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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 #1
42 32198
S S wrote:
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.
That will always be undefined behavior, regardless of what you try.
Modifying an object that was declared const is simply a no-no.

I know a way as written below

const char p[] = "hello";
const_cast<char &>(p[0]) = 'K'; //OK
Nope, not OK. It's undefined behavior. It may work on your platform -- for
now.

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.

Best

Kai-Uwe Bux

Oct 3 '06 #2
S S wrote:
Hi Everyone

I have

const char *p = "Hello";

So, here memory is not allocated by C++ compiler for p
p is only a pointer. The compiler does allocate storage for the pointer
itself.
and hence I cannot access p[0] to modify the contents to "Kello"
p[0] = 'K'; // error at runtime
The compiler allocates storage for the string literal, but as a constant. So
you can access it, but you're not allowed to write to it.
So I did

const char *p = new char[6];

But then how do I initialize it to "Hello"?
You can't directly initialize it. You can use strcpy to copy the literal
over to your array, like:

strcpy(p, "Hello");

However, that won't work without a cast unless you remove the 'const'.

Btw: Don't forget to delete your array as soon as you don't need it anymore.
My requirement is - I want a const char* initialised and later want to
modify the contents.
If you want to modify it, don't make it const.
I know a way as written below

const char p[] = "hello";
const_cast<char &>(p[0]) = 'K'; //OK
Why do you make the array const if you want to write to it? Just do:

char p[] = "hello";
p[0] = 'K';
But how to acheive this with pointers?
char p[] = "hello";
char* ptr = p;
ptr[0] = 'K';

But why do you actually want to have a pointer here?
What I know is - C compiler allocates memory when I do
const char* = "hello";

But C++ compiler does not do that.
There is no difference between the way C treats the above line of code and
the way C++ does.

Another question: Why do you use raw pointers to char instead of
std::string?
Oct 3 '06 #3
S S wrote:
>
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
No, that is not correct. First, you need to understand two things.
P is a pointer. It is a variable that holds the address of a char.
The memory for p itself is allocated in the context it was declared
(most likely local to a the function that contains it).

"Hello" in this context is allocated by the implementation at some
unspecified location as a array of (const) chars, this value is
converted to a pointer and stored in p.

First, your error should have happened NOT at runtime. The compiler
should reject at compile time the access violation of storing into
a const char.

Even if you were not to use const here:
char* p = "Hello";
p[0] = 'K';
is undefined behavior. The string literal memory is not allowed
to be changed (the fact the compiler lets this slip is historical).
>
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.
Yoj can't initalialize it, but you can copy into it.
strcpy(p, "hello");
>
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.
Nope, C and C++ actually behave identically in this aspect.
Oct 3 '06 #4
Firstly, and most importantly, in C++ you should be using std::string
to do this kind of stuff. This is especially true if you are having
difficulty with pointers and memory management, as you appear to.

S S wrote:
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
That's an arror at compile time, not runtime. You can't modify p[0] is
because p is a pointer to a const char.
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
That's not OK, that's very bad. If the program appeared to work, it's
only because you were unlucky. If you use const_cast to modify
something that is actually const then you get Undefined Behaviour
(which seems to be considered one of the 4 horsemen of the apocalypse
around these parts). In any case, don't do it.
>
But how to acheive this with pointers?
A couple of ways:

char *p = new char[6];
strncpy(p, "hello", 6);
const char *q = p; // if you really need a const char*
p[0] = 'K';
delete [] p;

or

char p[] = "hello";
const char *q = p; // if you really need a const char*
p[0] = 'K';

But you would be better off with:

std::string hello = "hello";
hello[0] = 'K';

much cleaner, much safer.

Oct 3 '06 #5

S S wrote:
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];
Why you did so?
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.
Why you want to make it const char* when you are going to modify it? If
you want some function to prevent from modifying it, write const char*
in the function signature. It will convert char* to const char*. const
char* (or any other const data member pointer) should be used when you
dont want it to change. Under very special cases, const_cast can be
used to remove const-ness, and don't make it a habit. If you need to
change the data, simply dont use const , and better use std::string
instead of char* .
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.
Neither C do it. None of them allocate memory for pointer. However both
do for the string.
Thanks
SS
Oct 3 '06 #6
S S

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

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];
memcpy(const_ca st<char*>(ptrc) ,"hello",6);
//memcpy((char*)p trc,"hello",6); // this also works
printf("%s\n",p trc); // hello
const_cast<char &>(ptrc[0]) = 'K'; //Kello

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???
Is my compiler wrong?

Thanks
SS

Oct 3 '06 #7
S S

Ron Natalie wrote:
S S wrote:

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

No, that is not correct. First, you need to understand two things.
P is a pointer. It is a variable that holds the address of a char.
The memory for p itself is allocated in the context it was declared
(most likely local to a the function that contains it).

"Hello" in this context is allocated by the implementation at some
unspecified location as a array of (const) chars, this value is
converted to a pointer and stored in p.

First, your error should have happened NOT at runtime. The compiler
should reject at compile time the access violation of storing into
a const char.

Even if you were not to use const here:
char* p = "Hello";
p[0] = 'K';
Actually that is what I meant , I put const there by mistake.
is undefined behavior. The string literal memory is not allowed
to be changed (the fact the compiler lets this slip is historical).
If we can not do p[0] = 'K'; in above case
then what would be the difference b/w
const char* p = "hello";
and
char* p ="hello";
both does not allow writing operation.

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.

Yoj can't initalialize it, but you can copy into it.
strcpy(p, "hello");
I can not copy into it as it is const memory. I have to do typecast as
I did in one of my later mails.
>

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.
Nope, C and C++ actually behave identically in this aspect.
Oct 3 '06 #8
S S

Pete C wrote:
Firstly, and most importantly, in C++ you should be using std::string
to do this kind of stuff. This is especially true if you are having
difficulty with pointers and memory management, as you appear to.

S S wrote:
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

That's an arror at compile time, not runtime. You can't modify p[0] is
because p is a pointer to a const char.
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

That's not OK, that's very bad. If the program appeared to work, it's
only because you were unlucky. If you use const_cast to modify
something that is actually const then you get Undefined Behaviour
(which seems to be considered one of the 4 horsemen of the apocalypse
around these parts). In any case, don't do it.
I think you are wrong here, when you see sizeof(p) here you will see
the size of string which means memory is allocated here and can always
be modified. IF I can not use const_cast which is actually const then
what is purpose of const_cast then.

But how to acheive this with pointers?

A couple of ways:

char *p = new char[6];
strncpy(p, "hello", 6);
const char *q = p; // if you really need a const char*
p[0] = 'K';
delete [] p;

or

char p[] = "hello";
const char *q = p; // if you really need a const char*
p[0] = 'K';

But you would be better off with:

std::string hello = "hello";
hello[0] = 'K';

much cleaner, much safer.
Oct 3 '06 #9
S S wrote:
Ron Natalie wrote:
>S S wrote:
>>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
No, that is not correct. First, you need to understand two things.
P is a pointer. It is a variable that holds the address of a char.
The memory for p itself is allocated in the context it was declared
(most likely local to a the function that contains it).

"Hello" in this context is allocated by the implementation at some
unspecified location as a array of (const) chars, this value is
converted to a pointer and stored in p.

First, your error should have happened NOT at runtime. The compiler
should reject at compile time the access violation of storing into
a const char.

Even if you were not to use const here:
char* p = "Hello";
p[0] = 'K';

Actually that is what I meant , I put const there by mistake.
>is undefined behavior. The string literal memory is not allowed
to be changed (the fact the compiler lets this slip is historical).

If we can not do p[0] = 'K'; in above case
then what would be the difference b/w
const char* p = "hello";
and
char* p ="hello";
both does not allow writing operation.
One would be a compile time error, and the other might be a runtime error.

Just make a non-const array, and initialize it.
char p[] = "hello";
Or if you did want to allocate storage yourself, you need to copy
the string into it afterwards.
char *p = new char[6];
strcpy(p,"hello ");

(there are better C++ ways of doing what you need to do, likely.)
Oct 3 '06 #10

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