converting a char* to a const char*

[Sean Farrow]

Hi:
What is best and safest way of converting a char* to a const char *? Can I
use const_cast?
Cheers
Sean.

[Pete Becker]

On 2008-08-06 13:47:32 -0400, kwikius <an**@servocomm .freeserve.co.u ksaid:

On Aug 6, 3:10Â*pm, "Sean Farrow" <sean.far...@se anfarrow.co.ukw rote:

>Hi:
it's a function call needing a const char*, I assume from your descriptio

n,

>then const_cast can be used.

Nope! Dont bother with it. make a copy of the string or use
std::string.

Wait, I'm confused. If the situation is this:

void f(const char *);

void g(char *p)
{
// how to call f with the character array that p points to?
}

Then the answer is far simpler: just do it. f(p) relies on the implicit
conversion of char* to const char*. No need for const_case, copying, or
std::string.

Of course, if the situation is something different, then the answer is
almost certainly different. <g>

--
Pete
Roundhouse Consulting, Ltd. (www.versatilecoding.com) Author of "The
Standard C++ Library Extensions: a Tutorial and Reference
(www.petebecker.com/tr1book)

[kwikius]

On Aug 6, 7:03*pm, Pete Becker <p...@versatile coding.comwrote :

On 2008-08-06 13:47:32 -0400, kwikius <a...@servocomm .freeserve.co.u ksaid:

On Aug 6, 3:10*pm, "Sean Farrow" <sean.far...@se anfarrow.co.ukw rote:

Hi:
it's a function call needing a const char*, I assume from your descriptio

n,

then const_cast can be used.

Nope! Dont bother with it. make a copy of the string or use
std::string.

Wait, I'm confused. If the situation is this:

void f(const char *);

void g(char *p)
{
* * * * // how to call f with the character array that p points to?

}

Then the answer is far simpler: just do it. f(p) relies on the implicit
conversion of char* to const char*. No need for const_case, copying, or
std::string.

hmm.. Apologies to the O.P. I wasnt aware that const_cast can be used
to cast to const. And rereading the post it seems that what he wanted.

The idea that its ok to cast away const in general use with const_cast
is bad. From time to time one gets an error message related to trying
to use a value that is passed as a constant ref sa non const. In this
situation one temptation is to use const_cast. However the real answer
is to try to find out what the problem is with the design or your
understanding of the function.

In writing a function Since the user of the function doesnt expect
the value to be changed in the function, there is no reason to use
const_cast. If you are going to write to a reference then make that
clear.

Problem is that const_cast sometimes looks like a nice easy solution
to problems, which it isnt.

regards
Andy Little

[Pete Becker]

On 2008-08-06 14:19:43 -0400, kwikius <an**@servocomm .freeserve.co.u ksaid:

>
Problem is that const_cast sometimes looks like a nice easy solution
to problems, which it isnt.

Agreed. It's an advanced feature, and beginners shouldn't use it.

--
Pete
Roundhouse Consulting, Ltd. (www.versatilecoding.com) Author of "The
Standard C++ Library Extensions: a Tutorial and Reference
(www.petebecker.com/tr1book)

[puzzlecracker]

>const_cast can be used to add orremove const;

How can const_cast add constantness? I was under impression that it
only remove constantness if object is "not really" constant.

[puzzlecracker]

>
For first-level const. *You can't pass a char** to a function
expecting a char const**.

First is the first level const?

[Ian Collins]

puzzlecracker wrote:

>const_cast can be used to add orremove const;

How can const_cast add constantness? I was under impression that it
only remove constantness if object is "not really" constant.

int n;
int* p = &n;
const int* cp = const_cast<cons t int*>(p);

The const_cast is superfluous here, but legal. I'm sure someone can
contrive a template example where it would be required!

--
Ian Collins.

[Greg Comeau]

In article <38************ *************** *******@k30g200 0hse.googlegrou ps.com>,
puzzlecracker <ir*********@gm ail.comwrote:

const_cast can be used to add orremove const;

How can const_cast add constantness? I was under impression that it
only remove constantness if object is "not really" constant.

Probbaly where I've seen it pop up most is in changing the
type of the "this" pointer to force a const overloaded function
to be called instead of the non-const one.

Usual caveats apply: consider using it this way wisely,
consider any other alternatives, blah blah.

Also, I forget if using it this way is undefined behavior.
--
Greg Comeau / 4.3.10.1 with C++0xisms now in beta!
Comeau C/C++ ONLINE == http://www.comeaucomputing.com/tryitout
World Class Compilers: Breathtaking C++, Amazing C99, Fabulous C90.
Comeau C/C++ with Dinkumware's Libraries... Have you tried it?

[Greg Comeau]

In article <ed************ *************** *******@2g2000h sn.googlegroups .com>,
puzzlecracker <ir*********@gm ail.comwrote:

>For first-level const. =A0You can't pass a char** to a function
expecting a char const**.

First is the first level const?

Dunno if this helps: http://www.comeaucomputing.com/techtalk/#deconstutoh
--
Greg Comeau / 4.3.10.1 with C++0xisms now in beta!
Comeau C/C++ ONLINE == http://www.comeaucomputing.com/tryitout
World Class Compilers: Breathtaking C++, Amazing C99, Fabulous C90.
Comeau C/C++ with Dinkumware's Libraries... Have you tried it?

[Juha Nieminen]

Sean Farrow wrote:

I've solved the issue, it was const_cast I needed, that's going to teach me
to rad error info properly.

Now that you have resolved the problem, could you also explain to us
the situation where you needed to use const_cast to *add* constness
(instead of removing it) so that also we can learn from it?

(Somehow I have the feeling that the const_cast is *not* the real
solution to your problem. It just happens to work, and masks the real
problem.)

[Juha Nieminen]

James Kanze wrote:

>You have got it backwards: const_cast is used to *remove*
constness, not to add it.

Usually. It can be used either way, and there are times when
you want to explicitly add the const, to control overload
resolution, or template instantiation, for example.

Isn't that what static_cast is for? const_cast sounds like the wrong
tool for the job (because common C++ wisdom dictates that static_cast is
good and const_cast should usually be avoided if possible).

You may as well say "if you want to add constness eg. for overload
resolution, use reinterpret_cas t". That's just wrong.