c equivalence of const_cast?

[Bj?rn]

I'm trying to convert some small piece of c++ code to c, only to ease
linking it to a third language (I'm not knowledgeable in either c or
c++).

What would be the the c equivalence of:
lpData=const_cast<char*>(Something);

Trying to compile this with gcc 3.2.3 (mingw special 20030504-1)

Thanks!
Björn

[Mike Deskevich]

i'm not a c++ guru, but i think you should be able to do:
lpData=(const char*)something;

[Keith Thompson]

"Mike Deskevich" <mi***********@gmail.com> writes:

i'm not a c++ guru, but i think you should be able to do:
lpData=(const char*)something;

Actually, I think he wants
lpData = (char*)something;

<OT>
A C++ const_cast is used to cast away constness. C++ has several
flavors of casts that are more restricted than C's "Just convert it,
trust me, I know what I'm doing" cast. (C++ has C-style casts as
well.)
</OT>

--
Keith Thompson (The_Other_Keith) ks***@mib.org <http://www.ghoti.net/~kst>
San Diego Supercomputer Center <*> <http://users.sdsc.edu/~kst>
We must do something. This is something. Therefore, we must do this.

[Mike Wahler]

"Bj?rn" <ss*****@hotmail.com> wrote in message
news:d5*************************@posting.google.co m...

I'm trying to convert some small piece of c++ code to c, only to ease
linking it to a third language (I'm not knowledgeable in either c or
c++).

What would be the the c equivalence of:
lpData=const_cast<char*>(Something);

lpData = (char*)Something;

(C doesn't have different 'flavors' of casts as does C++.)

-Mike

[Old Wolf]

ss*****@hotmail.com (Bj?rn) wrote:

I'm trying to convert some small piece of c++ code to c, only to ease
linking it to a third language (I'm not knowledgeable in either c or
c++).

What would be the the c equivalence of:
lpData=const_cast<char*>(Something);

lpData = (char *)Something;

Same goes for all the other C++ cast keywords

[Eric Schmidt]

lpData = (char*) (Something);

[Nicolas Pavlidis]

ss*****@hotmail.com (Bj?rn) writes:

I'm trying to convert some small piece of c++ code to c, only to ease
linking it to a third language (I'm not knowledgeable in either c or
c++).
In C doesn't exsist a const keyword, normally, some compiler support
const variables, like gcc, but IMHO it is the best way to remove the
const from the source, and replace it with defines.
What would be the the c equivalence of:
lpData=const_cast<char*>(Something);
If you have done the thing described above just remove the
const_cast<char*> statement, and assign Something to lpData:

lpData = Something;
Trying to compile this with gcc 3.2.3 (mingw special 20030504-1)

Thats because gcc supports (in C - Mode) const but not the C++
const_cast<..>

HTH && Kind regrads
icolas
--
| Nicolas Pavlidis | Elvis Presly: |\ |__ |
| Student of SE & KM | "Into the goto" | \|__| |
| pa****@sbox.tugraz.at | ICQ #320057056 | |
|-------------------University of Technology, Graz----------------|

[Keith Thompson]

Nicolas Pavlidis <pa****@sbox.tugraz.at> writes:

ss*****@hotmail.com (Bj?rn) writes:

I'm trying to convert some small piece of c++ code to c, only to ease
linking it to a third language (I'm not knowledgeable in either c or
c++).

In C doesn't exsist a const keyword, normally, some compiler support
const variables, like gcc, but IMHO it is the best way to remove the
const from the source, and replace it with defines.

C certainly does have a "const" keyword (though it doesn't have
"const_cast").

--
Keith Thompson (The_Other_Keith) ks***@mib.org <http://www.ghoti.net/~kst>
San Diego Supercomputer Center <*> <http://users.sdsc.edu/~kst>
We must do something. This is something. Therefore, we must do this.

[CBFalconer]

Nicolas Pavlidis wrote:

ss*****@hotmail.com (Bj?rn) writes:

I'm trying to convert some small piece of c++ code to c, only to
ease linking it to a third language (I'm not knowledgeable in
either c or c++).

In C doesn't exsist a const keyword, normally, some compiler
support const variables, like gcc, but IMHO it is the best way to
remove the const from the source, and replace it with defines.

What would be the the c equivalence of:
lpData=const_cast<char*>(Something);

If you have done the thing described above just remove the
const_cast<char*> statement, and assign Something to lpData:

lpData = Something;

Trying to compile this with gcc 3.2.3 (mingw special 20030504-1)

Thats because gcc supports (in C - Mode) const but not the C++
const_cast<..>

Before supplying flawed advice, I suggest you at least read the
other replies to this enquiry, some of which were quite accurate.

--
"I support the Red Sox and any team that beats the Yankees"

"Any baby snookums can be a Yankee fan, it takes real moral
fiber to be a Red Sox fan"

[Bj?rn]

Thank you for all your suggestions. Coming from Ada language I'm
always quite surprised of how easy it often is to type cast in C, good
or bad.

Björn

[Ben Pfaff]

ss*****@hotmail.com (Bj?rn) writes:

Thank you for all your suggestions. Coming from Ada language I'm
always quite surprised of how easy it often is to type cast in C, good
or bad.

All type casts are explicit in C. I don't see how that's "easy"
or "hard", it's just a matter of typing.
--
"Some people *are* arrogant, and others read the FAQ."
--Chris Dollin

[Nicolas Pavlidis]

Keith Thompson <ks***@mib.org> writes:

Nicolas Pavlidis <pa****@sbox.tugraz.at> writes:

ss*****@hotmail.com (Bj?rn) writes:

I'm trying to convert some small piece of c++ code to c, only to ease
linking it to a third language (I'm not knowledgeable in either c or
c++).

In C doesn't exsist a const keyword, normally, some compiler support
const variables, like gcc, but IMHO it is the best way to remove the
const from the source, and replace it with defines.

C certainly does have a "const" keyword (though it doesn't have
"const_cast").

Maybe I'm wrong, but AFAIK const is not supported by C89, if I'm wrong
tell me! So I recommended to use #defines instead of const becaue
support of C99 is not the best one with some compilers...

Kind regrads,
Nicolas
--
| Nicolas Pavlidis | Elvis Presly: |\ |__ |
| Student of SE & KM | "Into the goto" | \|__| |
| pa****@sbox.tugraz.at | ICQ #320057056 | |
|-------------------University of Technology, Graz----------------|

[Keith Thompson]

Ben Pfaff <bl*@cs.stanford.edu> writes:

ss*****@hotmail.com (Bj?rn) writes:

Thank you for all your suggestions. Coming from Ada language I'm
always quite surprised of how easy it often is to type cast in C, good
or bad.

All type casts are explicit in C. I don't see how that's "easy"
or "hard", it's just a matter of typing.

Part of the previous poster's surprise may have to do with a confusion
between casts and conversions. A cast is a syntactic construct that
looks like:
(type_name)expression
and specifies a conversion. There is no such thing as an implicit
cast. A conversion is an operation that happens at run time (or it
might happen during compilation if the optimizer is clever).

It's all too common to misuse the term "cast" or "type cast" to refer
to conversions rather than to the syntactic construct.

<OT>
Compared to Ada, C has far more cases of implicit conversions. In C,
you can convert any numeric type to any other numeric type just by
assigning the value; Ada requires such conversions to be done
explicitly. C also allows explicit conversions in more cases;
conversions between incompatible pointer types (that can lead to
undefined behavior in C) aren't allowed in Ada. Ada does have a type
punning mechanism called Unchecked_Conversion (often needed for
systems-level programming).
</OT>

--
Keith Thompson (The_Other_Keith) ks***@mib.org <http://www.ghoti.net/~kst>
San Diego Supercomputer Center <*> <http://users.sdsc.edu/~kst>
We must do something. This is something. Therefore, we must do this.

[Ben Pfaff]

Nicolas Pavlidis <pa****@sbox.tugraz.at> writes:

Keith Thompson <ks***@mib.org> writes:

Nicolas Pavlidis <pa****@sbox.tugraz.at> writes:

> ss*****@hotmail.com (Bj?rn) writes:
>> I'm trying to convert some small piece of c++ code to c, only to ease
>> linking it to a third language (I'm not knowledgeable in either c or
>> c++).
>
> In C doesn't exsist a const keyword, normally, some compiler support
> const variables, like gcc, but IMHO it is the best way to remove the
> const from the source, and replace it with defines.

C certainly does have a "const" keyword (though it doesn't have
"const_cast").

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

You are wrong. C89 includes const.
--
int main(void){char p[]="ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuv wxyz.\
\n",*q="kl BIcNBFr.NKEzjwCIxNJC";int i=sizeof p/2;char *strchr();int putchar(\
);while(*q){i+=strchr(p,*q++)-p;if(i>=(int)sizeof p)i-=sizeof p-1;putchar(p[i]\
);}return 0;}

[Jonathan Adams]

In article <2t*************@uni-berlin.de>,
Nicolas Pavlidis <pa****@sbox.tugraz.at> wrote:

Keith Thompson <ks***@mib.org> writes:

Nicolas Pavlidis <pa****@sbox.tugraz.at> writes:

ss*****@hotmail.com (Bj?rn) writes:
> I'm trying to convert some small piece of c++ code to c, only to ease
> linking it to a third language (I'm not knowledgeable in either c or
> c++).

In C doesn't exsist a const keyword, normally, some compiler support
const variables, like gcc, but IMHO it is the best way to remove the
const from the source, and replace it with defines.

C certainly does have a "const" keyword (though it doesn't have
"const_cast").

Maybe I'm wrong, but AFAIK const is not supported by C89, if I'm wrong
tell me! So I recommended to use #defines instead of const becaue
support of C99 is not the best one with some compilers...

Check your K&R 2nd edition. "const" was added as part of ANSI C -- i.e.
C89. It's even mentioned in the "changes from K&R C" appendix.

I think writing new code to be backwards compatible with pre-ANSI C
compilers is a bit silly nowadays.

Cheers,
- jonathan

[Mark McIntyre]

On Mon, 18 Oct 2004 10:45:34 -0700, in comp.lang.c , Ben Pfaff
<bl*@cs.stanford.edu> wrote:

ss*****@hotmail.com (Bj?rn) writes:

Thank you for all your suggestions. Coming from Ada language I'm
always quite surprised of how easy it often is to type cast in C, good
or bad.

All type casts are explicit in C. I don't see how that's "easy"
or "hard", it's just a matter of typing.

compared to C++, where the compiler tends to fight you every step of the
way, in C the compiler tends to treat casts as The Word From The
Mountain.... which is of course what makes them tricky.....

--
Mark McIntyre
CLC FAQ <http://www.eskimo.com/~scs/C-faq/top.html>
CLC readme: <http://www.ungerhu.com/jxh/clc.welcome.txt>
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[Mark McIntyre]

On 18 Oct 2004 20:34:44 +0200, in comp.lang.c , Nicolas Pavlidis
<pa****@sbox.tugraz.at> wrote:

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
CLC FAQ <http://www.eskimo.com/~scs/C-faq/top.html>
CLC readme: <http://www.ungerhu.com/jxh/clc.welcome.txt>
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[Ben Pfaff]

Mark McIntyre <ma**********@spamcop.net> writes:

On 18 Oct 2004 20:34:44 +0200, in comp.lang.c , Nicolas Pavlidis
<pa****@sbox.tugraz.at> wrote:

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.

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.
--
Bite me! said C.

[Mark McIntyre]

On 18 Oct 2004 20:34:44 +0200, in comp.lang.c , Nicolas Pavlidis
<pa****@sbox.tugraz.at> wrote:

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
CLC FAQ <http://www.eskimo.com/~scs/C-faq/top.html>
CLC readme: <http://www.ungerhu.com/jxh/clc.welcome.txt>
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[Chris Torek]

>Mark McIntyre <ma**********@spamcop.net> writes:

[const] just isn't the same keyword as in C++, which probably
confuses some people.

In article <news:87************@benpfaff.org>
Ben Pfaff <bl*@cs.stanford.edu> wrote: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:

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

but valid in C++ (because N is not a constant in C, but is in C++):

% cc -o t t.c
t.c:2: variable-size type declared outside of any function
% g++ -o t++ -x c++ t.c
% ./t++
% echo $status [examines the value returned from main()]
0
%
Contrariwise, the following works as C but not as C++:

/* file1.c */
const int N = 10; /* note lack of "extern" */
int f2(void);
int main(void) {
return f2();
}

/* file2.c -- a separate translation unit */
#include <stdio.h>
#include <stdlib.h>

extern const int N;

int f2(void) {
int i, *p, ret;

printf("N is %d (we assume it is at least 1)\n", N);
p = (int *)malloc(N * sizeof *p); /* cast required for C++ */
p[0] = 42;
for (i = 1; i < N; i++)
p[i] = p[0] * (5 - i);
ret = !p[N - 1];
free(p);
return ret;
}

Here, in C, N is shared across the two translation units, but in
C++ file2.c's extern "N" is missing:

% cc -o c file[12].c
% ./c
N is 10 (we assume it is at least 1)
% echo $status
0
% g++ -o c++ -x c++ file[12].c
/tmp/ccg7oeo1.o: In function `f2(void)':
/tmp/ccg7oeo1.o(.text+0xc): undefined reference to `N'
/tmp/ccg7oeo1.o(.text+0x22): undefined reference to `N'
/tmp/ccg7oeo1.o(.text+0x55): undefined reference to `N'
/tmp/ccg7oeo1.o(.text+0x88): undefined reference to `N'
%
--
In-Real-Life: Chris Torek, Wind River Systems
Salt Lake City, UT, USA (40°39.22'N, 111°50.29'W) [phone]
email: forget about it http://web.torek.net/torek/index.html
Reading email is like searching for food in the garbage, thanks to spammers.

[Ben Pfaff]

Chris Torek <no****@torek.net> writes:

In article <news:87************@benpfaff.org>
Ben Pfaff <bl*@cs.stanford.edu> wrote:

`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.

Oh, yes, I temporarily forgot about those. Thank you for
reminding me.
--
"Some people *are* arrogant, and others read the FAQ."
--Chris Dollin

[Old Wolf]

Nicolas Pavlidis <pa****@sbox.tugraz.at> wrote:

ss*****@hotmail.com (Bj?rn) writes:

I'm trying to convert some small piece of c++ code to c, only to ease
linking it to a third language (I'm not knowledgeable in either c or
c++).
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]

-----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA1

Chris Torek wrote:
|>Mark McIntyre <ma**********@spamcop.net> writes:
|>
|>>[const] just isn't the same keyword as in C++, which probably
|>>confuses some people.
|
|
| In article <news:87************@benpfaff.org>
| Ben Pfaff <bl*@cs.stanford.edu> wrote:
|
|>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]

"Nicolas Pavlidis" <pa****@sbox.tugraz.at> wrote in message
news:2t*************@uni-berlin.de...

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 Barts <ch************@gmail.com> writes:

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).

--
Keith Thompson (The_Other_Keith) ks***@mib.org <http://www.ghoti.net/~kst>
San Diego Supercomputer Center <*> <http://users.sdsc.edu/~kst>
We must do something. This is something. Therefore, we must do this.

[Mark McIntyre]

On 18 Oct 2004 20:34:44 +0200, in comp.lang.c , Nicolas Pavlidis
<pa****@sbox.tugraz.at> wrote:

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
CLC FAQ <http://www.eskimo.com/~scs/C-faq/top.html>
CLC readme: <http://www.ungerhu.com/jxh/clc.welcome.txt>
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[Mark McIntyre]

On Mon, 18 Oct 2004 13:00:00 -0700, in comp.lang.c , Ben Pfaff
<bl*@cs.stanford.edu> wrote:

Mark McIntyre <ma**********@spamcop.net> writes:

On 18 Oct 2004 20:34:44 +0200, in comp.lang.c , Nicolas Pavlidis
<pa****@sbox.tugraz.at> wrote:

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.

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.
--
Mark McIntyre
CLC FAQ <http://www.eskimo.com/~scs/C-faq/top.html>
CLC readme: <http://www.ungerhu.com/jxh/clc.welcome.txt>
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