Correct way to handle error in boost::filesystem::recursive_directory_iterator::increment()?

From: Dominique Pellé (dominique.pelle_at_[hidden])
Date: 2021-01-22 18:00:43

Hi

I see code like this in an application to iterate over
files in a directory with boost::filesystem (v1.70) without
using exceptions:

    boost::filesystem::path dir("somedir");
    boost::system::error_code error;
    boost::filesystem::directory_iterator node(dir, error), end;

    if (error) {
      return ...;
    }
    for (; node != end; node.increment(error)) {
      const boost::filesystem::path& path = node->path();
      // ...
    }

It appears to work... or mostly work since a crash
was reported and the stack points to this area of the
code. It's a rare crash that I could not reproduce myself.

I suspect that there is a bug in the above code and it
should instead be:

    boost::filesystem::path dir("somedir");
    boost::system::error_code error;
    boost::filesystem::directory_iterator node(dir, error), end;

    for (; !error && node != end; node.increment(error)) {
      const boost::filesystem::path& path = node->path();
      // ...
    }

In other words, I suspect that we need to check that
node.increment(error) in the for loop did not give an error
before checking the condition "node != end", or else we
may enter the loop in case of error and then it may access
node->path() with an invalid node iterator.

I could not find in the doc at ...
  https://www.boost.org/doc/libs/1_75_0/libs/filesystem/doc/reference.html
... what exactly happens when node.increment(error) gives
an error. Perhaps the doc deserves clarification or an example
illustrating how to reliably iterate over files without using
exceptions would be welcome.

Thanks
Dominique