NetNews Usenet Archive 1992 #30/NN_1992_30.iso/spool/comp/lang/cplus/18154

Text File | 1992-12-18 | 3.9 KB | 181 lines

Path: sparky!uunet!zaphod.mps.ohio-state.edu!saimiri.primate.wisc.edu!ames!olivea!charnel!sifon!CC.UMontreal.CA!IRO.UMontreal.CA!brisay.IRO.UMontreal.CA!laviers
From: [e-mail] (Cocotte Minute)
Newsgroups: comp.lang.c++
Subject: Can't I initialize a dynamically allocated object in its constructor?
Message-ID: <[e-mail]>
Date: 17 Dec 92 20:51:13 GMT
Sender: [e-mail]
Organization: Universite de Montreal
Lines: 170

Hello,

I have the following problem.

 I have created an class: Vector(k), it's
a vector of k dimension (of doubles). After a whole week of debugging,
I have come to the conclusion that you must not set the values of the elements
 of a class that are dynamically allocated (new) in the constructor.

In the following code, I simply add 3 vectors.
It seems that, in:

   v4=v1+v2+v3,


a temporary vector is constructed to put the result of v1+v2 .
If there is no initialization in the constructor , it works just fine,
if there is initialization (here, to [-1,-1,-1...]), we seem to loose the value
of v1+v2... why???


in this example: v1=[1,1,1],v2=[2,2,2],v3=[3,3,3]

without initialization
  (comment the line "for(int i=0;i<array_size;i++)item[i]=-1.0;")

v4=[6,6,6]; /* great! */

with initialization of vectors:

v4=[2,2,2]; /* not good: v4=[-1,-1,-1]+v3 */



Could this imply that the program gave the temporary (stack?) vector the
value v1+v2 _before_ finishing to allocate (construct) it's space?

Pohl (and others) seem to think that it is good practice to initialize
objects... and have no undefined values.

is this a bug in the compiler? I've only tried it on the C++ v2.1.1
from SGI. If you try this on any other compiler please tell me if you
get the same results.

Thank you very much for any help you can give me!

Sebastien Lavier
[e-mail]


-------------------------->8 snip, snip 8<-----------------------------

#include <stdio.h>

//-------------------------------------------------------------------
// Vector: vector of double
//-------------------------------------------------------------------

class Vector{

double     *item ;
int     size ;


public :

// constructors
Vector() { item = new double ; size = 1 ;}
Vector(int array_size);

// destructors
~Vector() { delete item ; }

// methods
int getsize() { return size ; }

void print(char *message);


// operators
Vector operator+ (Vector& b);
Vector& operator= (Vector& a);
double& operator[] (int i);
};


// constructor

Vector::Vector(int array_size) {
   item = new double[array_size] ;
   size = array_size ;
   printf("constructing\n");
   for(int i=0;i<array_size;i++)item[i]=-1.0;
}


Vector Vector::operator+ (Vector& a){
   if (getsize() != a.getsize()){
      printf("operator '+':Vectors are not of the same size.\n");
      exit(0);
   }
   Vector retVect(getsize()) ;
   for (int i = 0 ; i < a.getsize() ; i++){
      retVect.item[i] = item[i] + a.item[i] ;
   }
   return retVect ;
}



Vector& Vector::operator= (Vector& a){
   if (getsize() != a.getsize()){
      printf("operator '=':Vectors are not of the same size.\n");
      exit(0);
   }
   for (int i = 0 ; i < a.getsize() ; i++){
      item[i] = a.item[i] ;
   }
   return *this ;
}


double& Vector::operator[](int i){
   return item[i] ;
}


void  Vector::print(char *message) {
   printf("%s\n",message);
   for (int i = 0 ; i < size ; i++){
      printf("       [%d]=%lf\n", i, item[i]);
   }
}



main(){

Vector v1(3),v2(3),v3(3),v4(3);



v1[0]=1.0;
v1[1]=1.0;
v1[2]=1.0;

v2[0]=2.0;
v2[1]=2.0;
v2[2]=2.0;

v3[0]=3.0;
v3[1]=3.0;
v3[2]=3.0;

v4=v1+v2+v3;

v4.print("v4");

}

-------------------------->8 snip, snip 8<-----------------------------




--
_____________________________________________________________________________
|Sebastien Lavier [Calimero]    | Centre de recherche, Hopital du Sacre-Coeur
|[e-mail]       | "The opinions expressed here are mine and
|Universite de Montreal, Canada |   should also be yours!"