Help with destruction of an object created with the new operator- destructors

[Pablo]

Hello,
I am writing a windows application using C++ and BorlandBuilder 6
compiler. It is an event driven program and I need to create objects of
some classes written by me. One of the classes contains a pointer to int
values as a filed. In the definition (implementation) of constructor I
use this pointer to create table of int values with the new operator.
The number of elements of the table is provided by the user during
execution of the program (dynamic memory allocation).
After generating the objects and the tables, the results are displayed
in another form (another unit i.e. cpp file is created).

After seeing the results, the user may want to quit the program or
generate another object with the same name but the number of elements
in the table can be different (less or more elements) and again see the
results.
My question is:
when I should destroy the object and the table and release the memory
from the free store?

I know that once the memory is allocated with the new operator it has to
be released with the delete. I think that delete should be used in
definition of the destructor. I think that I should release allocated
memory before exiting the program. In my program the exiting is done by
pressing button and the code for Button1Click is simply execution of the
method Close() for the main form. So before exiting the code for
destructors is written there. The code obviously consists of delete
operator for releasing memory reserved for tables.

But if the user wants to generate the object again, I should release
the memory and destroy the object in order to create it again. Should I
call the destructors by writing the code i.e.

ClassName::~ClassName();

before starting the generation of the object again?

Regards
Pawel

 

[Kai-Uwe Bux]

Hello,
I am writing a windows application using C++ and BorlandBuilder 6
compiler. It is an event driven program and I need to create objects of
some classes written by me. One of the classes contains a pointer to int
values as a filed. In the definition (implementation) of constructor I
use this pointer to create table of int values with the new operator.
The number of elements of the table is provided by the user during
execution of the program (dynamic memory allocation).
After generating the objects and the tables, the results are displayed
in another form (another unit i.e. cpp file is created).

After seeing the results, the user may want to quit the program or
generate another object with the same name but the number of elements
in the table can be different (less or more elements) and again see the
results.
My question is:
when I should destroy the object and the table and release the memory
from the free store?

I know that once the memory is allocated with the new operator it has to
be released with the delete.

You said you allocate a table, then you want to use new[] and delete[]
accordingly. (However, see the final paragraph.)

I think that delete should be used in
definition of the destructor.

That is the standard way of doing it: new in constructors match a delete in
the destructor. This way, every object cleans up its own mess.

I think that I should release allocated
memory before exiting the program. In my program the exiting is done by
pressing button and the code for Button1Click is simply execution of the
method Close() for the main form. So before exiting the code for
destructors is written there. The code obviously consists of delete
operator for releasing memory reserved for tables.

I don't follow this: what does this Close() method do? Does it call exit()
or halt()? How does it terminate the program? Since Close() is not a
standard method, I have no way to tell whether object will be properly
destructed. If they are (and they should), then putting delete[] within the
destructor would be sufficient.

But if the user wants to generate the object again,

That would be impossible, you cannot resurrect objects. Only a different
object can be created replacing the dead body of the old one.

I should release
the memory and destroy the object in order to create it again. Should I
call the destructors by writing the code i.e.

ClassName::~ClassName();

before starting the generation of the object again?

No. The language does lifetime management for you in automatic and temporary
objects, and you do lifetime management for objects created via new and
new[] by calling delete and delete[]. Explicit destructor calls are rarely
what you want (I only use them when I write my own allocators replacing
std::allocator). In your case, it seems, undefined behavior would very
likely be the result of explictly calling the destructor.


Finally: you should not use int* for any of this anyway. The management of
pointers (especially arrays) is a little tricky when it comes to exception
safety. Just use std::vector<int> instead.


Best

Kai-Uwe Bux

 

[peter koch]

Pablo skrev:

Hello,
I am writing a windows application using C++ and BorlandBuilder 6
compiler. It is an event driven program and I need to create objects of
some classes written by me.
One of the classes contains a pointer to int
values as a filed. In the definition (implementation) of constructor I
use this pointer to create table of int values with the new operator.

This sounds like you should use std::vector
[snip]

After seeing the results, the user may want to quit the program or
generate another object with the same name but the number of elements
in the table can be different (less or more elements) and again see the
results.
My question is:
when I should destroy the object and the table and release the memory
from the free store?

You should most likely create a new element for each iteration. If the
only purpose is to save a little time creating and destroying objects,
then you definitely should create a new object every time. First
because the most important aspect of your code is clarity (e.g. it
should be easy to read by a fellow programmer), secondly because it
very well might be faster this way (or not measurably slower).

I know that once the memory is allocated with the new operator it has to
be released with the delete. I think that delete should be used in
definition of the destructor. delete calls the destructor.
I think that I should release allocated
memory before exiting the program. In my program the exiting is done by
pressing button and the code for Button1Click is simply execution of the
method Close() for the main form. So before exiting the code for
destructors is written there. The code obviously consists of delete
operator for releasing memory reserved for tables.

Again, use a std::vector and you will not have any dynamic memory in
your class.

But if the user wants to generate the object again, I should release
the memory and destroy the object in order to create it again. Should I
call the destructors by writing the code i.e.

ClassName::~ClassName();

before starting the generation of the object again?

(The syntax is object->~ClassName(); )
No - destructors should almost never be called directly. If you have
newed your object, delete object is enough.

/Peter

 

[Howard]

Hello,
I am writing a windows application using C++ and BorlandBuilder 6
compiler. It is an event driven program and I need to create objects of
some classes written by me. One of the classes contains a pointer to int
values as a filed. In the definition (implementation) of constructor I use
this pointer to create table of int values with the new operator. The
number of elements of the table is provided by the user during execution
of the program (dynamic memory allocation).
After generating the objects and the tables, the results are displayed in
another form (another unit i.e. cpp file is created).

After seeing the results, the user may want to quit the program or
generate another object with the same name but the number of elements
in the table can be different (less or more elements) and again see the
results.
My question is:
when I should destroy the object and the table and release the memory from
the free store?

I know that once the memory is allocated with the new operator it has to
be released with the delete. I think that delete should be used in
definition of the destructor. I think that I should release allocated
memory before exiting the program. In my program the exiting is done by
pressing button and the code for Button1Click is simply execution of the
method Close() for the main form. So before exiting the code for
destructors is written there. The code obviously consists of delete
operator for releasing memory reserved for tables.

But if the user wants to generate the object again, I should release
the memory and destroy the object in order to create it again. Should I
call the destructors by writing the code i.e.

ClassName::~ClassName();

before starting the generation of the object again?

If you use new[] to create an array of int's in the constructor, then your
should use delete[] to delete them in the destructor. (You'll probably also
want to add a copy-constructor and an assignment operator. Read up on the
"Rule of Three". A google search will find plenty of info on that.)

You have two options (at least) for handling creation of a new array.

When you want a new array, you could delete your instance of the class which
created the array, and then create a new instance of that class. When you
delete the old instance, its destructor will delete[] the array for you
(assuming you followed my earlier instruction). Then when you create the
new instance of the class, its constructor will allocate a new array using
new[] (according to your comments, at least).

Alternatively, you can add a function to that class which lets you delete
the old array and allocate a new one. Something like this:

MyClass::RecreateArray( int size )
{
delete [] myIntArray;
myIntArray = NULL;
myIntArray = new int[size];
}

(I set the pointer to NULL just in case the new[] fails, so that when the
destructor is called, the delete[] call inside the destructor won't attempt
to delete the array a second time.)

-Howard

 

[Howard]

MyClass::RecreateArray( int size )

oops, obviously that should be:

void MyClass::RecreateArray( int size )

{
delete [] myIntArray;
myIntArray = NULL;
myIntArray = new int[size];
}

 

[Pablo]

when I should destroy the object and the table and release the memory from
the free store?

If you use new[] to create an array of int's in the constructor, then your
should use delete[] to delete them in the destructor. (You'll probably also
want to add a copy-constructor and an assignment operator. Read up on the
"Rule of Three". A google search will find plenty of info on that.)

Thank you for the hint, I'll definitely do that. I'd like to thank all
of you who helped me understand problem of creation and destruction of
instances however, I have some more questions...

You have two options (at least) for handling creation of a new array.

When you want a new array, you could delete your instance of the class which
created the array, and then create a new instance of that class. When you
delete the old instance, its destructor will delete[] the array for you
(assuming you followed my earlier instruction). Then when you create the
new instance of the class, its constructor will allocate a new array using
new[] (according to your comments, at least).

But what would happen at the beginning, when the program runs and
creates the instance (and the array) for the first time?
Only the pointer is declared but the new didn't do anything yet. So the
pointer is pointing to NULL and the destruction will not take place - is
that correct way of thinking?

Regards
Pablo

 

[Howard]

when I should destroy the object and the table and release the memory
from the free store?

If you use new[] to create an array of int's in the constructor, then
your should use delete[] to delete them in the destructor. (You'll
probably also want to add a copy-constructor and an assignment operator.
Read up on the "Rule of Three". A google search will find plenty of info
on that.)

Thank you for the hint, I'll definitely do that. I'd like to thank all of
you who helped me understand problem of creation and destruction of
instances however, I have some more questions...

You have two options (at least) for handling creation of a new array.

When you want a new array, you could delete your instance of the class
which created the array, and then create a new instance of that class.
When you delete the old instance, its destructor will delete[] the array
for you (assuming you followed my earlier instruction). Then when you
create the new instance of the class, its constructor will allocate a new
array using new[] (according to your comments, at least).

But what would happen at the beginning, when the program runs and creates
the instance (and the array) for the first time?
Only the pointer is declared but the new didn't do anything yet. So the
pointer is pointing to NULL and the destruction will not take place - is
that correct way of thinking?

If the pointer is set to NULL in the constructor, any call to delete[] on
that NULL pointer will do nothing. It's perfectly safe.

-Howard