NetNews Offline 2/NetNews Offline Volume 2.iso/news/comp/lang/c++-part1/2804

Text File | 1996-08-06 | 3.3 KB | 89 lines

Path: atglab.bls.com!Alun.Champion
From: [e-mail] (Alun Champion)
Newsgroups: comp.lang.c++
Subject: Re: [Q] Dynamically allocating memory for a char*
Date: 19 Jan 1996 20:50:29 GMT
Organization: Computer People Inc.
Message-ID: <[e-mail]>
References: <4dmn1i$[e-mail]>
    <[e-mail]>
NNTP-Posting-Host: bstfirewall.bst.bls.com
In-reply-to: [e-mail]'s message of 19 Jan 1996 17:50:12 GMT

In article <[e-mail]> [e-mail] (Nitin More [CONTRACTOR]) writes:

: [original attribution not included in response]
: > I have a question about allocating memory for a string as it's passed
: > in by the user.  I'm using cin to get the info, but maybe this is
: > inappropriate.  I have included the relevant snippets of the file
: > below.  What I am trying to achieve is no limitation for the user when
: > they input a path + filename.  Is it possible to use char* foo and
: > then something like (I know this is wrong, but the idea of it ...)

: To read a string from cin, you need a place to store it (char *) with enough
: memory allocated.  Until you read the string, you won't know what is enough.
: This is a catch-22.  The best you can do is use a temporary variable with
: (reasonable) maximum possible size, read the string in the temporary variable,
: get the size plus one for the NULL character, allocate that much memory
: pointed to by your original variable and copy the string into it from the
: temporary variable.

: Since you may get into situation where the input string is bigger than the
: maximum size you have chosen, you could do the following.  Set the last char
: in the temporary variable to be NULL (i.e. 0) and check if it is still the
: same after reading the string.  If it is same then you are safe.  If not, the
: string read is bigger than your maximum size and you don't have the complete
: string.  [Worse yet, you might have overwritten memory of some other
: variable].  Either you can display an error and quit or reallocate bigger size
: (e.g., 2*maximum) for the temporary and try again.  Try until you succeed.

: Anybody else knows a better way?

Yes.
You could use a string class, which already deals with dynamic allocation of
strings. As most have stream operators defined on them, they work exactly
as desired.

or you could write the class yourself

or use a strstream:

   char ch;
   ostrstream ostr;

   // If strings are defined as white space delimited then you probably
   // want to gobble up any leading white space before entering the
   // reading loop. Maybe you should check cin.flags() & ios::skipws != 0
   while (cin >> ch) {
       if (isspace(ch)) {
           cin.putback(ch);
           break;
       }
       ostr << ch;
   }

   ostr << ends;
   char* filename = ostr.str();

   ... do stuff ...

   delete[] filename;       // Don't forget to clean up your responsibilites.

or if you can guarentee the filename will end at eof ('; which is rarely
the case ;') then more simply:

   ostrstream ostr;
   cin >> ostr.rdbuf();
   ostr << ends;
   char* filename = ostr.str();

   ... do stuff ...

   delete[] filename;       // Don't forget to clean up your responsibilites.

Regards

   -A.
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| A.Champion                |