How do I delete elements from an std::set that is stored inside a std::map?

Question

I have the following data structure stored in a class.

class MyClass {
 private:
  std::map<std::string, std::set<std::string>> myMap;
 public:
  void remove(std::string id); //trying to remove items from sets inside myMap
}

There is then a method to attempt to delete items from the sets. I tried the following 2 ways but neither worked. Approach 1, use for range.

for (auto pair : myMap) {
 auto set = pair.second;
 if (set.count(id)) {
  set.erase(id);
 }
}

Approach 2, use iterator.

auto it = myMap.begin();
while (it != myMap.end()) {
 auto set = it->second;
 if (set.count(id)) {
  set.erase(id);
 }
 it++;
}

What's the right way of removing elements from the sets inside a map in C++? Note that my code used to work when I had myMap defined as std::map<std::string, std::set<std::string>*> (pointers).

Answer 1

Suppose we have a std::map like this:

std::map<std::string, std::set<std::string>> myMap;
myMap["a"].insert("aaa");
myMap["a"].insert("abb");
myMap["a"].insert("acc");

myMap["b"].insert("aaa");
myMap["b"].insert("abb");
myMap["b"].insert("acc");

Then you can delete items from std::set by doing the following:

for (auto& i : myMap) {
    i.second.erase("aaa");
}

Demo

Why approach from the question is not working?

Because, by doing the following for(auto pair : myMap) {...} and auto set = pair.second; you are actually working on the copy of the data from myMap. So, instead you need to use references to the actual data like for(auto& pair : myMap) {...} and auto& set = pair.second;.

Furthermore, std::set::erase removes the data from the std::set if it exists so there is no need to manually check for the existence of the id.