Most efficient way to erase from a set while iterating over it

Question

What's the most efficient way to erase from a set while iterating over it? Here are two approaches I've thought of, which is the best between them? Is there another better way?

void WaitForFiles(std::set<string> files) {
  while (files.size() > 0) {
    std::set<string> found_files;
    for (const auto& file : files) {
      if (Exists(file)) {
        found_files.insert(file);
      }
    }
    for (const auto& found_file : found_files) {
      files.erase(file);
    }
  }
}

Using set_difference:

void WaitForFiles(std::set<string> files) {
  while (files.size() > 0) {
    std::set<string> found_files;
    for (const auto& file : files) {
      if (Exists(file)) {
        found_files.insert(file);
      }
    }
    std::set<string> difference;
    std::set_difference(files.begin(), files.end(),
                        found_files.begin(), found_files.end(),
                        std::inserter(difference, difference.end()));
    files = difference;
  }
}

Note that the following crashes:

void WaitForFiles(std::set<string> files) {
  while (files.size() > 0) {
    for (const auto& file : files) {  // <-- seg fault after first erase
      if (Exists(file)) {
        files.erase(file);
      }
    }
  }
}

For determining efficiency, keep in mind that in my case the file may take 30 minutes to come into existence, so the Exists function will get called many times and the files set will not change very often compared to the number of times the loop is iterated over.

Answer 1

It is undefined behaviour to erase from a set within a ranged-based for loop (even if it appears to work). Ranged-based for loops use an iterator internally and erasing the element invalidates the iterator.

But std::set::erase returns a valid iterator to the next element in the std::set so you can use an explicit iterator loop instead:

for(auto itr = files.cbegin(); itr != files.cend();) {
  if (exists(*itr)) {
    std::cout << "Found file: " << *itr << "\n";
    itr = files.erase(itr);
  } else
    ++itr;
}

Live demo.

Answer 2

With std::experimental::erase_if from v2 of the library fundamentals TS:

std::experimental::erase_if(files, Exists);

If Exists is overloaded or is a function template, use a lambda:

std::experimental::erase_if(files, [](const auto& f) { return Exists(f); });

This is already implemented in VS2015.