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2704. recursive_directory_iterator's members should require '*this is dereferenceable'

Section: 31.12.12.2 [fs.rec.dir.itr.members], 31.12.11.2 [fs.dir.itr.members] Status: C++17 Submitter: Eric Fiselier Opened: 2016-05-08 Last modified: 2017-07-30

Priority: 1

View all other issues in [fs.rec.dir.itr.members].

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Discussion:

In 31.12.12.2 [fs.rec.dir.itr.members] the following members are specified as having the requirement "*this != recursive_directory_iterator{}":

• options()

• depth()

• recursion_pending()

• operator++

• increment(...)

• pop()

• disable_recursion_pending()

This requirement is not strong enough since it still allows non-dereferenceable iterators to invoke these methods. For example:

recursive_directory_iterator it(".");
recursive_directory_iterator it_copy(it);
assert(it_copy.depth() == 0); // OK
++it;
assert(it_copy.depth() == ???); // Not OK
auto x = *it_copy; // Is this OK?

I believe these should instead require that *this is dereferenceable, however the current specification seems to say that all previous copies of it are still dereferenceable although not what they dereference to.

[fs.class.directory_iterator] p4:

The result of operator* on an end iterator is undefined behavior. For any other iterator value a const recursive_directory_entry& is returned. The result of operator-> on an end iterator is undefined behavior. For any other iterator value a const directory_entry* is returned.

Is the intention of this clause to make all non-end iterators dereferenceable?

One further complication with these methods comes from the specification of recursive_directory_iterator's copy/move constructors and assignment operators which specify the following post conditions:

• this->options() == rhs.options()

• this->depth() == rhs.depth()

• this->recursion_pending() == rhs.recursion_pending()

If rhs is the end iterator these post conditions are poorly stated.

[2016-06, Oulu — Daniel comments]

The loss of information caused by bullet three of the suggested wording below is corrected by 2726⁽ⁱ⁾'s wording.

Voted to Ready 7-0 Monday morning in Oulu

Proposed resolution:

This wording is relative to N4582.

[Drafting note: I have not attempted to fix the specification of the copy/move constructors and assignment operators for recursive_directory_iterator]

[Drafting note: increment directly specifies "Effects: As specified by Input iterators (24.2.3)", so no additional specification is needed.]

1. Change [fs.class.directory_iterator] p4 as indicated:

   -4- The result of operator* on an end iterator is undefined behavior. For any other iterator value a const directory_entry& is returned. The result of operator-> on an end iterator is undefined behavior. For any other iterator value a const directory_entry* is returnedThe end iterator is not dereferenceable.

2. Add a new bullet after the class synopsis in 31.12.12 [fs.class.rec.dir.itr]:

   -?- Calling options, depth, recursion_pending, pop or disable_recursion_pending on an iterator that is not dereferencable results in undefined behavior.

3. Change 31.12.12.2 [fs.rec.dir.itr.members] as indicated:

   directory_options options() const;
   

   -17- Requires: *this != recursive_directory_iterator().

   […]

   int depth() const;
   

   -20- Requires: *this != recursive_directory_iterator().

   […]

   bool recursion_pending() const;
   

   -23- Requires: *this != recursive_directory_iterator().

   […]

   recursive_directory_iterator& operator++();
   recursive_directory_iterator& increment(error_code& ec) noexcept;
   

   -26- Requires: *this != recursive_directory_iterator().

   […]

   void pop();
   

   -30- Requires: *this != recursive_directory_iterator().

   […]

   void disable_recursion_pending();
   

   -32- Requires: *this != recursive_directory_iterator().

   […]