Memory Cleanup problem

[Jonathan Ames]

Moving to C++ from Java, I'm still confused by some aspects of memory
cleanup operations.

For example, let's say I have a class MovingObject which maintains a
pointer to another class MovementAlgorit hm.

MovingObject has a method
SetMovementAlgo rithm(MovementA lgorithm* movementAlgorit hm)

if the body of this method reads
{
this->movementAlgori thm = movementAlgorit hm;
}

is this a memory leak every time a new MovementAlgorit hm is set (other
than the first time)? Because the previous movement algorithm isn't
deleted?

What is the correct way to handle this problem?

If I change the body of the method to read as follows :

if (this->movementAlgori thm != NULL) {
delete this->movementAlgori thm;
}
this->movementAlgori thm = movementAlgorit hm;

I get program crash. All of the constructors for MovingObject
initialize the pointer variable either to NULL or through the new
operator. Can anyone explain this?

[John Harrison]

On Sat, 31 Jul 2004 18:58:25 +0100, Jonathan Ames <am**@reb.org > wrote:

Moving to C++ from Java, I'm still confused by some aspects of memory
cleanup operations.

For example, let's say I have a class MovingObject which maintains a
pointer to another class MovementAlgorit hm.

MovingObject has a method
SetMovementAlgo rithm(MovementA lgorithm* movementAlgorit hm)

if the body of this method reads
{
this->movementAlgori thm = movementAlgorit hm;
}

is this a memory leak every time a new MovementAlgorit hm is set (other
than the first time)? Because the previous movement algorithm isn't
deleted?
There is no memory leak here because there is no memory allocated here,
it's the way that you *use* SetMovementAlgo rithm that causes the memory
leak.

I think you are using it something like this

MovingObject x;
x.SetMovementAl gorithm(new MovementAlgorit hm); // 1
....
x.SetMovementAl gorithm(new MovementAlgorit hm); // 2

Here there is a memory leak, because the only pointer pointing to the
object allocated at step 1 has been overwritten at step 2, and so that
object can never be deleted.

But in this similar code there is no memory leak

MovingObject x;
MovementAlgorit hm y1;
x.SetMovementAl gorithm(&y1);
....
MovementAlgorit hm y2;
x.SetMovementAl gorithm(&y2);

What is the correct way to handle this problem?

It depends.
If I change the body of the method to read as follows :

if (this->movementAlgori thm != NULL) {
delete this->movementAlgori thm;
}
this->movementAlgori thm = movementAlgorit hm;

I get program crash. All of the constructors for MovingObject
initialize the pointer variable either to NULL or through the new
operator. Can anyone explain this?

Not without seeing more code. I guess the most likely reason for the crash
is that you are deleteing the same memory twice because you have not
defined a suitable copy constructor or assignment operator for the class
MovingObject. When a MovingObject object is copied you are copying the
movementAlgorit hm data member and hence end up deleting the same memory
twice. But that is only a guess.

The solution to all these problems is to use pointer in a less cavalier
fashion. I don't know exactly what you are tring to achieve but here are
some suggestions.

1) To you really need to use pointers at all? Could you not define
MovingObject like this

class MovingObject
{
...
MovementAlgorit hm movementAlgorit hm;
};

2) If you really do need pointers then read up on the 'rule of three'. as
soon as you include a pointer in a class you almost always need to define
proper copy constructors and assignment operators for that class. I
suspect that you are not doing that, although I don't know because you
didn't post the code.

3) As soon as you start allocating objects using new, then you must think
about whose responsiblity it is to free that pointer. In other words who
owns the pointer. The way you have described your MovingObject class so
far it sounds as if the MovementAlgorit hm object is allocated outside the
MovingObject class but a MovingObject object takes ownership of the
pointer when SetMovementAlgo rithm is called. Are you happy and clear about
that?

4) Learn about smart pointers, which take care of many of the problems in
correctly freeing allocated objects automatically. For instance have a
look at the shared_ptr class available from boost
http://www.boost.org/libs/smart_ptr/smart_ptr.htm. It could be exactly
what you need.

If all this doesn't help then post the definitions for Movingbject and
MovementAlgorit hm and I'll take another look.

john

[Victor Bazarov]

"Jonathan Ames" <am**@reb.org > wrote...

Moving to C++ from Java, I'm still confused by some aspects of memory
cleanup operations.

For example, let's say I have a class MovingObject which maintains a
pointer to another class MovementAlgorit hm.

MovingObject has a method
SetMovementAlgo rithm(MovementA lgorithm* movementAlgorit hm)

if the body of this method reads
{
this->movementAlgori thm = movementAlgorit hm;
}

is this a memory leak every time a new MovementAlgorit hm is set (other
than the first time)? Because the previous movement algorithm isn't
deleted?
Probably. That is, if 'movementAlgori thm' in fact _was_ allocated by
somebody using 'new'. A 'MovingObject' apparently takes ownership of
the pointer (and is responsible for releasing the memory from there on)
so if you simply copy the value, the previous value is usually lost.

What is the correct way to handle this problem?
Depends on the situation.

If I change the body of the method to read as follows :

if (this->movementAlgori thm != NULL) {
delete this->movementAlgori thm;
There is no need for the 'if' here. 'delete NULL' does nothing.
}
this->movementAlgori thm = movementAlgorit hm;

I get program crash.
It is more likely because the memory to which the 'movementAlgori thm'
points when you try to delete it, wasn't allocated using 'new'.
All of the constructors for MovingObject
initialize the pointer variable either to NULL or through the new
operator. Can anyone explain this?

I could guess. If all of them were in fact allocated using 'new',
then the problem can be in deleting the same object more than once.
Of course, it's just as good a guess as anybody else's who hasn't
seen the complete code.

C++ pointers are tricky. Pointers to dynamically allocated object
are even trickier. They require a lot of attention. That's why it
is often recommended that beginners don't use them.

Victor

[Mateusz £oskot]

On 7/31/2004 7:58 PM, Jonathan Ames wrote:

if the body of this method reads
{
this->movementAlgori thm = movementAlgorit hm;
}

is this a memory leak every time a new MovementAlgorit hm is set (other
than the first time)? Because the previous movement algorithm isn't
deleted?
If the first MovementAlgorit hm object was created dynamicaly on the
heap somewhere in your program, you have to place somewhere delete
action on it, because leaks will occur.

So, one of the possible solution would be to check if
the local member of MovingObject (pointer to MovementAlgorit hm)
is null or not:

{
// Destroy existing algo. object
if (this->movementAlgori thm != NULL)
delete this->movementAlgori thm;

// Assign new one
this->movementAlgori thm = movementAlgorit hm;
}

But you have to track movementAlgorit hm member NULL'ity,
I mean you have to assign NULL to it in MovingObject constructor
or initialization list and if you don't assign new algo. but you have
i.e. method called DetachAlgoFromM ovingObject(); which clears
it you have to again assign NULL so when you assign new algo.
to the object again, NULL'ity of MovingObject assumption will
be true, so no destruction of algorithm will be performed.

I don't know your all code but I think that the statement
this->myPointer is on oe Java habit ;-)
If you don't have any pointer casts (to base or derived class)
and dynamic (virtual) polymorphism I think
that you can simply write the code above this way:

{
// Destroy existing algo. object
if (movementAlgori thm != NULL)
delete this->movementAlgori thm;

// Assign new one
movementAlgorit hm = movementAlgorit hm;
}

What is the correct way to handle this problem?
The above is only my idea, but I don't guarantee it is the
most correct way.
If I change the body of the method to read as follows :

if (this->movementAlgori thm != NULL) {
delete this->movementAlgori thm;
}
this->movementAlgori thm = movementAlgorit hm;
Hehehehe !
I've just seen it ;-)) after I wrote my idea, so
I leave it and post on group but it's funny.
I didn't read whole your post before I write my answer ;-)
I get program crash. All of the constructors for MovingObject
initialize the pointer variable either to NULL or through the new
operator. Can anyone explain this?

You have to assign NULL to pointers by yourself.
There is know default constructor for pointer which would
do it for you.

Greets

--

Mateusz £oskot
mateusz at loskot dot net

[Mateusz £oskot]

On 7/31/2004 8:24 PM, Victor Bazarov wrote:

If I change the body of the method to read as follows :

if (this->movementAlgori thm != NULL) {
delete this->movementAlgori thm;

There is no need for the 'if' here. 'delete NULL' does nothing.

It's new information for me ;-)
Thanks a lot. It seems to be obvious but untill now I thought
deleting null will couse some not-defined state of program.

Thanks ;-))
--

Mateusz £oskot
mateusz at loskot dot net

[Paul]

"John Harrison" <jo************ *@hotmail.com> wrote in message
news:opsb0k3qjt 212331@andronic us...

On Sat, 31 Jul 2004 18:58:25 +0100, Jonathan Ames <am**@reb.org > wrote:

I think you are using it something like this

MovingObject x;
x.SetMovementAl gorithm(new MovementAlgorit hm); // 1
...
x.SetMovementAl gorithm(new MovementAlgorit hm); // 2

Which is a major stumbling block for most Java programmers that I've come in
contact with who are learning C++. They "new" everything under the sun in
their C++ programs, thinking that "new" in C++ means the same thing as "new"
in Java.

Since in Java, every object, except the basic types, must be created using
"new", they believe the same thing is the case for C++. I've cringed when
I've taken a look at some C++ programs written by Java programmers...

Paul

[John Harrison]

On Sun, 01 Aug 2004 04:22:48 GMT, Paul <pa**@paul.co m> wrote:

"John Harrison" <jo************ *@hotmail.com> wrote in message
news:opsb0k3qjt 212331@andronic us...

On Sat, 31 Jul 2004 18:58:25 +0100, Jonathan Ames <am**@reb.org > wrote:

I think you are using it something like this

MovingObject x;
x.SetMovementAl gorithm(new MovementAlgorit hm); // 1
...
x.SetMovementAl gorithm(new MovementAlgorit hm); // 2

Which is a major stumbling block for most Java programmers that I've
come in
contact with who are learning C++. They "new" everything under the sun
in
their C++ programs, thinking that "new" in C++ means the same thing as
"new"
in Java.

Since in Java, every object, except the basic types, must be created
using
"new", they believe the same thing is the case for C++. I've cringed
when
I've taken a look at some C++ programs written by Java programmers...

Paul

Not only that, but also the lack of consideration for pointer ownership
issues. Not surprisingly, when you've been spoilt with Java's garbage
collection, it's a big adjustment when you start coding C++.

I wonder if there is a good C++ for Java programmers book to recommend.

John

[Mateusz £oskot]

On 8/1/2004 10:01 AM, John Harrison wrote:

I wonder if there is a good C++ for Java programmers book to recommend.

I know one book touching this topic:

"Programmin g with Objects: A Comparative Presentation of Object-Oriented
Programming with C++ and Java"

by Avinash C. Kak

ISBN:0471268526

John Wiley & Sons (c) 2003 (1115 pages)
I haven't read it but I know it is on the market.
The title suggest it explains the differences between
C++ and Java and instructs how to solve some problems in
C++ in comparsion to Java.

Greets

--

Mateusz £oskot
mateusz at loskot dot net

[Peter van Merkerk]

Paul wrote:

"John Harrison" <jo************ *@hotmail.com> wrote in message
news:opsb0k3qjt 212331@andronic us...

On Sat, 31 Jul 2004 18:58:25 +0100, Jonathan Ames <am**@reb.org > wrote:

I think you are using it something like this

MovingObjec t x;
x.SetMovement Algorithm(new MovementAlgorit hm); // 1
...
x.SetMovement Algorithm(new MovementAlgorit hm); // 2

Which is a major stumbling block for most Java programmers that I've come in
contact with who are learning C++. They "new" everything under the sun in
their C++ programs, thinking that "new" in C++ means the same thing as "new"
in Java.

Since in Java, every object, except the basic types, must be created using
"new", they believe the same thing is the case for C++. I've cringed when
I've taken a look at some C++ programs written by Java programmers...

It is unfortunate the designers of the Java language choose a syntax
similar to C++. The different semantics combined with similar syntax
makes the crossover between these two languages unnecessarily hard.

--
Peter van Merkerk
peter.van.merke rk(at)dse.nl

[Jonathan Ames]

On Sat, 31 Jul 2004 18:58:25 +0100, Jonathan Ames <am**@reb.org >
wrote:

Moving to C++ from Java, I'm still confused by some aspects of memory
cleanup operations.

For example, let's say I have a class MovingObject which maintains a
pointer to another class MovementAlgorit hm.

MovingObject has a method
SetMovementAlg orithm(Movement Algorithm* movementAlgorit hm)

if the body of this method reads
{
this->movementAlgori thm = movementAlgorit hm;
}

is this a memory leak every time a new MovementAlgorit hm is set (other
than the first time)? Because the previous movement algorithm isn't
deleted?

What is the correct way to handle this problem?

If I change the body of the method to read as follows :

if (this->movementAlgori thm != NULL) {
delete this->movementAlgori thm;
}
this->movementAlgori thm = movementAlgorit hm;

I get program crash. All of the constructors for MovingObject
initialize the pointer variable either to NULL or through the new
operator. Can anyone explain this?

Thanks all for the replies. They were very helpful. I've solved the
immediate problem and now realise that I have some further studying
and reflecting to do on my use of C++ idioms.