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From: K�r�at on 26 May 2008 18:01
Hi,

The code below sometimes results access violation at hash_map::erase :

char * popItem (int nKey)
{
char * item = NULL;

stdext::hash_map <int, char * >::iterator hmIterator =
m_hmItemMap.find (nKey);

if (hmIterator != m_hmItemMap.end())
{
item = hmIterator->second;

m_hmItemMap.erase (nKey);
}

return item;
}

I can't find any mistake, what is wrong?

Thanks


From: Giovanni Dicanio on 26 May 2008 18:11

"K�r�at" <xx(a)yy.com> ha scritto nel messaggio
news:%23JKVVw3vIHA.2064(a)TK2MSFTNGP05.phx.gbl...


> The code below sometimes results access violation at hash_map::erase :
>
> char * popItem (int nKey)
> {
> char * item = NULL;
[...]

Could you try using std::string instead of char* ?

Giovanni



From: Brian Muth on 26 May 2008 20:43
I'm wondering if the problem lies in code which are not not showing.

Where is the char* value portion of the hash_map being allocated? By any chance are you allocated in on the stack?



From: Giovanni Dicanio on 27 May 2008 09:57

"K�r�at" <xx(a)yy.com> ha scritto nel messaggio
news:%23JKVVw3vIHA.2064(a)TK2MSFTNGP05.phx.gbl...


> The code below sometimes results access violation at hash_map::erase :
>
> char * popItem (int nKey)
> {
> char * item = NULL;
>
> stdext::hash_map <int, char * >::iterator hmIterator =
> m_hmItemMap.find (nKey);

It seems that you did follow-up neither Brian nor me.

However, I still believe that if you use std::string instead of raw char*
pointers, you will have no access violation.

You may consider the following simple example code, which uses std::string
instead of char* (I renamed your popItem() method as Extract()):


<code>

#include <hash_map>
#include <string>

class IntToString
{
public:


void Insert( int key, const std::string & value )
{
typedef std::pair< int, std::string > IntStringPair;

m_map.insert( IntStringPair( key, value ) );
}


std::string Extract( int key )
{
std::string item;

IntToStringMap::iterator it = m_map.find( key );
if ( it != m_map.end() )
{
item = it->second;

m_map.erase( key );
}
else
{
// Not in map: throw an exception
throw std::invalid_argument( "Key not presenet in map." );
}

return item;
}

// ...
// ... other members
// ...

private:

typedef stdext::hash_map< int, std::string > IntToStringMap;
IntToStringMap m_map;
};

</code>


Test like this:

<code>

IntToString map;
map.Insert( 1, "January" );
map.Insert( 2, "February" );
map.Insert( 5, "May" );

cout << map.Extract(5) << endl;
cout << map.Extract(1) << endl;
cout << map.Extract(2) << endl;

</code>


HTH,
Giovanni


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