Assigning values via pointers in C/C++? (Visual Studio.NET 2003)

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Usenet User - 09 Nov 2007 20:32 GMT

I am using Visual Studio.NET 2003, and to my surprise, the below code
in a simple console application does not work for me:

char *str = "hello!";
*str = 'H'; <-- error
str[0] = 'H'; <-- error

The above code compiles just fine, but I am getting runtime errors.
If managed extensions are used, I'm getting "Objest reference is not
set to an object." If I disable managed extensions, I get "Access
violation" errors.

Any suggestions?

TIA!

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John Hensley - 09 Nov 2007 21:18 GMT

The string "Hello!" is a const value that is located in a read-only area of
memory. When you try to change the first character of the string with *str =
'H' and str[0]=h you are attempting to overwrite read-only memory and that is
why you are getting an exception.

Use something like this if you want to be able to overwrite your string.

char *tmp[] = "Hello!"
char *str = tmp;
*str = 'H';
str[0] = 'H';

Signature

John Hensley
www.resqware.com

> I am using Visual Studio.NET 2003, and to my surprise, the below code
> in a simple console application does not work for me:
[quoted text clipped - 11 lines]
>
> TIA!

Reply to this Message

John Hensley - 09 Nov 2007 21:25 GMT

char *tmp[] = "Hello!"

should have been

char tmp[] = "Hello!"

Signature

John Hensley
www.resqware.com

> The string "Hello!" is a const value that is located in a read-only area of
> memory. When you try to change the first character of the string with *str =
[quoted text clipped - 23 lines]
> >
> > TIA!

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Larry Smith - 09 Nov 2007 21:31 GMT

> char *tmp[] = "Hello!"
>
> should have been
>
> char tmp[] = "Hello!"

You don't need "tmp". This will do fine:

char str[] = "Hello!";
*str = 'H';
str[0] = 'H';

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Usenet User - 09 Nov 2007 23:57 GMT

>> char *tmp[] = "Hello!"
>>
[quoted text clipped - 7 lines]
>*str = 'H';
>str[0] = 'H';

Doh!

Thanks , everyone! I suspected something like this was going on...

-UU

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John Hensley - 09 Nov 2007 21:27 GMT

The line:
char *tmp[] = "Hello!"
should be:
char tmp[] = "Hello!"

Signature

John Hensley
www.resqware.com

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Larry Smith - 09 Nov 2007 21:19 GMT

>I am using Visual Studio.NET 2003, and to my surprise, the below code
> in a simple console application does not work for me:
[quoted text clipped - 7 lines]
> set to an object." If I disable managed extensions, I get "Access
> violation" errors.

You can't assign to a string literal. It's type is "const char[]". Your
declaration should therefore be:

const char *str = "hello!";

The "const" can be omitted for backwards compatibility with C but it's still
illegal to write to this space. You should therefore specify the "const". If
you wnat to modify it then copy it instead:

char str[] = "hello!";

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David Wilkinson - 09 Nov 2007 22:06 GMT

> I am using Visual Studio.NET 2003, and to my surprise, the below code
> in a simple console application does not work for me:
[quoted text clipped - 9 lines]
>
> Any suggestions?

UU:

When you write

char *str = "hello!";

the string is (or at least may be) in read-only memory. That is why you
get a runtime error when you try to change it. In fact in C++ you should
write

const char *str = "hello!";

and then

*str = 'H';

will fail to compile. The form

char *str = "hello!";

is only allowed to provide compatibility with legacy C code. Don't do it!

Signature

David Wilkinson
Visual C++ MVP

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Assigning values via pointers in C/C++? (Visual Studio.NET 2003)

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