C / C++ / MFC

19-Dec-02 10:44

Hi --

I have a simple question about dynamic memory and vectors:

Consider the vector class:

class vector
{

private:
int rows ;
double *vec ;

public:
vector(void) { rows=0 ; vec=0; } // default constructor

vector(int rows) // constructor
{
vec = new double[rows] ;
}

~vector(void) {} ; // destructor

} ;

when i use the class in MAIN() like:

vector* A ;
A = new vector(15) ;

I get an error. It's giving a "null reference error" and saying that the object has not been instantiated..

I don't get what I'm doing wrong?

Thanks

KBL

Christian Graus19-Dec-02 10:53

19-Dec-02 10:53

I'm confused, are you using the vector class which comes with C++, or writing your own ? A vector is a dynamic array.

You're also not deleting your double * in your destructor.

Christian

No offense, but I don't really want to encourage the creation of another VB developer.
- Larry Antram 22 Oct 2002

C# will attract all comers, where VB is for IT Journalists and managers - Michael
P Butler 05-12-2002

Again, you can screw up a C/C++ program just as easily as a VB program. OK, maybe not
as easily, but it's certainly doable. - Jamie Nordmeyer - 15-Nov-2002

KBL19-Dec-02 12:00

19-Dec-02 12:00

i'm writing my own vector class, but using dynamic arrays to do it. as you say, a vector is just a dynamic array, but i thought it was nice to wrap it up in a class so i can do stuff like this in main():

vector* a = new vector(10) ;
a->sort() ;
a->rank() ;

etc..

Christian Graus19-Dec-02 12:34

19-Dec-02 12:34

KBL wrote:
but i thought it was nice to wrap it up in a class so i can do stuff like this in main():

?How disturbing. I would not do it this way, if you really want to do this I would do two things.

1. I would call it something else, so that the two never clash
2. I would contain a vector and expose the methods to do what I wanted. Then you can use the algorithms that come with the STL instead of writing your own.

Christian

No offense, but I don't really want to encourage the creation of another VB developer.
- Larry Antram 22 Oct 2002

C# will attract all comers, where VB is for IT Journalists and managers - Michael
P Butler 05-12-2002

Again, you can screw up a C/C++ program just as easily as a VB program. OK, maybe not
as easily, but it's certainly doable. - Jamie Nordmeyer - 15-Nov-2002

KBL19-Dec-02 12:55

19-Dec-02 12:55

in this case i have some reasons for writing my own. i'm a mathematician/statistician trying to learn c++ (over c and fortran like languages), and i just don't know enough about the efficiency of all the stl functions. i already wrote my functions but now i wanted to wrap the whole thing up in a class.

Christian Graus19-Dec-02 13:00

19-Dec-02 13:00

The efficiency of STL functions is documented in the standard, plus you have the source code for vector, so you can look for yourself. I certainly respect the desire to write your own as a way to learn. You should make your container expose iterators so you can use the standard algorithms all the same, and you should most certainly not call it 'vector'.

Christian

No offense, but I don't really want to encourage the creation of another VB developer.
- Larry Antram 22 Oct 2002

C# will attract all comers, where VB is for IT Journalists and managers - Michael
P Butler 05-12-2002

Again, you can screw up a C/C++ program just as easily as a VB program. OK, maybe not
as easily, but it's certainly doable. - Jamie Nordmeyer - 15-Nov-2002

KBL19-Dec-02 13:07

19-Dec-02 13:07

thanks, that makes alot of sense. i'll take a look at the stl functions.

do you think it's the name clash that gives me problems, or is it the second contructor. should the constructor look like this rather than what i had before (i changed the name to myvector):

myvector(int n)
{ rows=n ;
vec = new double[rows] ;
}

Christian Graus19-Dec-02 13:40

19-Dec-02 13:40

I doubt the name is the problem, it's more a potential problem. The constructor is a safer bet, the new one looks much better.

Christian

No offense, but I don't really want to encourage the creation of another VB developer.
- Larry Antram 22 Oct 2002

C# will attract all comers, where VB is for IT Journalists and managers - Michael
P Butler 05-12-2002

Again, you can screw up a C/C++ program just as easily as a VB program. OK, maybe not
as easily, but it's certainly doable. - Jamie Nordmeyer - 15-Nov-2002

KBL19-Dec-02 12:57

19-Dec-02 12:57

i'll definitely rename it and change my destructor.

Alvaro Mendez19-Dec-02 10:55

Alvaro Mendez

19-Dec-02 10:55

1. Where is the error occurring -- what line?
2. Did you notice that in your second constructor you don't initialize your "rows" member?
3. I recommend that you declare and initialize A in the same line: vector* A = new vector(15);. It's less code and it's safer.

Regards,
Alvaro

Well done is better than well said. -- Benjamin Franklin
(I actually prefer medium-well.)

KBL19-Dec-02 12:03

19-Dec-02 12:03

It's occuring the constructor. That is, in the line:

vec = new double[15] ;

KBL19-Dec-02 12:07

19-Dec-02 12:07

do i need to intialize it in the second constructor when i have it as a member variable in the class?

KBL19-Dec-02 12:16

19-Dec-02 12:16

so my class should be something like this

class vector
{

private:
int rows ;
double *vec ;

public:
vector(void) { rows=0 ; vec=0; } // default constructor

vector(int n) // constructor
{
rows=n ;
vec = new double[rows] ;
}

~vector(void) {delete vec} ; // destructor

} ;

KBL19-Dec-02 13:01

19-Dec-02 13:01

is this how you would initialize it:

vector(int n) ;
{
rows=n ;
vec = new double[rows] ;
}

Help: Vector class won't work

Gary Kirkham20-Dec-02 3:52

Gary Kirkham

20-Dec-02 3:52

Other than not setting rows in you second constructor and not deleting vec in your destructor, there is nothing wrong with your code.

I created a simple console app and cut and pasted your code...no errors.

#include "stdafx.h"<br />
<br />
class vector<br />
{ <br />
<br />
private:<br />
int rows ; <br />
double *vec ; <br />
<br />
public: <br />
vector(void) { rows=0 ; vec=0; } // default constructor<br />
<br />
vector(int rows) // constructor<br />
{ <br />
vec = new double[rows] ;<br />
}<br />
<br />
~vector(void) {} ; // destructor<br />
<br />
} ;<br />
<br />
int main(int argc, char* argv[])<br />
{<br />
	vector* A ; <br />
	A = new vector(15) ;<br />
<br />
	return 0;<br />
}

just a memory leak

There has to be something else...Is this your entire code?

I think this is a good learning exercise...I would advise you to persue it and not spin your wheels over in STL land;)

Gary Kirkham

A working Program is one that has only unobserved bugs

KBL19-Dec-02 10:42