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babis85@gmail.com
Guest
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 Posted: Sun Oct 30, 2005 11:29 am Post subject: I cannot deallocate space. |
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Hello, i have a problem, which i cannot actually understand.
I have defined a class, let's call it, X and i have a variable X* tmp,
whom i allocate space with new.Then i use
stack<X* > mystack;
and do
mystack.push(tmp);
And then, i do
tmp = mystack.top();
if (tmp != NULL)
delete tmp;
mystack.pop();
and i get segmentation fault.
Even if i do
if (mystack.top() != NULL)
delete mystack.top();
mystack.pop();
i get the same message.
How can, finally, deallocate the tmp's space ?
Thanks...
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TIT
Guest
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| Posted: Sun Oct 30, 2005 11:54 am Post subject: Re: I cannot deallocate space. |
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[email]babis85 (AT) gmail (DOT) com[/email] sade:
| Quote: | Hello, i have a problem, which i cannot actually understand.
I have defined a class, let's call it, X and i have a variable X* tmp,
whom i allocate space with new.Then i use
stack<X* > mystack;
|
Despite what you say, the presented code lacks the class and
the actual allocation.
This works flawless if you ignore potential exceptions:
#include <stack>
class X {};
int main() {
std::stack<X*> mystack;
X * tmp = new X;
mystack.push(tmp);
tmp = mystack.top();
mystack.pop();
delete tmp;
return 0;
}
| Quote: | and do
mystack.push(tmp);
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Do you allocate before or after the above statement?
| Quote: | And then, i do
tmp = mystack.top();
if (tmp != NULL)
delete tmp;
mystack.pop();
and i get segmentation fault.
|
Most likely because you have pushed something other than the
allocated pointer, or already deleted it, or haven't pushed
anything at all. And notice that you can do this:
X * tmp = 0;
delete tmp;
Copy and paste the code that you're trying to run here.
TIT
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Valentin Samko
Guest
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| Posted: Sun Oct 30, 2005 11:55 am Post subject: Re: I cannot deallocate space. |
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[email]babis85 (AT) gmail (DOT) com[/email] wrote:
| Quote: | Hello, i have a problem, which i cannot actually understand.
I have defined a class, let's call it, X and i have a variable X* tmp,
whom i allocate space with new.Then i use
stack<X* > mystack;
and do
mystack.push(tmp);
|
What's tmp? Are you sure, you are not pushing the same pointer twice?
| Quote: |
And then, i do
tmp = mystack.top();
if (tmp != NULL)
delete tmp;
mystack.pop();
and i get segmentation fault.
Even if i do
if (mystack.top() != NULL)
delete mystack.top();
mystack.pop();
i get the same message.
|
Can you post the code which compiles and generates a segmentation fault?
--
Valentin Samko - http://www.valentinsamko.com
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gary
Guest
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| Posted: Sun Oct 30, 2005 11:57 am Post subject: Re: I cannot deallocate space. |
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On 30 Oct 2005 03:29:22 -0800
[email]babis85 (AT) gmail (DOT) com[/email] wrote:
| Quote: | Hello, i have a problem, which i cannot actually understand.
I have defined a class, let's call it, X and i have a variable X* tmp,
whom i allocate space with new.Then i use
stack<X* > mystack;
and do
mystack.push(tmp);
tmp is allocated by new operator? |
| Quote: | And then, i do
tmp = mystack.top();
if (tmp != NULL)
delete tmp;
|
top() is not equal to pop(). When using top(), the top element is not deleted from the stack. But even so, in my test there is no error.
| Quote: | mystack.pop();
After this sentence, do "delete tmp". |
| Quote: | and i get segmentation fault.
Even if i do
if (mystack.top() != NULL)
delete mystack.top();
mystack.pop();
i get the same message.
How can, finally, deallocate the tmp's space ?
Thanks...
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Jim Langston
Guest
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| Posted: Mon Oct 31, 2005 12:48 am Post subject: Re: I cannot deallocate space. |
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<babis85 (AT) gmail (DOT) com> wrote
| Quote: | Hello, i have a problem, which i cannot actually understand.
I have defined a class, let's call it, X and i have a variable X* tmp,
whom i allocate space with new.Then i use
stack<X* > mystack;
and do
mystack.push(tmp);
And then, i do
tmp = mystack.top();
if (tmp != NULL)
delete tmp;
mystack.pop();
|
Here you are popping something you already deleted. Pop it first then
delete.
tmp = mystack.top();
if (tmp != NULL)
{
mystack.pop();
delete tmp;
}
| Quote: |
and i get segmentation fault.
Even if i do
if (mystack.top() != NULL)
delete mystack.top();
mystack.pop();
i get the same message.
How can, finally, deallocate the tmp's space ?
Thanks...
|
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Old Wolf
Guest
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| Posted: Mon Oct 31, 2005 3:30 am Post subject: Re: I cannot deallocate space. |
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[email]babis85 (AT) gmail (DOT) com[/email] wrote:
| Quote: | Hello, i have a problem, which i cannot actually understand.
I have defined a class, let's call it, X and i have a variable X* tmp,
whom i allocate space with new.Then i use
stack<X* > mystack;
and do
mystack.push(tmp);
And then, i do
tmp = mystack.top();
if (tmp != NULL)
delete tmp;
mystack.pop();
and i get segmentation fault.
|
You have to do the pop() before you do the delete, eg:
tmp = mystack.pop();
delete tmp;
If you still get errors after making this change, then you
have an error elsewhere in your code. See if you can post
a compilable program that demonstrates the error.
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TIT
Guest
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| Posted: Mon Oct 31, 2005 5:25 am Post subject: Re: I cannot deallocate space. |
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Old Wolf sade:
| Quote: | babis85 (AT) gmail (DOT) com wrote:
Hello, i have a problem, which i cannot actually understand.
I have defined a class, let's call it, X and i have a variable X* tmp,
whom i allocate space with new.Then i use
stack<X* > mystack;
and do
mystack.push(tmp);
And then, i do
tmp = mystack.top();
if (tmp != NULL)
delete tmp;
mystack.pop();
and i get segmentation fault.
You have to do the pop() before you do the delete, eg:
tmp = mystack.pop();
delete tmp;
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Assuming he's using std::stack<> then that won't even compile
since std::stack<>::pop() returns void.
TIT
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TIT
Guest
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| Posted: Mon Oct 31, 2005 5:30 am Post subject: Re: I cannot deallocate space. |
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Jim Langston sade:
| Quote: | babis85 (AT) gmail (DOT) com> wrote in message
news:1130671762.158221.188790 (AT) o13g2000cwo (DOT) googlegroups.com...
Hello, i have a problem, which i cannot actually understand.
I have defined a class, let's call it, X and i have a variable X* tmp,
whom i allocate space with new.Then i use
stack<X* > mystack;
and do
mystack.push(tmp);
And then, i do
tmp = mystack.top();
if (tmp != NULL)
delete tmp;
mystack.pop();
Here you are popping something you already deleted. Pop it first then
delete.
tmp = mystack.top();
if (tmp != NULL)
{
mystack.pop();
delete tmp;
}
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It doesn't matter if he pops before or after he deletes the top.
TIT
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TIT
Guest
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| Posted: Mon Oct 31, 2005 5:56 am Post subject: Re: I cannot deallocate space. |
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TIT sade:
| Quote: | Jim Langston sade:
[email]babis85 (AT) gmail (DOT) com[/email]> wrote in message
news:1130671762.158221.188790 (AT) o13g2000cwo (DOT) googlegroups.com...
Hello, i have a problem, which i cannot actually understand.
I have defined a class, let's call it, X and i have a variable X* tmp,
whom i allocate space with new.Then i use
stack<X* > mystack;
and do
mystack.push(tmp);
And then, i do
tmp = mystack.top();
if (tmp != NULL)
delete tmp;
mystack.pop();
Here you are popping something you already deleted. Pop it first then
delete.
tmp = mystack.top();
if (tmp != NULL)
{
mystack.pop();
delete tmp;
}
It doesn't matter if he pops before or after he deletes the top.
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Sorry, that should read:
It doesn't matter if he pops before of after he deletes the already
aquired top value.
TIT
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savagesmc
Guest
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| Posted: Mon Oct 31, 2005 6:00 pm Post subject: Re: I cannot deallocate space. |
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You can't do the NULL check on mystack.top(). What you want to do is
check to see if mystack is empty or not before calling top to get a
pointer to delete. Something like:
while (!mystack.empty())
{
delete mystack.top();
mystack.pop();
}
This will work assuming that you actually inserted valid pointers into
the container.
Hope that helps.
Steve
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Old Wolf
Guest
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| Posted: Mon Oct 31, 2005 8:12 pm Post subject: Re: I cannot deallocate space. |
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TIT wrote:
| Quote: | Old Wolf sade:
[email]babis85 (AT) gmail (DOT) com[/email] wrote:
tmp = mystack.top();
if (tmp != NULL)
delete tmp;
mystack.pop();
and i get segmentation fault.
You have to do the pop() before you do the delete, eg:
tmp = mystack.pop();
delete tmp;
Assuming he's using std::stack<> then that won't even compile
since std::stack<>::pop() returns void.
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Let me try again:
tmp = mystack.top();
mystack.pop();
delete tmp;
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