STD Map clear() strange behaviour

Question

I defined a map in this way:

map<unsigned int, map<unsigned int, std::shared_ptr<MyObject>>> map;

The first map is preinitialized with some keys and empty maps (the inner maps).

I have a piece of code which operates with this map:

for(auto mapElement : map){
  //cout << "1) " << mapElement.second.size() << endl;
  if(mapElement.second.size()>0){
    // do something
  }
  mapElement.second.clear();
  cout << "2) " << mapElement.second.size() << endl;
}
for(auto mapElement : overwrittenMsgs){
  cout << "3) " << mapElement.second.size() << endl;
}

This a possible output of one iteration:

1) 2
2) 0
1) 1
2) 0
3) 2
3) 1

So it seems that clear() is not really working.

I can fix the problem by replacing mapElement.second.clear(); with map.at(mapElement.first).clear();.

What's the reason of this behaviour?

Answer 1

It' because you loop with copies. Change the loop to use references instead:

for(auto& mapElement : map){ ... }