NetNews Offline 2/NetNews Offline Volume 2.iso/news/comp/lang/c++-part1/691

Text File | 1996-08-06 | 3.3 KB | 97 lines

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From: [e-mail] (Nitin More [CONTRACTOR])
Newsgroups: comp.lang.c++
Subject: Re: Help: How to initialize objects created by new class[x]
Date: 05 Jan 1996 21:08:53 GMT
Organization: SunSoft
Message-ID: <[e-mail]>
References: <4cicd6$[e-mail]>
    <[e-mail]>
NNTP-Posting-Host: more.eng.sun.com
In-reply-to: Enno Sandner's message of Fri, 05 Jan 1996 11:15:35 +0100

In article <[e-mail]> Enno Sandner <[e-mail]> writes:

$  From: Enno Sandner <[e-mail]>
$  Newsgroups: comp.lang.c++
$  Date: Fri, 05 Jan 1996 11:15:35 +0100
$  Organization: Fachbereich Informatik, TH Darmstadt

$  [e-mail] wrote:
$  >
$  > Let say I have the following class
$  >
$  > My_class {
$  >   public:
$  >     int (int i=0) : a(i), b(i) {}
$  >   private:
$  >     int a, b;
$  > }
$  >
$  > For the following statement, default constructor is involved upon
$  > the creation of the object.
$  >
$  >            My_class* my_ptr= new My_class;
$  >
$  > However,  if I use
$  >                          My_class* my_ptr= new My_class[10];
$  >
$  > to allocate 10 objects of class My_class, then C++ would not
$  > call any constructor to do any initialization.  All the objects
$  > allocated are not initializated at all.
$  >
$  > I wonder what is the proper way to perform object initialization
$  > whenever an array of objects are created?  As long as a constructor
$  > is involved for each of the object in the array, I don't care if all the
$  > objects are initializated the same or not.
$  >
$  > If there is no way to get constructor to initialize an array of objects
$  > upon creation.  Does that means one should not do so?
$  >

$  That's not true. The expression 'new My_class[10]' should call the
$  default-ctor of 'My_class' for each element.
$  (I will assume the class looks like
$
$           class My_class {
$             public:
$               My_class (int i=0) : a(i), b(i) {}
$             private:
$               int a, b;
$           };
$
$  ).
$  The problems start when your class doesn't provide a default-ctor
$  or you want to initialize elements of the array with different values.
$  Here you are out of luck, because C++ doesn't provide an appropriate
$  syntax. You'll have to use some hack like using the placement-new/delete
$  operators.

$       Enno

You can define your class as follows:

           class My_class {
             public:
               My_class () { init(); }
               My_class (int i) { init(i); }
             private:
               int a, b;
               void init(int i=0) { a = i; b = i; }
           };

In other words,
(1) define an init method.
(2) define a default constructor to call init.
(3) Remove default argument from your original constructor so that the compiler
    can distinguish between the default and the other constructor.
(4) Call the new init method from your original constructor.

-Nitin
--
----------------------------------------------------------------------
Nitin More
SunSoft, Bldg 16  Off: [phone]
Menlo Park, CA    Fax: [phone]   e-mail: [e-mail]
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