Re: [PATCH] libstdc++/71005 fix post-increment for filesystem iterators

[Jonathan Wakely <jwakely at redhat dot com>]

On 10/05/16 12:25 +0100, Jonathan Wakely wrote:
> Eric pointed out that the post-increment operator on filesystem
> directory iterators was returning the incremented value, because the
> returnedobject shared its state with the incremented iterator.
>
> This makes post-increment return a proxy, which contains the
> directory_entry value before the increment.
>
> 	PR libstdc++/71005
> 	* include/experimental/bits/fs_dir.h (__directory_iterator_proxy):
> 	New type.
> 	(directory_iterator::operator++(int)): Return proxy.
> 	(recursive_directory_iterator::operator++(int)): Likewise.
> 	* testsuite/experimental/filesystem/iterators/directory_iterator.cc:
> 	Test post-increment.
> 	* testsuite/experimental/filesystem/iterators/
> 	recursive_directory_iterator.cc: Likewise.
>
> Tested x86_64-linux, committed to trunk. Backports to follow.

After discussion with Eric we came up with this optimization, so that
the common case of do_something(*it++) can move the directory_entry
out of the proxy instead of copying it.

Overloading on ref-qualifiers allows this (IIUC invalid) case to still
DTRT:

   auto it2 = it++;   // it2 is actually a proxy, not an iterator
   auto ent1 = *it2;
   auto ent2 = *it2;  // doesn't access moved-from value

commit 49003a7c6896980c797d7fa2f404a2620f8ed389
Author: redi <redi@138bc75d-0d04-0410-961f-82ee72b054a4>
Date:   Tue May 10 13:09:22 2016 +0000

    Optimize __directory_iterator_proxy for the common case
    
    	* include/experimental/bits/fs_dir.h (__directory_iterator_proxy):
    	Overload operator* to move from rvalues.
    
    git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@236078 138bc75d-0d04-0410-961f-82ee72b054a4

diff --git a/libstdc++-v3/include/experimental/bits/fs_dir.h b/libstdc++-v3/include/experimental/bits/fs_dir.h
index 5fd41c2..011d398 100644
--- a/libstdc++-v3/include/experimental/bits/fs_dir.h
+++ b/libstdc++-v3/include/experimental/bits/fs_dir.h
@@ -158,7 +158,9 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
 
   struct __directory_iterator_proxy
   {
-    const directory_entry& operator*() const noexcept { return _M_entry; }
+    const directory_entry& operator*() const& noexcept { return _M_entry; }
+
+    directory_entry operator*() && noexcept { return std::move(_M_entry); }
 
   private:
     friend class directory_iterator;