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Max
Guest
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 Posted: Sun Dec 18, 2005 12:54 am Post subject: delete this in destructor |
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I vaguely remember a discussion about this, but can't find it back. Can
I deallocate the memory of a class by having "delete this" in the
destructor? Eg:
class EG
{
private:
int somevariable;
public:
EG(int i){somevariable=i;}
~EG(){delete this;}
}
void function(void)
{
EG eg = new EG(2);
....
}
Will the allocated memory get freed when the function terminates or will
there go something very wrong when this happens?
Regards,
--
Max
C++U2
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Rob
Guest
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| Posted: Sun Dec 18, 2005 2:24 am Post subject: Re: delete this in destructor |
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Max wrote:
| Quote: | I vaguely remember a discussion about this, but can't find it back.
Can I deallocate the memory of a class by having "delete this" in the
destructor? Eg:
class EG
{
private:
int somevariable;
public:
EG(int i){somevariable=i;}
~EG(){delete this;}
}
void function(void)
{
EG eg = new EG(2);
....
}
Will the allocated memory get freed when the function terminates or
will there go something very wrong when this happens?
|
The memory will not be freed when the function terminates, and
something very wrong will probably happen when (or if) the object is
eventually destroyed.
The destructor is invoked in the process of destroying an object (and
deleting an object is one way to destroy it). Placing "delete this;"
into the destructor effectively attempts to destroy the object while
already in the process of destroying it, and results in undefined
behaviour.
Also, within function, eg should be declared as a pointer.
To achieve effective cleanup, three common approaches are;
class EG
{
private:
int somevariable;
public:
EG(int i){somevariable=i;}
~EG(){}
}
void function(void)
{
EG eg(2);
....
// eg will be destroyed as the function returns
}
void AlternateFunction()
{
EG *eg = new EG(2);
delete eg;
}
#include <memory>
void SaferAlternateFunction()
{
std::auto_ptr<EG> eg(new EG(2));
...
}
The first and third forms are safe, as they implicitly destroy the
object. In the first form, the destructor is automatically invoked.
In the third form, the std::auto_ptr<EG> object is invoked as the
function returned, and it's destructor destroys the object created with
operator new.
The second form (with a manual deletion) is unsafer: both because of
potential to forget to delete the object before returning, and because
it takes more effort to ensure the object is destroyed if an exception
is thrown.
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Max
Guest
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| Posted: Sun Dec 18, 2005 5:04 pm Post subject: Re: delete this in destructor |
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Thanks for your answers Rob.
Rob wrote:
| Quote: |
The memory will not be freed when the function terminates, and
something very wrong will probably happen when (or if) the object is
eventually destroyed.
The destructor is invoked in the process of destroying an object (and
deleting an object is one way to destroy it). Placing "delete this;"
into the destructor effectively attempts to destroy the object while
already in the process of destroying it, and results in undefined
behaviour.
[...] |
I was afraid for that.
What I was trying to do is open a file in the constructor and close it
in the destructor. This works fine if you only open the file once and
let it close when you leave the scope of the object. But when the file
opening happens in a loop I only can make it work when I use the
new/delete aproach of the object. And as you stated, that mechanism is
less save. So for that reason I hoped that the destructor could free the
memory, although it was his own memory space too. I understand this will
not work, what is a pitty, because I don't like the try / catch and
finally constructs around the file handling in my functions.
Thanks,
--
Max
C++U2
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Alan Bellingham
Guest
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| Posted: Sun Dec 18, 2005 5:33 pm Post subject: Re: delete this in destructor |
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Max <nomail (AT) 4me (DOT) nl> wrote:
| Quote: | I was afraid for that.
|
Why?
| Quote: | What I was trying to do is open a file in the constructor and close it
in the destructor. This works fine if you only open the file once and
let it close when you leave the scope of the object.
|
So, something like:
{
MyFile f("C:\temptest"); // open
} // close
| Quote: | But when the file
opening happens in a loop I only can make it work when I use the
new/delete aproach of the object.
|
What? What's wrong with
for (int i = 0; i != 10; ++i)
{
MyFile f("C:\temptest"); // open
} // close
| Quote: | not work, what is a pitty, because I don't like the try / catch and
finally constructs around the file handling in my functions.
|
If you think you need try/catch/finally, then you're going the wrong
way.
Oh, and in
MyFile* f = new MyFile("");
...
delete f;
the delete statement does _TWO_ things. It calls the objects destructor,
and then it releases the memory.
Alan Bellingham
--
Team Thai Kingdom
<url:http://www.borland.com/newsgroups/> Borland newsgroup descriptions
<url:http://www.borland.com/newsgroups/netiquette.html> netiquette
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Max
Guest
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| Posted: Sun Dec 18, 2005 6:15 pm Post subject: Re: delete this in destructor |
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Alan Bellingham wrote:
[...]
| Quote: | What? What's wrong with
for (int i = 0; i != 10; ++i)
{
MyFile f("C:\temptest"); // open
} // close
Thanks Alan, I just didn't realize the file would close each loop and I |
thought a new instance of the file object would be created, using up
more and more memory.
[...]
| Quote: |
If you think you need try/catch/finally, then you're going the wrong
way.
Oh, and in
MyFile* f = new MyFile("");
...
delete f;
the delete statement does _TWO_ things. It calls the objects destructor,
and then it releases the memory.
Alan Bellingham
|
Best regards,
--
Max
C++U2
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Alan Bellingham
Guest
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| Posted: Sun Dec 18, 2005 7:39 pm Post subject: Re: delete this in destructor |
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Max <nomail (AT) 4me (DOT) nl> wrote:
| Quote: | Thanks Alan, I just didn't realize the file would close each loop and I
thought a new instance of the file object would be created, using up
more and more memory.
|
Ah, I thought you were working under some form of misconception. Glad to
have cleared it up.
Alan Bellingham
--
Team Thai Kingdom
<url:http://www.borland.com/newsgroups/> Borland newsgroup descriptions
<url:http://www.borland.com/newsgroups/netiquette.html> netiquette
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Remy Lebeau (TeamB)
Guest
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| Posted: Mon Dec 19, 2005 6:52 pm Post subject: Re: delete this in destructor |
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"Max" <nomail (AT) 4me (DOT) nl> wrote
| Quote: | Can I deallocate the memory of a class by having "delete this"
in the destructor?
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No. That would be very dangerous to do. The class is already in a state of
being destroyed. There is no need to 'delete' it again.
Gambit
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