Writing a map into a file C++

I have two methods of doing this. First one:

string output;
for (auto& kv : stored) {
  output.append(kv.second + "\n");
}
insert(file, output);

where insert() is:

void insert(string file, string str) {
  ifstream stream(file);
  stream << str;
  stream.close();
}

Second method:

ifstream stream(file);
for(auto& kv : stored) {
  stream << kv.second;
}
stream.close();

where in both cases, stored is of type map<int, string>

Which method is more efficient?

EDIT: For the people who are telling me this is a duplicate, I want to know what's the most efficient method to use out of the two, not how to do it.