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gfiuni2
Guest
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 Posted: Thu Aug 24, 2006 9:10 am Post subject: Valid pointer to map element after erase |
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Hi,
Given the code below:
#include <iostream.h>
#include <string>
#include <map>
int main()
{
map<int, string > m;
string *ptr;
m[1]="one";
m[2]="two";
map<int, string >::iterator it=m.find(1);
ptr=&(it->second);
m.erase(m.find(2));
cout << *ptr << endl;
}
Is ptr a valid pointer after erasing elements from map?
Thanks in advance,
Jose Luis. |
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Kai-Uwe Bux
Guest
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| Posted: Thu Aug 24, 2006 9:10 am Post subject: Re: Valid pointer to map element after erase |
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gfiuni2 wrote:
| Quote: |
Hi,
Given the code below:
#include <iostream.h
|
This header never was part of the standard. Upgrade to:
#include <iostream>
| Quote: | #include <string
#include <map
int main()
{
map<int, string > m;
string *ptr;
m[1]="one";
m[2]="two";
map<int, string >::iterator it=m.find(1);
ptr=&(it->second);
m.erase(m.find(2));
cout << *ptr << endl;
}
Is ptr a valid pointer after erasing elements from map?
|
According to [23.1.2/8] erase (for associative containers) does not
invalidate references and iterators to elements other than the erased one.
It does not mention pointers, which appears to be an oversight. Pointers
are mentioned along with references and iterators where invalidation is
mentioned elsewhere (e.g., the standard says that swap for containers does
not invalidate pointers, reference, and iterators into a container). So
formally, your program may be undefined; but that appears to be a defect in
the standard and no implementation would actually invalidate the pointer
ptr in your code.
Best
Kai-Uwe Bux |
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Guest
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| Posted: Thu Aug 24, 2006 9:10 am Post subject: Re: Valid pointer to map element after erase |
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gfiuni2 wrote:
| Quote: | Hi,
Given the code below:
#include <iostream.h
#include <string
#include <map
int main()
{
map<int, string > m;
string *ptr;
m[1]="one";
m[2]="two";
map<int, string >::iterator it=m.find(1);
ptr=&(it->second);
m.erase(m.find(2));
cout << *ptr << endl;
}
Is ptr a valid pointer after erasing elements from map?
Thanks in advance,
Jose Luis.
|
No, it will be invalid.
The erase is going to destroy the string object stored in the map.
You can try this by adding a user defined object to the map and then
calling erase. The destructor will get called when erase is called.
amir kamerkar |
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Kai-Uwe Bux
Guest
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| Posted: Thu Aug 24, 2006 9:10 am Post subject: Re: Valid pointer to map element after erase |
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amirkam1 (AT) yahoo (DOT) com wrote:
| Quote: | gfiuni2 wrote:
Hi,
Given the code below:
#include <iostream.h
#include <string
#include <map
int main()
{
map<int, string > m;
string *ptr;
m[1]="one";
m[2]="two";
map<int, string >::iterator it=m.find(1);
ptr=&(it->second);
m.erase(m.find(2));
cout << *ptr << endl;
}
Is ptr a valid pointer after erasing elements from map?
Thanks in advance,
Jose Luis.
No, it will be invalid.
The erase is going to destroy the string object stored in the map.
|
Did you notice that ptr points to the string whose key is 1 whereas the item
erased is the one whose key is 2?
| Quote: | You can try this by adding a user defined object to the map and then
calling erase. The destructor will get called when erase is called.
|
Best
Kai-Uwe Bux |
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