memory allocation problem

[chandra sekhar]

Hi All,
I am trying to allocate memory for member variable in a static
method belongs to same class.. the allocation is o.k and when try to
access the member variable I find that it is a null pointer.

Here is the code ...

#include <iostream.h>


class A
{
public:
A():i(0){}
~A(){ if(i) { delete i; i = 0;}}
static void create(A** obj)
{
*obj = new A();
int* ii = 0;
(*obj)->get_i(ii);
ii = new int;
}
void get_i(int*& g_out)
{
g_out = i;
}
private:
int* i;
};

int main()
{
A* ob = 0;
A::create(&ob);
int* i = 0;
ob->get_i(i);
try
{
cout << " value " << *i << endl;
}catch(...)
{
// thows the null pointer exception
}
if( ob ) { delete ob; ob = 0;}
return 0;
}

Any suggestions in this regard is welcome.

Thanks
Chandra-

[John Harrison]

Re: memory allocation problem


"chandra sekhar" <ccadmin@sisl.c o.in> wrote in message
news:403350FD.D A178334@sisl.co .in...[color=blue]
> Hi All,
> I am trying to allocate memory for member variable in a static
> method belongs to same class.. the allocation is o.k and when try to
> access the member variable I find that it is a null pointer.
>
> Here is the code ...
>
> #include <iostream.h>
>
>
> class A
> {
> public:
> A():i(0){}
> ~A(){ if(i) { delete i; i = 0;}}
> static void create(A** obj)
> {
> *obj = new A();
> int* ii = 0;
> (*obj)->get_i(ii);
> ii = new int;
> }
> void get_i(int*& g_out)
> {
> g_out = i;
> }
> private:
> int* i;
> };
>
> int main()
> {
> A* ob = 0;
> A::create(&ob);
> int* i = 0;
> ob->get_i(i);
> try
> {
> cout << " value " << *i << endl;
> }catch(...)
> {
> // thows the null pointer exception
> }
> if( ob ) { delete ob; ob = 0;}
> return 0;
> }
>
> Any suggestions in this regard is welcome.
>
> Thanks
> Chandra-
>[/color]

Nowhere in the code above do you assign to the member variable i, so it
stays NULL. Where did you think you were changing member variable i?

john

[chandra sekhar]

Re: memory allocation problem

To change the contents of the variable memory for the variable should
exists.. I am checking the memory existance by calling get_i in the main()
Here the problem is the memory for the variable is not existing inspite of
allocation in the static method.

Thanks
Chandra

John Harrison wrote:
[color=blue]
> "chandra sekhar" <ccadmin@sisl.c o.in> wrote in message
> news:403350FD.D A178334@sisl.co .in...[color=green]
> > Hi All,
> > I am trying to allocate memory for member variable in a static
> > method belongs to same class.. the allocation is o.k and when try to
> > access the member variable I find that it is a null pointer.
> >
> > Here is the code ...
> >
> > #include <iostream.h>
> >
> >
> > class A
> > {
> > public:
> > A():i(0){}
> > ~A(){ if(i) { delete i; i = 0;}}
> > static void create(A** obj)
> > {
> > *obj = new A();
> > int* ii = 0;
> > (*obj)->get_i(ii);
> > ii = new int;
> > }
> > void get_i(int*& g_out)
> > {
> > g_out = i;
> > }
> > private:
> > int* i;
> > };
> >
> > int main()
> > {
> > A* ob = 0;
> > A::create(&ob);
> > int* i = 0;
> > ob->get_i(i);
> > try
> > {
> > cout << " value " << *i << endl;
> > }catch(...)
> > {
> > // thows the null pointer exception
> > }
> > if( ob ) { delete ob; ob = 0;}
> > return 0;
> > }
> >
> > Any suggestions in this regard is welcome.
> >
> > Thanks
> > Chandra-
> >[/color]
>
> Nowhere in the code above do you assign to the member variable i, so it
> stays NULL. Where did you think you were changing member variable i?
>
> john[/color]

[Patrik Stellmann]

Re: memory allocation problem

>>> static void create(A** obj)[color=blue][color=green][color=darkred]
>>> {
>>> *obj = new A();
>>> int* ii = 0;
>>> (*obj)->get_i(ii);
>>> ii = new int;
>>> }
>>> void get_i(int*& g_out)
>>> {
>>> g_out = i;
>>> }[/color][/color][/color]
you functions writes the value of i to g_out but what you need is i
itself (or a pointer to it). So something (extremly 'ugly') like
int** get_i()
{
return &i;
}
static void create(A** obj)
{
*obj = new A();
int** ii = (*obj)->get_i(ii);
*ii = new int;
}

would probably do. This is - of course - very bad design since you pass
out a pointer to a member variable and there are most likely better ways
to achieve what you're looking for. Maybe a set_i function but the
decision also depends on what else you're doing with this class...

[Rolf Magnus]

Re: memory allocation problem

chandra sekhar wrote:
[color=blue]
> Hi All,
> I am trying to allocate memory for member variable in a static
> method belongs to same class.. the allocation is o.k and when try to
> access the member variable I find that it is a null pointer.
>
> Here is the code ...
>
> #include <iostream.h>
>
>
> class A
> {
> public:
> A():i(0){}
> ~A(){ if(i) { delete i; i = 0;}}
> static void create(A** obj)
> {
> *obj = new A();
> int* ii = 0;
> (*obj)->get_i(ii);
> ii = new int;
> }
> void get_i(int*& g_out)
> {
> g_out = i;
> }
> private:
> int* i;
> };
>
> int main()
> {
> A* ob = 0;
> A::create(&ob);
> int* i = 0;
> ob->get_i(i);
> try
> {
> cout << " value " << *i << endl;
> }catch(...)
> {
> // thows the null pointer exception
> }
> if( ob ) { delete ob; ob = 0;}
> return 0;
> }
>
> Any suggestions in this regard is welcome.[/color]

Your above code is extremely obfuscated. However, you never set the i
member of your object and neither the local i variable in main() to a
valid memory address, so it still is a null pointer in the cout line.

[John Harrison]

Re: memory allocation problem


"chandra sekhar" <ccadmin@sisl.c o.in> wrote in message
news:403353BF.5 9176919@sisl.co .in...[color=blue]
> To change the contents of the variable memory for the variable should
> exists.. I am checking the memory existance by calling get_i in the main()
> Here the problem is the memory for the variable is not existing inspite of
> allocation in the static method.
>
> Thanks
> Chandra[/color]

Nothing in the static method allocates memory and assigns it to i.

I cannot understand why you think it should.

You need to do something like this

static void create(A** obj)
{
*obj = new A();
(*obj)-> i = new int; // allocate some memory and assign it to i
}

john