resizable array of pointers

[Mark]

i need to make a class that can store anything (integers,objec ts,etc)
it needs to maintain a list of pointers to these objects
the list needs to be resized to accomodate more elements
i can't use any special classes

i just can't seem to figure it out.

i've got a pointer,

void *storage

which will point to the start of the list

then there will be one pointer, every 4 bytes (sizeof(void*))

which i *think* i can access by doing

storage[index*sizeof(vo id*)]

i'm not sure if I need to cast that to a void pointer or not, since
that's already what it's defined as...

but then to resize this beast, i'm completely lost

i create a new void pointer..

void* temp = new void*[length]

where length is the new size (how many pointers it will hold)

which should allocate length*4 bytes of memory...

and then i tried doing

for(int i=0; i<count; i++)
{
temp[i] = storage[i*size];
}

to copy the pointers over
where count is the number of elements currently in the stash, but my
compiler complains...

`void*' is not a pointer-to-object type
pointer of type `void *' used in arithmetic

and...i don't really know what else to do.

any help?

[David W]

Re: resizable array of pointers

"Mark" <mnbayazit@gmai l.comwrote in message
news:1160084668 .203213.75030@c 28g2000cwb.goog legroups.com...

i need to make a class that can store anything (integers,objec ts,etc)
it needs to maintain a list of pointers to these objects
the list needs to be resized to accomodate more elements
i can't use any special classes
>
i just can't seem to figure it out.
>
i've got a pointer,
>
void *storage

A template would be much better. Then you can store an array of T* for almost any type T. If you use
void* you'll need to cast from void* to the correct type whenever you read the array, which is
pretty horrible. Anyway, I'll press on with your non-template version.

which will point to the start of the list

It should be void **storage, not void*. Remember, the elements of the array are themselves void *,
and 'storage' is a pointer to the first one, so it's void **.

then there will be one pointer, every 4 bytes (sizeof(void*))

Forget bytes. You don't need to think about 'em.

which i *think* i can access by doing
>
storage[index*sizeof(vo id*)]

No, you can't dereference a void *, because then you'd get void, which doesn't exist and has no
defined size to do arithmetic on. It works without arithmetic if 'storage' is void**, since
dereferencing that gives void*, which does exist and has a size, i.e., storage[index] is all you
need.

i'm not sure if I need to cast that to a void pointer or not, since
that's already what it's defined as...

It shouldn't be defined as that, though.

but then to resize this beast, i'm completely lost
>
i create a new void pointer..
>
void* temp = new void*[length]

No, void **temp = new void*[length];

where length is the new size (how many pointers it will hold)
>
which should allocate length*4 bytes of memory...

Forget bytes.

and then i tried doing
>
for(int i=0; i<count; i++)
{
temp[i] = storage[i*size];

temp[i] = storage[i];

}
>
to copy the pointers over
where count is the number of elements currently in the stash, but my
compiler complains...
>
`void*' is not a pointer-to-object type

That's right. Again, void** would fix that.

pointer of type `void *' used in arithmetic
>
and...i don't really know what else to do.
>
any help?

DW

[David W]

Re: resizable array of pointers

"David W" <no@email.provi dedwrote in message news:WdgVg.1631 5$b6.167047@nas al.pacific.net. au...

"Mark" <mnbayazit@gmai l.comwrote in message
news:1160084668 .203213.75030@c 28g2000cwb.goog legroups.com...

i need to make a class that can store anything (integers,objec ts,etc)
it needs to maintain a list of pointers to these objects
the list needs to be resized to accomodate more elements
i can't use any special classes

i just can't seem to figure it out.

i've got a pointer,

void *storage

>
A template would be much better. Then you can store an array of T* for almost any type T. If you

use

void* you'll need to cast from void* to the correct type whenever you read the array, which is
pretty horrible. Anyway, I'll press on with your non-template version.

I was assuming here that in a given array the pointers are all to the same type. If the same array
has pointers to different types then a template wouldn't work. Of course, with an array of void * to
a mix of types you'd always have to know which void * pointed to which type and cast it
appropriately when you read it.

DW

[peter koch]

Re: resizable array of pointers


Mark skrev:

i need to make a class that can store anything (integers,objec ts,etc)
it needs to maintain a list of pointers to these objects
the list needs to be resized to accomodate more elements
i can't use any special classes
>
i just can't seem to figure it out.
>

[snip]

>
any help?

Sure... std::vector<>

/Peter

[Ole Nielsby]

Re: resizable array of pointers


Mark <mnbayazit@gmai l.comwrote:

i need to make a class that can store anything (integers,objec ts,etc)
it needs to maintain a list of pointers to these objects
the list needs to be resized to accomodate more elements

You might want to use a vector<void*of the Standard
Template Library (STL). Vectors are essentially resizeable arrays.

Your online docs - or STL tutorials - will have vector examples
you can work from.

At first glimse, it can seem a bit strange that you have to deal
with iterators instead of pointers, but the iterators are really just
pointers that are dressed up fancy for mating with STL algorithms.

HTH/ON

[Mark]

Re: resizable array of pointers


peter koch wrote:

>
Sure... std::vector<>
>
/Peter

Mark wrote:

i can't use any special classes

i was referring specifically to vectors and such.
but thanks for trying.

[Mark]

Re: resizable array of pointers


David W wrote:

"Mark" <mnbayazit@gmai l.comwrote in message
news:1160084668 .203213.75030@c 28g2000cwb.goog legroups.com...

i need to make a class that can store anything (integers,objec ts,etc)
it needs to maintain a list of pointers to these objects
the list needs to be resized to accomodate more elements
i can't use any special classes

i just can't seem to figure it out.

i've got a pointer,

void *storage

>
A template would be much better. Then you can store an array of T* for almost any type T. If you use
void* you'll need to cast from void* to the correct type whenever you read the array, which is
pretty horrible. Anyway, I'll press on with your non-template version.
>

which will point to the start of the list

>
It should be void **storage, not void*. Remember, the elements of the array are themselves void *,
and 'storage' is a pointer to the first one, so it's void **.
>

then there will be one pointer, every 4 bytes (sizeof(void*))

>
Forget bytes. You don't need to think about 'em.
>

which i *think* i can access by doing

storage[index*sizeof(vo id*)]

>
No, you can't dereference a void *, because then you'd get void, which doesn't exist and has no
defined size to do arithmetic on. It works without arithmetic if 'storage' is void**, since
dereferencing that gives void*, which does exist and has a size, i.e., storage[index] is all you
need.
>

i'm not sure if I need to cast that to a void pointer or not, since
that's already what it's defined as...

>
It shouldn't be defined as that, though.
>

but then to resize this beast, i'm completely lost

i create a new void pointer..

void* temp = new void*[length]

>
No, void **temp = new void*[length];
>

where length is the new size (how many pointers it will hold)

which should allocate length*4 bytes of memory...

>
Forget bytes.
>

and then i tried doing

for(int i=0; i<count; i++)
{
temp[i] = storage[i*size];

>
temp[i] = storage[i];
>

}

to copy the pointers over
where count is the number of elements currently in the stash, but my
compiler complains...

`void*' is not a pointer-to-object type

>
That's right. Again, void** would fix that.
>

pointer of type `void *' used in arithmetic

and...i don't really know what else to do.

any help?

>
DW

ahh.. i tried a double pointer the first time around, which compiled,
but then i ran into another problem down the road.

i thought i might just be able to just have a pointer to the start of
the list, but i guess what i had was a pointer to a list of voids,
which don't have a size, when i needed a pointer to a list of void*'s..


kinda tough to think about when you have all these asterisks and
brackets laying around.

but thank you, this really helped clear things up. much appreciated.

void ObStash::inflat e()
{
cout << "inflating" ;
length += increment;
void** temp = new void*[length];
for(int i=0; i<count; i++)
{
temp[i] = storage[i];
}
cout << "inflated";
}

now i'm having a problem adding to the list (it crashes after printing
"inflated")

int ObStash::add(vo id *p)
{
if( count >= length ) inflate();
storage[count] = p;
count++;
}

but, i'll see if i can work this out first before i ask any more
questions.

the reason i was so fixated on size was because i had previously made a
stash where all the data was copied into the list, so it was very
dependant on the size of the object.

i agree that templates would be a much better solution..but.. that isn't
an option right now.

the lists actually aren't mixed btw :) it would be crazy to try and
keep track of what each thing was pointing to..

[Mark]

Re: resizable array of pointers


Mark wrote:

David W wrote:

"Mark" <mnbayazit@gmai l.comwrote in message
news:1160084668 .203213.75030@c 28g2000cwb.goog legroups.com...

i need to make a class that can store anything (integers,objec ts,etc)
it needs to maintain a list of pointers to these objects
the list needs to be resized to accomodate more elements
i can't use any special classes
>
i just can't seem to figure it out.
>
i've got a pointer,
>
void *storage

A template would be much better. Then you can store an array of T* for almost any type T. If you use
void* you'll need to cast from void* to the correct type whenever you read the array, which is
pretty horrible. Anyway, I'll press on with your non-template version.

which will point to the start of the list

It should be void **storage, not void*. Remember, the elements of the array are themselves void *,
and 'storage' is a pointer to the first one, so it's void **.

then there will be one pointer, every 4 bytes (sizeof(void*))

Forget bytes. You don't need to think about 'em.

which i *think* i can access by doing
>
storage[index*sizeof(vo id*)]

No, you can't dereference a void *, because then you'd get void, which doesn't exist and has no
defined size to do arithmetic on. It works without arithmetic if 'storage' is void**, since
dereferencing that gives void*, which does exist and has a size, i.e., storage[index] is all you
need.

i'm not sure if I need to cast that to a void pointer or not, since
that's already what it's defined as...

It shouldn't be defined as that, though.

but then to resize this beast, i'm completely lost
>
i create a new void pointer..
>
void* temp = new void*[length]

No, void **temp = new void*[length];

where length is the new size (how many pointers it will hold)
>
which should allocate length*4 bytes of memory...

Forget bytes.

and then i tried doing
>
for(int i=0; i<count; i++)
{
temp[i] = storage[i*size];

temp[i] = storage[i];

}
>
to copy the pointers over
where count is the number of elements currently in the stash, but my
compiler complains...
>
`void*' is not a pointer-to-object type

That's right. Again, void** would fix that.

pointer of type `void *' used in arithmetic
>
and...i don't really know what else to do.
>
any help?

DW

>
ahh.. i tried a double pointer the first time around, which compiled,
but then i ran into another problem down the road.
>
i thought i might just be able to just have a pointer to the start of
the list, but i guess what i had was a pointer to a list of voids,
which don't have a size, when i needed a pointer to a list of void*'s..
>
>
kinda tough to think about when you have all these asterisks and
brackets laying around.
>
but thank you, this really helped clear things up. much appreciated.
>
void ObStash::inflat e()
{
cout << "inflating" ;
length += increment;
void** temp = new void*[length];
for(int i=0; i<count; i++)
{
temp[i] = storage[i];
}
cout << "inflated";
}
>
now i'm having a problem adding to the list (it crashes after printing
"inflated")
>
int ObStash::add(vo id *p)
{
if( count >= length ) inflate();
storage[count] = p;
count++;
}
>
but, i'll see if i can work this out first before i ask any more
questions.
>
the reason i was so fixated on size was because i had previously made a
stash where all the data was copied into the list, so it was very
dependant on the size of the object.
>
i agree that templates would be a much better solution..but.. that isn't
an option right now.
>
the lists actually aren't mixed btw :) it would be crazy to try and
keep track of what each thing was pointing to..

hahaha..

oops.

forgot the two most important lines.

void ObStash::inflat e()
{
cout << "inflating" ;
length += increment;
void** temp = new void*[length];
for(int i=0; i<count; i++)
{
temp[i] = storage[i];
}
delete [] storage;
storage = temp;
cout << "inflated";
}

(delete, and storage = temp)

i am deleting it properly, right?

[Thomas J. Gritzan]

Re: resizable array of pointers

Mark schrieb:

hahaha..
>
oops.
>
forgot the two most important lines.
>
void ObStash::inflat e()
{
cout << "inflating" ;
length += increment;
void** temp = new void*[length];
for(int i=0; i<count; i++)
{
temp[i] = storage[i];
}
delete [] storage;
storage = temp;
cout << "inflated";
}
>
(delete, and storage = temp)
>
i am deleting it properly, right?
>

You forgot the class definition and all the other things. How we are
supposted to know what the class is doing and what could be wrong?

--
Thomas

http://www.netmeister.org/news/learn2quote.html

[Mark]

Re: resizable array of pointers


David W wrote:

"Mark" <mnbayazit@gmai l.comwrote in message
news:1160084668 .203213.75030@c 28g2000cwb.goog legroups.com...

i need to make a class that can store anything (integers,objec ts,etc)
it needs to maintain a list of pointers to these objects
the list needs to be resized to accomodate more elements
i can't use any special classes

i just can't seem to figure it out.

i've got a pointer,

void *storage

>
A template would be much better. Then you can store an array of T* for almost any type T. If you use
void* you'll need to cast from void* to the correct type whenever you read the array, which is
pretty horrible. Anyway, I'll press on with your non-template version.
>

which will point to the start of the list

>
It should be void **storage, not void*. Remember, the elements of the array are themselves void *,
and 'storage' is a pointer to the first one, so it's void **.
>

then there will be one pointer, every 4 bytes (sizeof(void*))

>
Forget bytes. You don't need to think about 'em.
>

which i *think* i can access by doing

storage[index*sizeof(vo id*)]

>
No, you can't dereference a void *, because then you'd get void, which doesn't exist and has no
defined size to do arithmetic on. It works without arithmetic if 'storage' is void**, since
dereferencing that gives void*, which does exist and has a size, i.e., storage[index] is all you
need.
>

i'm not sure if I need to cast that to a void pointer or not, since
that's already what it's defined as...

>
It shouldn't be defined as that, though.
>

but then to resize this beast, i'm completely lost

i create a new void pointer..

void* temp = new void*[length]

>
No, void **temp = new void*[length];
>

where length is the new size (how many pointers it will hold)

which should allocate length*4 bytes of memory...

>
Forget bytes.
>

and then i tried doing

for(int i=0; i<count; i++)
{
temp[i] = storage[i*size];

>
temp[i] = storage[i];
>

}

to copy the pointers over
where count is the number of elements currently in the stash, but my
compiler complains...

`void*' is not a pointer-to-object type

>
That's right. Again, void** would fix that.
>

pointer of type `void *' used in arithmetic

and...i don't really know what else to do.

any help?

>
DW

okay..maybe you can help me here,

the names are printing out properly, but the numbers (scores) are
garbage.

void Student::print( )
{
cout << " name: " << name << endl;
int sum = 0;
for(int i=0; i<scores.getCou nt(); i++)
{
int score = *(int*)scores.f etch(i);
cout << " score" << i+1 << ": " << score << endl;
sum += score;
}
if( scores.getCount () 0 )
cout << " gpa: " << sum/(float)scores.g etCount() << endl;
else
cout << " no scores" << endl;
}

fetched like so

void* ObStash::fetch( int index)
{
return storage[index];
}

[Mark]

Re: resizable array of pointers


Thomas J. Gritzan wrote:

Mark schrieb:

hahaha..

oops.

forgot the two most important lines.

void ObStash::inflat e()
{
cout << "inflating" ;
length += increment;
void** temp = new void*[length];
for(int i=0; i<count; i++)
{
temp[i] = storage[i];
}
delete [] storage;
storage = temp;
cout << "inflated";
}

(delete, and storage = temp)

i am deleting it properly, right?

>
You forgot the class definition and all the other things. How we are
supposted to know what the class is doing and what could be wrong?
>
--
Thomas
http://www.netmeister.org/news/learn2quote.html

that part seems to be working (or at least not crashing)
storage is defined as
void** storage;
i'm just asking if that will delete all the pointers in storage,
without leaving anything behind.

but, if you really want to see the class definition

// opstash.h
#ifndef OBSTASH_H
#define OBSTASH_H

const int increment = 100;
const int size = sizeof(void*);

class ObStash
{
int length;
int count;
void** storage;

public:
ObStash();
int add(void* p);
void* fetch(int index);
~ObStash();
int getCount();
void inflate();
};
#endif


// obstash.cpp
#include "ObStash.h"
#include <iostream>
#include "Student.h"
//#include <cassert>
using namespace std;



ObStash::ObStas h() : count(0), length(0)
{}

ObStash::~ObSta sh()
{
// delete all the pointers...
}



void ObStash::inflat e()
{
length += increment;
void** temp = new void*[length];
for(int i=0; i<count; i++)
{
temp[i] = storage[i];
}
delete [] storage;
storage = temp;
}

int ObStash::add(vo id *p)
{
if( count >= length ) inflate();
storage[count] = p;
cout << count << ":" << storage[count] << endl;
count++;
}

void* ObStash::fetch( int index)
{
return storage[index];
}

int ObStash::getCou nt() {
return count;
}



andddd...since i'll be adding students (as well as integers) to this
stash...

// student.h
#ifndef STUDENT_H
#define STUDENT_H
#include <string>
#include "ObStash.h"
using namespace std;

class Student
{
string name;
ObStash scores;

public:
Student(string name);
void print();
void cleanUp();
};

#endif

// student.cpp
#include "student.h"
#include <iostream>

using namespace std;

Student::Studen t(string name) : name(name)
{
int score;
while(1)
{
cout << " score" << scores.getCount ()+1 << ": ";
cin >score;
if( cin.fail() )
{
cin.clear();
string s;
getline(cin,s);
cout << " * bad input" << endl;
continue;
}
if( score == -1 )
break;
else if( score < 1 || score 5 ) {
cout << " * wrong input" << endl;
continue;
}
scores.add(&sco re);
};
}

void Student::print( )
{
cout << " name: " << name << endl;
int sum = 0;
for(int i=0; i<scores.getCou nt(); i++)
{
int score = *(int*)scores.f etch(i);
cout << " score" << i+1 << ": " << score << endl;
sum += score;
}
if( scores.getCount () 0 )
cout << " gpa: " << sum/(float)scores.g etCount() << endl;
else
cout << " no scores" << endl;
}

[David W]

Re: resizable array of pointers

"Mark" <mnbayazit@gmai l.comwrote in message
news:1160098225 .299548.71890@m 7g2000cwm.googl egroups.com...

>
>
hahaha..
>
oops.
>
forgot the two most important lines.
>
void ObStash::inflat e()
{
cout << "inflating" ;
length += increment;
void** temp = new void*[length];
for(int i=0; i<count; i++)
{
temp[i] = storage[i];
}
delete [] storage;
storage = temp;
cout << "inflated";
}
>
(delete, and storage = temp)
>
i am deleting it properly, right?

Yes. new[] requires delete[].

DW

[David W]

Re: resizable array of pointers

"Mark" <mnbayazit@gmai l.comwrote in message
news:1160098972 .858209.218710@ m73g2000cwd.goo glegroups.com.. .

>
>
okay..maybe you can help me here,
>
the names are printing out properly, but the numbers (scores) are
garbage.
>
void Student::print( )
{
cout << " name: " << name << endl;
int sum = 0;
for(int i=0; i<scores.getCou nt(); i++)
{
int score = *(int*)scores.f etch(i);
cout << " score" << i+1 << ": " << score << endl;
sum += score;
}
if( scores.getCount () 0 )
cout << " gpa: " << sum/(float)scores.g etCount() << endl;
else
cout << " no scores" << endl;
}
>
fetched like so
>
void* ObStash::fetch( int index)
{
return storage[index];
}

My guess is that the numbers you are putting in there no longer exist, but you didn't include that
code. If the ints are ordinary local variables they may have gone out of scope. You may need to
create each int to be stored with 'new' if that's the case (and delete later).

DW

[Mark]

Re: resizable array of pointers


David W wrote:

"Mark" <mnbayazit@gmai l.comwrote in message
news:1160098972 .858209.218710@ m73g2000cwd.goo glegroups.com.. .

okay..maybe you can help me here,

the names are printing out properly, but the numbers (scores) are
garbage.

void Student::print( )
{
cout << " name: " << name << endl;
int sum = 0;
for(int i=0; i<scores.getCou nt(); i++)
{
int score = *(int*)scores.f etch(i);
cout << " score" << i+1 << ": " << score << endl;
sum += score;
}
if( scores.getCount () 0 )
cout << " gpa: " << sum/(float)scores.g etCount() << endl;
else
cout << " no scores" << endl;
}

fetched like so

void* ObStash::fetch( int index)
{
return storage[index];
}

>
My guess is that the numbers you are putting in there no longer exist, but you didn't include that
code. If the ints are ordinary local variables they may have gone out of scope. You may need to
create each int to be stored with 'new' if that's the case (and delete later).
>
DW

okay, that worked like a charm!

now..last question..i hope.

i've got my destructor

ObStash::~ObSta sh()
{
delete [] storage;
}

but will that be sufficient to prevent leakage?

since it's an array of pointers. i figure i would have to delete each
pointer first, and then delete the array, since it's declared as void
**storage, however.. i can't delete each pointer because they delete
void* is undefined!

but, from my understanding, it's not necessarily delete integers
anyways..they'l l take care of themselves..but the Student class
however, is another story.

class Student
{
string name;
ObStash scores;

public:
Student(string name);
void print();
void cleanUp();
};

the string has it's own destructor i believe, and should clean up after
it's self...

the obstash's destructor should clean up the scores too i think..

but what about when

ObStash students;

goes out of scope?

do i need to call the destructor for each object individually or
anything..? (delete each student manually) or..will they all die
gracefully?

[Mark]

Re: resizable array of pointers


Mark wrote:

David W wrote:

"Mark" <mnbayazit@gmai l.comwrote in message
news:1160098972 .858209.218710@ m73g2000cwd.goo glegroups.com.. .

>
>
okay..maybe you can help me here,
>
the names are printing out properly, but the numbers (scores) are
garbage.
>
void Student::print( )
{
cout << " name: " << name << endl;
int sum = 0;
for(int i=0; i<scores.getCou nt(); i++)
{
int score = *(int*)scores.f etch(i);
cout << " score" << i+1 << ": " << score << endl;
sum += score;
}
if( scores.getCount () 0 )
cout << " gpa: " << sum/(float)scores.g etCount() << endl;
else
cout << " no scores" << endl;
}
>
fetched like so
>
void* ObStash::fetch( int index)
{
return storage[index];
}

My guess is that the numbers you are putting in there no longer exist, but you didn't include that
code. If the ints are ordinary local variables they may have gone out of scope. You may need to
create each int to be stored with 'new' if that's the case (and delete later).

DW

>
okay, that worked like a charm!
>
now..last question..i hope.
>
i've got my destructor
>
ObStash::~ObSta sh()
{
delete [] storage;
}
>
but will that be sufficient to prevent leakage?
>
since it's an array of pointers. i figure i would have to delete each
pointer first, and then delete the array, since it's declared as void
**storage, however.. i can't delete each pointer because they delete
void* is undefined!
>
but, from my understanding, it's not necessarily delete integers
anyways..they'l l take care of themselves..but the Student class
however, is another story.
>
class Student
{
string name;
ObStash scores;
>
public:
Student(string name);
void print();
void cleanUp();
};
>
the string has it's own destructor i believe, and should clean up after
it's self...
>
the obstash's destructor should clean up the scores too i think..
>
but what about when
>
ObStash students;
>
goes out of scope?
>
do i need to call the destructor for each object individually or
anything..? (delete each student manually) or..will they all die
gracefully?

i know i'm spamming, but there's no edit button that i've discovered.

the destructor doesn't seem to be called for each student, when obstash
students goes out of scope

Student::~Stude nt()
{
cout << "killing student " << name << endl;
}

int main(int argc, char *argv[])
{

{
ObStash students;

// snip
}
// destructors ought to be called here?
system("PAUSE") ;
return EXIT_SUCCESS;
}



in fact.. only the destructor for ObStash students is called, the
ObStash scores are not destructed either! i'm assuming they're not
being destructed because the students aren't being destructed.

modifying it as follows..

int main(int argc, char *argv[])
{

{
ObStash students;
// snip
for(int i=0; i<students.getC ount(); i++) {
delete (Student*)stude nts.fetch(i);
}
}

system("PAUSE") ;
return EXIT_SUCCESS;
}


everything seems to get destructed. i'm not missing anything am i?

thank you all for your help :)

i have one class in java, and one class in c++ this semester.. i'm
really starting to appreciate java..kinda.

to delete a list in java, all you have to do is set the head to null (i
think), and let the garbage collector clean up the huge amount of data
you just tossed into space.