C++ MapType::iterator using updated value

Question

In my C++ code, I access a map through iterator. Update the map if necessary and re-assign it to class variable. In proceeding statements, I want to use updated map value again. Should I load the map again, refresh the iterator? etc. e.g. the map is:

MapType tbl = device->trust();
MapType::iterator iter_trust = tbl.begin();
tbl.insert(std::pair<int, double> (node->getId().id(), 0.1));

to execute the following on updated value, what should I do?

iter_trust = tbl.find(node->getId().id());

What is your question? Whether the iterator is invalidated? All the iterator invalidation rules are [here](http://stackoverflow.com/questions/6438086/iterator-invalidation-rules). — Lightness Races in Orbit, Sep 15 '11 at 09:22
BTW, your `tbl` is a _copy_ of the original map. Did you want that? — Lightness Races in Orbit, Sep 15 '11 at 09:24

score 1 · Accepted Answer · edited Sep 15 '11 at 09:23

1

MapType tbl = device->trust();
MapType::iterator iter_trust = tbl.find(node->getId().id());

if (iter_trust == tbl.end()) {
   tbl.insert(std::make_pair(node->getId().id(), 0.1));
   iter_trust = tbl.find(node->getId().id());
}
else {
   tbl[node->getId().id()] = 0.1;
}

So you'll be sure you upgrade.

edited Sep 15 '11 at 09:23

Lightness Races in Orbit

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answered Sep 15 '11 at 09:07

Miguel Angel

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thanks, of course the map is updated. and i would not necessarily be using the last inserted pair in next line. should i reload the map and refresh the iterator? – Kashif Sep 15 '11 at 09:09
OK, I edited my code (add line 5) for update the iterator value. – Miguel Angel Sep 15 '11 at 09:12
Sorry Kashif, I didn't read your real cuestion. No, you shouldn't update the map or the iterator. – Miguel Angel Sep 15 '11 at 09:17
OK, that was obvious, just wanted to make sure... Thanks – Kashif Sep 15 '11 at 09:26

score 0 · Answer 2 · answered Sep 15 '11 at 08:51

std::map::insert returns an iterator to the newly inserted element. If your MapType is some typedef to a std::map, you can do

std::pair<MapType::iterator, bool> result = tbl.insert(std::make_pair(node->getId().id(), 0.1));
MapType::iterator iter_trust = result.first;

and go on.

Btw, note the use of std::make_pair, which is usually simpler than using the pair constructor directly.

Also, note that map::insert returns a pair of an iterator and a boolean. The iterator gives you the position of the inserted object, the boolean indicates if the insertion was successful (i.e. the key didn't previously exist in the map)

thanks, but the map is updated. and i would not necessarily be using the last inserted pair in next line. should i reload the map and refresh the iterator? — Kashif, Sep 15 '11 at 09:03

C++ MapType::iterator using updated value

2 Answers2