NetNews Offline 2/NetNews Offline Volume 2.iso/news/comp/lang/c++-part1/2781

Text File | 1996-08-06 | 4.3 KB | 111 lines

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From: [e-mail] (Nitin More [CONTRACTOR])
Newsgroups: comp.lang.c++
Subject: Re: [Q] Dynamically allocating memory for a char*
Date: 19 Jan 1996 17:50:12 GMT
Organization: SunSoft
Message-ID: <[e-mail]>
References: <4dmn1i$[e-mail]>
NNTP-Posting-Host: more.eng.sun.com
In-reply-to: [e-mail]'s message of Fri, 19 Jan 1996 00:00:49 GMT


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?

-Nitin


In article <4dmn1i$[e-mail]> [e-mail] (Felipe Jordao) writes:

> From: [e-mail] (Felipe Jordao)
> Newsgroups: comp.lang.c++
> Date: Fri, 19 Jan 1996 00:00:49 GMT
> Organization: HAC
>
> Hi,
>
> 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 ...)
>
> cin >> malloc(sizeof(infile));             :0
>
> Here is the actual code
> ----------------------------------------------------------------------
>
> main()
> {
>  ifstream InputFile;     // Input file stream
>  ofstream OutputFile;    // Output file stream
>  .
>  .
>  .
>  OpenFiles(InputFile, OutputFile);
>  .
>  .
> }
>
>
> void OpenFiles(ifstream& InputFile, ofstream& OutputFile)
> {
>  char infile[30];        // <---- RIGHT HERE.  this works as it is,
>  char outfile[30];       //       but I would like to use
>                          //       char *infile, *outfile.  When
>                          //       I try, I have not found a way to
>                          //       dynamically allocate memory when the
>                          //       user inputs the file.  I know I can
>                          //       use something like infile = new char[X]
>                          //       but that still leave me with the X limit.
>
>  cout << "Enter the path and filename of" << endl
>       << "the input file (*.csv): ";
>  cin  >> infile;         // I would like to allocate memory right after
>                          // user inputs the name of the file.  If I use
>                          // *infile and leave it like this it crashes
>                          // because of unallocated space
>
>  InputFile.open(infile);       // Test to see if the input file opens
>  if (!InputFile) {
>     cerr << "Cannot open input file!" << endl;
>     exit(1);
>     }
>
>
>  cout << "Enter the path and filename of" << endl  // get output file
>       << "the output file (*.bcp): ";              // from user
>  cin  >> outfile;
>
>  OutputFile.open(outfile);    // Test to see if the output file opens
>  if (!OutputFile) {
>     cerr << "Cannot open output file!" << endl;
>     exit(1);
>     }
> }
>
--
----------------------------------------------------------------------
Nitin More
SunSoft, Bldg 16  Off: [phone]
Menlo Park, CA    Fax: [phone]   e-mail: [e-mail]
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