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const char* = new char[6]

S S
Hi Everyone

I have

const char *p = "Hello";

So, here memory is not allocated by C++ compiler for p and hence I
cannot access p[0] to modify the contents to "Kello"
p[0] = 'K'; // error at runtime

So I did

const char *p = new char[6];

But then how do I initialize it to "Hello"? My requirement is - I want
a const char* initialised and later want to modify the contents.

I know a way as written below

const char p[] = "hello";
const_cast<char &>(p[0]) = 'K'; //OK

But how to acheive this with pointers?

What I know is - C compiler allocates memory when I do
const char* = "hello";

But C++ compiler does not do that. Any help is welcome.

Thanks
SS

Oct 3 '06
42 32208
S S posted:
>This behaviour of this statement is well-defined, as it does not modify
const data.

Yes, but it would have been undefined if I would have written
char const *p = new char const[6];

You are correct.

Can you please give 2 syntaxes in the given context where
we strip away constness

First of all, let's start off with guinea pig: a const pointer to a const
int:

int const *const p;
const int *const p; /* These two are the same */

1- for pointer itself is const

This would only make sense if you want to yield an L-value, so I will cast
to a reference type. (Unless you cast to a reference type, a cast always
yields an R-value in C++.)

const_cast<int const*&>(p)

2- data is const

To yield an R-value: const_cast<int* >(p)

or,

To yield an L-value: const_cast<int* const&>(p)

(Not that I didn't write const_cast<int* constfor the first one -- reason
being that it would have been redundant because the cast yields an R-
value.)

3- both
Yield an L-value:

const_cast<int* &>(p)

--

Frederick Gotham
Oct 3 '06 #31
S S posted:
Incorrect, type of "hello" is const char[]
that can be confirmed when you overload the function
void func(char* str); //1st fxn
void func(const char* str); //2nd fxn
func("hello"); // calls the 2nd fxn
That is indeed quite a funky example... I've got a question for comp.std.c++.

--

Frederick Gotham
Oct 3 '06 #32
S S

Frederick Gotham wrote:
S S posted:
This behaviour of this statement is well-defined, as it does not modify
const data.
Yes, but it would have been undefined if I would have written
char const *p = new char const[6];


You are correct.

Can you please give 2 syntaxes in the given context where
we strip away constness


First of all, let's start off with guinea pig: a const pointer to a const
int:

int const *const p;
const int *const p; /* These two are the same */

1- for pointer itself is const


This would only make sense if you want to yield an L-value, so I will cast
to a reference type. (Unless you cast to a reference type, a cast always
yields an R-value in C++.)

const_cast<int const*&>(p)

2- data is const


To yield an R-value: const_cast<int* >(p)

or,

To yield an L-value: const_cast<int* const&>(p)

(Not that I didn't write const_cast<int* constfor the first one -- reason
being that it would have been redundant because the cast yields an R-
value.)

3- both

Yield an L-value:

const_cast<int* &>(p)
Bingo!!!
Thanks for explanation, that is what I was most confused about.

--

Frederick Gotham
Oct 3 '06 #33
benben schrieb:
Either do

char p[] = {'h', 'e', 'l', 'l', 'o', 0};
p[0] = 'K'; // OK
Same as this (which is shorter & easier to read):
char p[] = "hello";
p[0] = 'K';
or

char* p = new char[6];

if (p != 0)
{
No need to check the pointer. It can't be null here.
strcpy(p, "hello");
p[0] = 'K'; // OK
delete[] p;
}
Or even better, use std::string

std::string str = "Hello";
str[0] = 'K'; // also ok

Notice that any of the examples (especially the last one) are elegant,
and above all, correct, compared to your solutions with const_cast.
If the OP wants a constant string:

const std::string str = "Hello";

He should start using the C++ features (strings and IO-streams) and come
back to plain array and pointers only when he really needs the better
performance.

--
Thomas
http://www.netmeister.org/news/learn2quote.html
Oct 3 '06 #34
S S

Frederick Gotham wrote:
Rolf Magnus posted:
You would be correct to think that the altering of a string literal
produces undefined behaviour, but nonetheless, string literals are not
const.
I'll answer that with a quote from the standard:

2.13.4 String literals

[...]
An ordinary string literal has type "array of n const char" and
static storage duration (3.7), where n is the size of the string as
defined below, and is initialized with the given characters.


I stand corrected.

void Func(char (&str)[6]) {}
Put const and compile error will go away
void Func(const char (&str)[6]) {}
I did not get what you actually wanted to say here.
>
int main()
{
Func("Hello"); /* Compile ERROR */
}

--

Frederick Gotham
Oct 4 '06 #35
S S

Frederick Gotham wrote:
S S posted:
I am sorry, please read my first line as
char * p = "hello";
in my previous mail


Extremely il-advised. You're storing the address of non-modifiable data in a
pointer to non-const.

But I am able to get the desired result by the following way , but I am
amazed how it has worked?

const char* ptrc = new char[6];


Here you store the address of non-const data in a pointer to const. Be
consistent! Either use:

char const *const p = new char const[6];
This is not compiling, saying uninitialized const in `new' of `const
char'
How to initialize it
If I give
char const *const p = new char const[6]("hello");
It compiles fine but if I try to print p, it does not show value hello,
jus blank line
Any idea Frederick?
>
or:

char *const p = new char[6];

memcpy(const_ca st<char*>(ptrc) ,"hello",6);


This behaviour of this statement is well-defined, as it does not modify const
data.

//memcpy((char*)p trc,"hello",6); // this also works


Yes, this is equivalent.

printf("%s\n",p trc); // hello


You're mixing C and C++ all over the place! If you're hell-bent on using C
functions in C++ code, you must change:

#include <stdio.h>

printf(...

to:

#include <cstdio>

std::printf(...

const_cast<char &>(ptrc[0]) = 'K'; //Kello


Again, the behaviour is well-defined because the data is ours to modify.

My question is "How I am able to modify the constness of memory by
using 2nd statement which actually is supposed to remove the constness
of pointers only???


Your question is flawed. The following denotes a const pointer:

char *const p;

The following two denote a pointer to const:

char const *p;
const char *p;

The following two denote a const pointer to const:

char const *const p;
const char *const p;

A "const_cast " can be used to strip away either of the constnesses (i.e.
whether the pointer itself is const, or whether the data it points to may be
modified by the pointer in question.)
Is my compiler wrong?

You'll get your head around all this soon enough. Keep asking questions until
you're absolutely certain you know what's going on -- that's what sets the
good programmers from the great programmers.

--

Frederick Gotham
Oct 4 '06 #36
S S wrote:
>
Frederick Gotham wrote:
>Rolf Magnus posted:
>You would be correct to think that the altering of a string literal
produces undefined behaviour, but nonetheless, string literals are not
const.

I'll answer that with a quote from the standard:

2.13.4 String literals

[...]
An ordinary string literal has type "array of n const char" and
static storage duration (3.7), where n is the size of the string as
defined below, and is initialized with the given characters.


I stand corrected.

void Func(char (&str)[6]) {}

Put const and compile error will go away
Yes. That was the point.
void Func(const char (&str)[6]) {}
I did not get what you actually wanted to say here.
He wanted to say that string literals are const. The fact that the above
fails without const proves it.
> int main()
{
Func("Hello"); /* Compile ERROR */
}

--

Frederick Gotham
Oct 4 '06 #37
S S posted:
> char const *const p = new char const[6];

This is not compiling, saying uninitialized const in `new' of `const
char' How to initialize it If I give char const *const p = new char
const[6]("hello"); It compiles fine but if I try to print p, it does not
show value hello, jus blank line Any idea Frederick?

Wups, you're right, you must initialise a const object:

int main()
{
int const i; /* Compile ERROR */
}

The only way in which you can initialise an array when using new is to
default-initialise it, which is done as follows:

int *p = new int[3]();

If you stick anything inside those brackets, you've got a syntax error. If
your compiler allows it, then it's either broken or possibly has some sort
of non-Standard extension enabled.

Please do more snipping in future when replying.

--

Frederick Gotham
Oct 4 '06 #38
S S

Frederick Gotham wrote:
S S posted:
char const *const p = new char const[6];
This is not compiling, saying uninitialized const in `new' of `const
char' How to initialize it If I give char const *const p = new char
const[6]("hello"); It compiles fine but if I try to print p, it does not
show value hello, jus blank line Any idea Frederick?


Wups, you're right, you must initialise a const object:

int main()
{
int const i; /* Compile ERROR */
}

The only way in which you can initialise an array when using new is to
default-initialise it, which is done as follows:

int *p = new int[3]();
Even if you do not put brackets () and write
int *p = new int[3];
then also it does the default initialisation for all 3 members of
array, any significance of brackets?
Thanks
>
If you stick anything inside those brackets, you've got a syntax error. If
your compiler allows it, then it's either broken or possibly has some sort
of non-Standard extension enabled.

Please do more snipping in future when replying.

--

Frederick Gotham
Oct 5 '06 #39
S S

Frederick Gotham wrote:
S S posted:
char const *const p = new char const[6];
This is not compiling, saying uninitialized const in `new' of `const
char' How to initialize it If I give char const *const p = new char
const[6]("hello"); It compiles fine but if I try to print p, it does not
show value hello, jus blank line Any idea Frederick?


Wups, you're right, you must initialise a const object:

int main()
{
int const i; /* Compile ERROR */
}

The only way in which you can initialise an array when using new is to
default-initialise it, which is done as follows:

int *p = new int[3]();

If you stick anything inside those brackets, you've got a syntax error. If
your compiler allows it, then it's either broken or possibly has some sort
of non-Standard extension enabled.
You are wrong here
If we stick inside those brackets, the corrosponding ctor will be
called. Example is pasted below.
#include<iostre am>

class A {
public:
A() {a = 10;}
A(int b) {a = b;}
void dis() const { printf("%d\n",a );}
private:
int a;
};

int main()
{
A const* p = new A const[3](5); // not a default ctor
p->dis();
(p+1)->dis();
(p+2)->dis();
(*p).dis();
(*(p+1)).dis();
(*(p+2)).dis();
p[0].dis();
p[1].dis();
p[2].dis();
return 0;
}

Output will be
5
5
5
5
5
5
5
5
5
>
Please do more snipping in future when replying.

--

Frederick Gotham
Oct 5 '06 #40

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