[PATCH] Make std::match_results::_M_resize more useful

[Jonathan Wakely]

As both callers of match_results::_M_resize(unsigned) immediately follow
it with a loop to update the value of each sub_match, that behaviour can
be moved into _M_resize itself. The first caller fills the container
with unmatched subs, which can be done with vector::assign, and the
second caller clears the container to establish a specific state, which
can be provided by a new member function specific to that purpose.

Tangentially, I also noticed that match_results::max_size() doesn't
account for the three special sub_match objects that are always present
in a fully established result state. This patch also fixes that.

        * include/bits/regex.h (match_results::max_size()): Adjust return
        value to account for prefix/suffix/unmatched subs.
        (match_results::_M_resize(unsigned int)): Use _Base_type::assign to
        reset the contained sub matches.
        (match_results::_M_establish_failed_match(_Bi_iter)): Add new member
        function to set result state following a failed match.
        * include/bits/regex.tcc (__regex_algo_impl): Remove loop to set
        sub_match states after _M_resize. Use _M_establish_failed_match.

Tested x86_64-linux.

Any objections?



commit 6a96a528102e1062dd2627b7eb8ddf26c43bc1e7
Author: Jonathan Wakely <jwak...@redhat.com>
Date:   Tue Dec 4 13:21:26 2018 +0000

    Make std::match_results::_M_resize more useful
    
    As both callers of match_results::_M_resize(unsigned) immediately follow
    it with a loop to update the value of each sub_match, that behaviour can
    be moved into _M_resize itself. The first caller fills the container
    with unmatched subs, which can be done with vector::assign, and the
    second caller clears the container to establish a specific state, which
    can be provided by a new member function specific to that purpose.
    
    Tangentially, I also noticed that match_results::max_size() doesn't
    account for the three special sub_match objects that are always present
    in a fully established result state. This patch also fixes that.
    
            * include/bits/regex.h (match_results::max_size()): Adjust return
            value to account for prefix/suffix/unmatched subs.
            (match_results::_M_resize(unsigned int)): Use _Base_type::assign to
            reset the contained sub matches.
            (match_results::_M_establish_failed_match(_Bi_iter)): Add new member
            function to set result state following a failed match.
            * include/bits/regex.tcc (__regex_algo_impl): Remove loop to set
            sub_match states after _M_resize. Use _M_establish_failed_match.

diff --git a/libstdc++-v3/include/bits/regex.h 
b/libstdc++-v3/include/bits/regex.h
index af6fe3f0d79..d36ee33033d 100644
--- a/libstdc++-v3/include/bits/regex.h
+++ b/libstdc++-v3/include/bits/regex.h
@@ -1698,7 +1698,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
 
       size_type
       max_size() const noexcept
-      { return _Base_type::max_size(); }
+      { return _Base_type::max_size() - 3; }
 
       /**
        * @brief Indicates if the %match_results contains no results.
@@ -1942,9 +1942,20 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
                                    const basic_regex<_Cp, _Rp>&,
                                    regex_constants::match_flag_type);
 
+      // Reset contents to __size unmatched sub_match objects
+      // (plus additional objects for prefix, suffix and unmatched sub).
       void
       _M_resize(unsigned int __size)
-      { _Base_type::resize(__size + 3); }
+      { _Base_type::assign(__size + 3, sub_match<_Bi_iter>{}); }
+
+      // Set state to a failed match for the given past-the-end iterator.
+      void
+      _M_establish_failed_match(_Bi_iter __end)
+      {
+       sub_match<_Bi_iter> __sm;
+       __sm.first = __sm.second = __end;
+       _Base_type::assign(3, __sm);
+      }
 
       const_reference
       _M_unmatched_sub() const
diff --git a/libstdc++-v3/include/bits/regex.tcc 
b/libstdc++-v3/include/bits/regex.tcc
index dcf660902bc..de0ce795b84 100644
--- a/libstdc++-v3/include/bits/regex.tcc
+++ b/libstdc++-v3/include/bits/regex.tcc
@@ -57,8 +57,6 @@ namespace __detail
       typename match_results<_BiIter, _Alloc>::_Base_type& __res = __m;
       __m._M_begin = __s;
       __m._M_resize(__re._M_automaton->_M_sub_count());
-      for (auto& __it : __res)
-       __it.matched = false;
 
       bool __ret;
       if ((__re.flags() & regex_constants::__polynomial)
@@ -109,12 +107,7 @@ namespace __detail
        }
       else
        {
-         __m._M_resize(0);
-         for (auto& __it : __res)
-           {
-             __it.matched = false;
-             __it.first = __it.second = __e;
-           }
+         __m._M_establish_failed_match(__e);
        }
       return __ret;
     }