Re: help needed on stl string ( reserve vs resize )

[Claudio Bley <bley at CS dot uni-magdeburg dot de>]

On Mon, May 17, 2004 at 02:59:19PM +0530, Jyotirmoy Das wrote:
> Hi All,
>   I have the following sample code (try.cpp) which crashes on Linux AS
> 2.1. I am using gcc 2.96. Problem is that after using reserve, capacity
> remains same. As a result, I am not able to the string. 
>   If I use resize in place of reserve, it works fine. Can someone
> explain this behavior? Moreover, I do not want to use resize, as it will
> change the length of string.
> 
> TIA, 
> Jyoti
> 
> ==================================================================
> [jdas@linux2 cpp]$ cat try.cpp
> 
> #include <string>

#include <iostream>

> using namespace std;
> 
> int main()
> {
>   string * m;
>    m = new string();
>   cout << "length=" << m->length() << "  capacity=" << m->capacity() <<
> endl;
>   //m->resize(10); 
>   m->reserve(10);  // In my program, this does not make an impact on 
> 			// capacity 

This is certainly a bug. You should use a GCC release, Redhat's GCC 2.96 is 
known to be buggy and not supported by the GCC team in any way -- you could 
try reporting it to Redhat; maybe they'll fix it. But beware if you're 
going to install a newer GCC: any version < 2.96 or >= 3.x is incompatible 
with your system compiler and libraries regarding C++ support. So, I'd 
recommend installing it side-by-side and not replacing it.

Your program works with GCC 3.4, BTW. Plus 3.4 is better, less buggy, has
a lot more features (like pre-compiled header files), is brand new ;-), 
produces better code and so on...

>   cout << "length=" << m->length() << "  capacity=" << m->capacity() <<
> endl;

>   char * str = const_cast<char *> (m->c_str());
>   *str++ = 'a'; // since my capacity is still the zero, so it dumps core
>   *str++ = 'b';
>   *str++ = 'c';
>   *str = '\0';

Ouch, DONT DO THAT. Writing to a pointer of type const char. This is evil. It can 
break easily. E.g. m->length () is still 0. Why do you use std::string at all? If 
you really want to manipulate a c-string by pointer arithmetic just use a char array.

Or just use:

    m->reserve (10);
    m->append (1, 'a');
    m->append (1, 'b');
    // etc.
    cout << string=" << *m;

> Reading specs from /usr/lib/gcc-lib/i386-redhat-linux/2.96/specs
> gcc version 2.96 20000731 (Red Hat Linux 7.2 2.96-108.1)

It seems you failed to keep your system up-to-date, i.e. installing
all the (security) advisories from Redhat. See https://rhn.redhat.com/errata/RHBA-2002-200.html
and https://rhn.redhat.com/errata/rh72-errata.html and install *all* the packages. Back then 
when I was using Redhat there was an utitilty called up2date IIRC which should ease the 
pain sorting out the dependencies and installing the packages. You could also try upgrading 
your distro; it's rather old, isn't it?!

Cheers.

-- 
Claudio