CODEWITHSUNDEEP
c++qtiterator
ASten
ASten

Reputation: 804

Is it correct to walk through QMap with iterators and erasing/adding items?

Is it correctly possible to walk sequently through QMap with help of iterators, and doing such actions: removing some items and adding new ones?

For example:

for( QMap<key_t,val_t>::iterator it = map.begin();
     it != map.end();
     ++it )
{
     if( it->value == something )
     {
          map.erase(it);
          map.insert(it->key+10,it->value); 
     } 
}

It seems, that nothing will be done wrong, I'm asking to be sure. (I have no enough time to check it).

UPD Will solve with QMap::unite():

for( QMap<key_t,val_t>::iterator it = map.begin();
     it != map.end();
     ++it )
{
     if( it->value == something )
     {
          tmp_map.insert(it->key+10,it->value); 
          map.erase(it);
     } 
}
map.unite(tmp_map);

Thanks for answers!

Upvotes: 7

Views: 11289

Answers (4)

Some programmer dude
Some programmer dude

Reputation: 409176

Think about it a little while... You are iterating over a collection, removing an item in the middle and adding another item somewhere else. Will the iterators still be correct? Will the "next" iterator really be the next item?

In general it is not a good idea to change a collection you are iterating over. If you need to then use a temporary collection and copy selected items over to that, and clear the real collection and move the items from the temporary collection over to the real one.

In your case though, why not use QMap::find to search for something, and if found erase it and add the new item, and do it in a loop until something is not found anymore?

Upvotes: 4

Mike Seymour
Mike Seymour

Reputation: 254461

The iterator will be invalidated by erase, and so cannot safely be used or incremented afterwards. The following should work:

for( QMap<key_t,val_t>::iterator it = map.begin(); it != map.end(); ) 
{
    if( it->value == something ) 
    {
        map.insert(it.key()+10,it.value()); 
        it = map.erase(it);
    } else {
        ++it;
    }
}

Upvotes: 14

Bo Persson
Bo Persson

Reputation: 92261

I would expect it to be invalid after map.erase(it), in which case it->value and ++it will not work.

Upvotes: 2

cmannett85
cmannett85

Reputation: 22346

You have to 'reset' your iterator to the one returned by erase and insert. It's fine in principle though.

Upvotes: 1

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