Casting from const pair<const unsigned char, size_t> to constpair<unsigned char, size_t>

**=?UTF-8?B?RXJpayBXaWtzdHLDtm0=?=** · Oct 12 '08, 12:25 PM

Re: Casting from const pair<const unsigned char*, size_t>* to constpair<un signed char*, size_t>*

On 2008-10-12 13:16, Alex Vinokur wrote:

Hi,
>
Is it possible to do C++-casting from
const pair<const unsigned char*, size_t>*
to
const pair<unsigned char*, size_t>*
?

const_cast might work, but I don't think this counts as casting away
constness, you should probably use reinterpret_cas t.

--
Erik WikstrÃ¶m

**Barry** · Oct 12 '08, 12:25 PM

Re: Casting from const pair<const unsigned char*, size_t>* to constpair<un signed char*, size_t>*

On Oct 12, 7:16 pm, Alex Vinokur <ale...@users.s ourceforge.netw rote:

Hi,
>
Is it possible to do C++-casting from
const pair<const unsigned char*, size_t>*
to
const pair<unsigned char*, size_t>*
?
>

const pair<const unsigned char*, size_t>* p1 = 0;
const pair<unsigned char*, size_t>* p2 =
reinterpret_cas t<const pair<unsigned char*, size_t>*>(p1);

but reinterpre_cast should be avoided if possible.
*Practically*, I wonder you can just do it this way:

char* s = const_cast<char *>(p1->first);

Or can you tell me the scenario you are in?

--
Best Regards
Barry

**Alex Vinokur** · Oct 12 '08, 01:15 PM

Re: Casting from const pair<const unsigned char*, size_t>* to constpair<un signed char*, size_t>*

On Oct 12, 2:22 pm, Barry <dhb2...@gmail. comwrote:

On Oct 12, 7:16 pm, Alex Vinokur <ale...@users.s ourceforge.netw rote:
>

Hi,

>

Is it possible to do C++-casting from
const pair<const unsigned char*, size_t>*
to
const pair<unsigned char*, size_t>*
?

>
const pair<const unsigned char*, size_t>* p1 = 0;
const pair<unsigned char*, size_t>* p2 =
reinterpret_cas t<const pair<unsigned char*, size_t>*>(p1);
>
but reinterpre_cast should be avoided if possible.
*Practically*, I wonder you can just do it this way:
>
char* s = const_cast<char *>(p1->first);
>
Or can you tell me the scenario you are in?
>

I have function foo1 (const pair<unsigned char*, size_t>* p);
I need also function foo2 (const pair<const unsigned char*, size_t>*
p) that does the same thing as foo1().

Currently
void foo2 (const pair<const unsigned char*, size_t>* p)
{
// I would like to use here C++-style casting
const pair<unsigned char*, size_t>* p1 = ( const pair<unsigned
char*, size_t>* ) p;
foo1(p1);
}

Alex Vinokur

**Sam** · Oct 12 '08, 01:15 PM

Re: Casting from const pair<const unsigned char*, size_t>* to constpair<un signed char*, size_t>*

Alex Vinokur writes:

Hi,
>
Is it possible to do C++-casting from
const pair<const unsigned char*, size_t>*
to
const pair<unsigned char*, size_t>*
?

Yes, if you write your own conversion function. Although, if you are
attempting to do something like this, then it's fairly likely that whatever
you're really trying to do, you're doing it the wrong way.

-----BEGIN PGP SIGNATURE-----
Version: GnuPG v1.4.9 (GNU/Linux)

iEYEABECAAYFAkj x9tEACgkQx9p3GY HlUOKbtQCdHLZUG km+k5aHKdQtEiQt cc3o
vaMAniIYUdDc0XE sfWITCcbqSe9GLs me
=goWJ
-----END PGP SIGNATURE-----

**Barry** · Oct 12 '08, 01:55 PM

Re: Casting from const pair<const unsigned char*, size_t>* to constpair<un signed char*, size_t>*

On Oct 12, 9:05 pm, Alex Vinokur <ale...@users.s ourceforge.netw rote:

On Oct 12, 2:22 pm, Barry <dhb2...@gmail. comwrote:
>
>
>

On Oct 12, 7:16 pm, Alex Vinokur <ale...@users.s ourceforge.netw rote:

>

Hi,

>

Is it possible to do C++-casting from
const pair<const unsigned char*, size_t>*
to
const pair<unsigned char*, size_t>*
?

>

const pair<const unsigned char*, size_t>* p1 = 0;
const pair<unsigned char*, size_t>* p2 =
reinterpret_cas t<const pair<unsigned char*, size_t>*>(p1);

>

but reinterpre_cast should be avoided if possible.
*Practically*, I wonder you can just do it this way:

>

char* s = const_cast<char *>(p1->first);

>

Or can you tell me the scenario you are in?

>
I have function foo1 (const pair<unsigned char*, size_t>* p);
I need also function foo2 (const pair<const unsigned char*, size_t>*
p) that does the same thing as foo1().
>
Currently
void foo2 (const pair<const unsigned char*, size_t>* p)
{
// I would like to use here C++-style casting
const pair<unsigned char*, size_t>* p1 = ( const pair<unsigned
char*, size_t>* ) p;
foo1(p1);
>
}
>

I'm sorry that my previous post misled you.
I think sam got my answer.

What I asked is that I was confused that why you need
such conversion. And I was expecting to see if there's some
way to avoid such conversion.

If you just wanted to learn the language. OK, "reinterpret_ca st"
as C++-style cast, or just use C-style cast. While in practice,
avoid doing this.

--
Best Regards
Barry

**peter koch** · Oct 12 '08, 02:15 PM

Re: Casting from const pair<const unsigned char*, size_t>* to constpair<un signed char*, size_t>*

On 12 Okt., 13:16, Alex Vinokur <ale...@users.s ourceforge.netw rote:

Hi,
>
Is it possible to do C++-casting from
const pair<const unsigned char*, size_t>*
to
const pair<unsigned char*, size_t>*
?
>
Alex Vinokur

Only a reinterpret_cas t works: the two types are unrelated. Why do you
want to do this? It would be better to fix your design instead.

/Peter

**James Kanze** · Oct 13 '08, 09:35 AM

Re: Casting from const pair<const unsigned char*, size_t>* to constpair<un signed char*, size_t>*

On Oct 12, 4:08 pm, peter koch <peter.koch.lar ...@gmail.comwr ote:

On 12 Okt., 13:16, Alex Vinokur <ale...@users.s ourceforge.netw rote:

Is it possible to do C++-casting from
const pair<const unsigned char*, size_t>*
to
const pair<unsigned char*, size_t>*
?

Only a reinterpret_cas t works: the two types are unrelated.
Why do you want to do this? It would be better to fix your
design instead.

Reinterpret_cas t doesn't work. You can't do a reinterpret_cas t
to or from a user defined type, and instantiations of std::pair
are considered user defined types.

What he can do is construct a new std::pair, e.g.:

std::pair< unsigned char*, size_t const p2
= std::make_pair(
const_cast< unsigned char* >( p1.first ),
p2.second ) ;

--
James Kanze (GABI Software) email:james.kan ze@gmail.com
Conseils en informatique orientée objet/
Beratung in objektorientier ter Datenverarbeitu ng
9 place Sémard, 78210 St.-Cyr-l'École, France, +33 (0)1 30 23 00 34

**Vidar Hasfjord** · Oct 13 '08, 09:35 AM

Re: Casting from const pair<const unsigned char*, size_t>* to constpair<un signed char*, size_t>*

On Oct 12, 2:05 pm, Alex Vinokur <ale...@users.s ourceforge.netw rote:

I have function foo1 (const pair<unsigned char*, size_t>* p);
I need also function foo2 (const pair<const unsigned char*, size_t>*
p) that does the same thing as foo1().

template <class T>
void foo_impl (const pair <T, size_t>* p) {...}

typedef pair <const unsigned char*, size_tpair1;
typedef pair <unsigned char*, size_tpair2;

void foo1 (const pair1* p) {foo_impl (p);}
void foo2 (const pair2* p) {foo_impl (p);}

Regards,
Vidar Hasfjord

**=?UTF-8?B?RXJpayBXaWtzdHLDtm0=?=** · Oct 13 '08, 03:55 PM

Re: Casting from const pair<const unsigned char*, size_t>* to constpair<un signed char*, size_t>*

On 2008-10-13 11:27, James Kanze wrote:

On Oct 12, 4:08 pm, peter koch <peter.koch.lar ...@gmail.comwr ote:

>On 12 Okt., 13:16, Alex Vinokur <ale...@users.s ourceforge.netw rote:

Is it possible to do C++-casting from
const pair<const unsigned char*, size_t>*
to
const pair<unsigned char*, size_t>*
?

>

>Only a reinterpret_cas t works: the two types are unrelated.
>Why do you want to do this? It would be better to fix your
>design instead.

>
Reinterpret_cas t doesn't work. You can't do a reinterpret_cas t
to or from a user defined type, and instantiations of std::pair
are considered user defined types.

He wanted to cast from a pointer to a user defined type to a pointer to
another user defined type, which is allowed.

--
Erik WikstrÃ¶m

**James Kanze** · Oct 13 '08, 05:15 PM

Re: Casting from const pair<const unsigned char*, size_t>* to constpair<un signed char*, size_t>*

On Oct 13, 5:48 pm, Erik Wikström <Erik-wikst...@telia. comwrote:

On 2008-10-13 11:27, James Kanze wrote:
>

On Oct 12, 4:08 pm, peter koch <peter.koch.lar ...@gmail.comwr ote:

On 12 Okt., 13:16, Alex Vinokur <ale...@users.s ourceforge.netw rote:
Is it possible to do C++-casting from
const pair<const unsigned char*, size_t>*
to
const pair<unsigned char*, size_t>*
?

Only a reinterpret_cas t works: the two types are unrelated.
Why do you want to do this? It would be better to fix your
design instead.

Reinterpret_cas t doesn't work. You can't do a reinterpret_cas t
to or from a user defined type, and instantiations of std::pair
are considered user defined types.

He wanted to cast from a pointer to a user defined type to a
pointer to another user defined type, which is allowed.

Yep. I missed the trailing * in the original posting. In that
case, you can use reinterpret_cas t to replace a compile time
error with runtime undefined behavior. Otherwise, you do need
to create a new object, using the technique I described.

(Also, it seems sort of strange to have pointers to an
std::pair. The object definitely has value semantics.)

--
James Kanze (GABI Software) email:james.kan ze@gmail.com
Conseils en informatique orientée objet/
Beratung in objektorientier ter Datenverarbeitu ng
9 place Sémard, 78210 St.-Cyr-l'École, France, +33 (0)1 30 23 00 34

Casting from const pair<const unsigned char, size_t> to constpair<unsigned char, size_t>