99 compiler.)

|
| const int N = 10;
| int a[N];
| int main(void) {
| /* ... code using the array "a" ... */
| return 0;
| }
|

~From what I read of the post you snipped, it seems that Nicloas Pavlidis
should say that `const'-ness in C isn't as strong as it is in C++ or
Pascal or Ada, for example, and that if he wants to avoid /all/
potential surprises arising from the different semantics, he should use
`#define's in C where he would use constant variables in Ada.
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[Mike Wahler]

Maybe I'm wrong, but AFAIK const is not supported by C89, if I'm wrong
tell me!

You're wrong. See 3.1.1 and 3.5.3

-Mike

 

[Keith Thompson]

Chris Torek wrote: [...]
| In particular, the following is invalid in C:

Is it still invalid under C99, with VLAs? (I understand that `C'
defaults to C89/C90, but this could possibly confuse someone who
actually had a C99 compiler.)

|
| const int N = 10;
| int a[N];
| int main(void) {
| /* ... code using the array "a" ... */
| return 0;
| }
|

A C99 variable length array cannot have static storage duration
(C99 6.7.5.2p2).

 

[Mark McIntyre]

Maybe I'm wrong, but AFAIK const is not supported by C89, if I'm wrong
tell me!

You're wrong. It just isn't the same keyword as in C++, which probably
confuses some people.

So I recommended to use #defines instead of const

#defines are (in C) totally different to consts. I do agree tho that tehy
behave more like C++'s const objects.

 

[Mark McIntyre]

It isn't? `const' has largely the same meaning in C and in C++
as far as I know.

C++ consts are actually constants. Whereas C's const objects are merely
advisory.