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

P: n/a
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
Oct 12 '08 #1
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10 Replies


P: n/a
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_cast.

--
Erik Wikström
Oct 12 '08 #2

P: n/a
On Oct 12, 7:16*pm, Alex Vinokur <ale...@users.sourceforge.netwrote:
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_cast<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
Oct 12 '08 #3

P: n/a
On Oct 12, 2:22*pm, Barry <dhb2...@gmail.comwrote:
On Oct 12, 7:16*pm, Alex Vinokur <ale...@users.sourceforge.netwrote:
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_cast<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
Oct 12 '08 #4

P: n/a
Sam
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.

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Oct 12 '08 #5

P: n/a
On Oct 12, 9:05*pm, Alex Vinokur <ale...@users.sourceforge.netwrote:
On Oct 12, 2:22*pm, Barry <dhb2...@gmail.comwrote:
On Oct 12, 7:16*pm, Alex Vinokur <ale...@users.sourceforge.netwrote:
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_cast<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_cast"
as C++-style cast, or just use C-style cast. While in practice,
avoid doing this.

--
Best Regards
Barry

Oct 12 '08 #6

P: n/a
On 12 Okt., 13:16, Alex Vinokur <ale...@users.sourceforge.netwrote:
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_cast works: the two types are unrelated. Why do you
want to do this? It would be better to fix your design instead.

/Peter
Oct 12 '08 #7

P: n/a
On Oct 12, 4:08 pm, peter koch <peter.koch.lar...@gmail.comwrote:
On 12 Okt., 13:16, Alex Vinokur <ale...@users.sourceforge.netwrote:
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_cast works: the two types are unrelated.
Why do you want to do this? It would be better to fix your
design instead.
Reinterpret_cast doesn't work. You can't do a reinterpret_cast
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:ja*********@gmail.com
Conseils en informatique orientée objet/
Beratung in objektorientierter Datenverarbeitung
9 place Sémard, 78210 St.-Cyr-l'École, France, +33 (0)1 30 23 00 34
Oct 13 '08 #8

P: n/a
On Oct 12, 2:05*pm, Alex Vinokur <ale...@users.sourceforge.netwrote:
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
Oct 13 '08 #9

P: n/a
On 2008-10-13 11:27, James Kanze wrote:
On Oct 12, 4:08 pm, peter koch <peter.koch.lar...@gmail.comwrote:
>On 12 Okt., 13:16, Alex Vinokur <ale...@users.sourceforge.netwrote:
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_cast works: the two types are unrelated.
Why do you want to do this? It would be better to fix your
design instead.

Reinterpret_cast doesn't work. You can't do a reinterpret_cast
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 Wikström
Oct 13 '08 #10

P: n/a
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.comwrote:
On 12 Okt., 13:16, Alex Vinokur <ale...@users.sourceforge.netwrote:
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_cast works: the two types are unrelated.
Why do you want to do this? It would be better to fix your
design instead.
Reinterpret_cast doesn't work. You can't do a reinterpret_cast
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_cast 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:ja*********@gmail.com
Conseils en informatique orientée objet/
Beratung in objektorientierter Datenverarbeitung
9 place Sémard, 78210 St.-Cyr-l'École, France, +33 (0)1 30 23 00 34
Oct 13 '08 #11

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