Erasing map elements in a while loop

[indikau]

I have a map with following signature.

map<const char*, int, Comparator> map_Keys; //Comparator is defined elsewhere 

I need to clear the map after the use of it. Does following is valid for that?

map<const char*, int, Comparator>::iterator ite = map_Keys.begin();

while ( ite != map_Keys.end() )
{
    delete [] ite->first;
    ite->first = NULL;

    map_Keys.erase(ite); //I'm not sure does this advances the ite for next element
}

[NVTKrishna]

No it will not advances, You have to post increment the iterator explicitly.

map<const char*, int, Comparator>::iterator ite = map_Keys.begin();

while ( ite != map_Keys.end() )
{
    delete [] ite->first;
    ite->first = NULL;

    map_Keys.erase(ite++);
}

[Peter87]

erase returns an iterator to the element after so you can write

map<const char*, int, Comparator>::iterator ite = map_Keys.begin();

while ( ite != map_Keys.end() )
{
    delete [] ite->first;

    ite = map_Keys.erase(ite);
}

[indikau]

Thanks. So what is the best way to implement the clearing of map?

 map_Keys.erase(ite++);

or

ite = map_Keys.erase(ite);

Does erasing will return next element as vectors & deque?

In the reference section I found that map & set do not return an iterator. But vector & deque does return an iterator on erase.

[Peter87]

In the reference section I found that map & set do not return an iterator.

This was changed in C++11

[moorecm]

FYI, the best way to clear a map is to call clear().

[indikau]

Thanks Peter. I'm not using the C++11 yet. I would expect ite = map_Keys.erase(ite); will work.

Anyway map.clear() will free the char buffers I created on heap?

[Peter87]

no clear() will not call delete[].

If the keys are strings why not use std::string instead? If it's just some char data array why not use std::vector? That way you don't need use a custom comparator (unless you have special needs).

[moorecm]

Right, I missed the fact that you wanted to delete the pointers and then clear the map. That is a valid approach--to loop once to delete and then call clear.

However, using std::string is what I would do.

[indikau]

Yes, I did the correct way of clearing the char keys of the map (in the very first code). Char keys are used to gain more speed.

Thanks for the help.

[Cubbi]

char keys are used to gain more speed

More speed compared to what, lookup in a map with std::string keys? That does not sound plausible. Do you have a test case?

[indikau]

No, I don't use std::string throughout my program. Hence I don't want to keep it as strings in map and avoiding string creations.