NetNews Offline 2/NetNews Offline Volume 2.iso/news/comp/lang/c++-part2/15974

Text File | 1996-08-05 | 3.7 KB | 85 lines

Newsgroups: comp.lang.c++
Path: netcom.com!marnold
From: [e-mail] (Matt Arnold)
Subject: Re: Simple elastic list in C++...
Message-ID: <[e-mail]>
Organization: NETCOM On-line Communication Services (408 261-4700 guest)
References: <4ka4d5$[e-mail]>
Date: Mon, 8 Apr 1996 07:38:39 GMT
Sender: [e-mail]

[e-mail] (Steven Camhi) writes:

>As a programmer old to C, new to C++, I have seen no 'simple' answer to the
>following situation.   In most cases, the easiest, simplest way (for me, that
>is), to create a growable list in straight ANSI C, has been the following:
[snip]
>What is *the best* way of doing dynamically growable lists in C++, without a
>headache?

Uh, use STL?

>Perhaps there is no *best* way, what is the way to approach the
>problem of manufacturing a simple list class.  One way I have seen is the use
>of the STL vector classes; thats been my frame of reference.

There's not many ways to do it other than the how the vector class in STL
is implemented.  That's the basic model when the base implementation is an
array (contiguous items in memory).  You resize the block holding the array,
move items up and down, etc..

The next technique would be linked list.  Again, not to many variations
there.  The STL implementation (the list class) is as good as any other.
The only real choice is whether you use a singly- or doubly-linked list.
STL uses a doubly-linked list, I believe.

>I've seen some examples on the net use realloc(), and I've heard thats a major
>no-no!  Whats the right way?

Mixing C++ new/delete with realloc() is the no-no.  Using reallac() itself
is OK---although it's only meant to be used with memory returned by malloc(),
of course.  realloc() and malloc() are part of the standard C library.  All
C++ programs are C programs---there's nothing stopping you from using the
standard C memory allocation routines in a C++ program if you want to.  Just
#include "alloc.h" and code away.  You just have to be careful not to use
operator::delete() on memory you got from malloc(), etc..

This is an example of the "no-no's" you've heard of...

   Object* p = new Object();
   realloc(p);   // no, NO!

There's no garantee that new() and realloc() use the same underlying
memory manager.  Passing realloc() an address obtained from malloc()
is just like passing it garbage.  You're likely to corrupt new's heap,
the C library's heap, or both.

This is fine...

   MY_DATA* p = (MY_DATA*)malloc(sizeof(MY_DATA));
   p = (MY_DATA*)realloc(p, sizeof(MY_DATA) + 42);


Question:  If you need lists, what's wrong with using STL?  Taking care of
these kinds of programming issues in a standardized way is what something
like STL is all about.  Why not use it?

'Course, I have found that trying to create container classes in C++ to
be a very good learning experience.  I encourage you to persue that if
this is what you are after.  However, for the people who created STL at
HP, this was their *job*, or at least a very important "extra-curricular"
activity.  They've done a very good job with it, I think.

For learning STL, I've found the book "The C++ Programmer's Guide to the
Standard Template Library" from IDG books to be quite good.  It comes
with the HP version of STL.

Regards,
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Matt Arnold                       |        | ||| | |||| |  | | || ||
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