Re: [patch] fix libstdc++/56278

[François Dumont <frs dot dumont at gmail dot com>]

On 02/11/2013 01:21 AM, Jonathan Wakely wrote:
>          PR libstdc++/56278
>          * include/bits/hashtable_policy.h (_Hash_code_base): Make default
>          constructor public.
>          * testsuite/23_containers/unordered_set/56278.cc: New.
>
> Tested x86_64-linux, committed to trunk.
    In fact we do not need the default constructor to be public. It is 
the static assertion in hashtable that is not accurate enough. 
std::is_default_constructible is in fact a 
std::is_publicly_default_constructible which is more than we need. Here 
is a patch that restore default constructor protected and check that 
_Hash_code_base is default constructible through inheritance.

    Of course this is not mandatory but I think _Hash_code_base would 
be cleaner this way only exposing as public what is required by C++11. 
It can also wait for 4.9.

    Tested under Linux x86_64.

2013-02-12  François Dumont  <fdumont@gcc.gnu.org>

    * include/bits/hashtable_policy.h (_Hash_code_base): Restore
    default constructor protected.
    * include/bits/hashtable.h: static assert that _Hash_code_base has
    a default constructor available through inheritance.

François

Index: include/bits/hashtable_policy.h
===================================================================
--- include/bits/hashtable_policy.h	(revision 195955)
+++ include/bits/hashtable_policy.h	(working copy)
@@ -918,15 +918,13 @@
       using __ebo_extract_key = _Hashtable_ebo_helper<0, _ExtractKey>;
       using __ebo_hash = _Hashtable_ebo_helper<1, _Hash>;
 
-    public:
-      // We need the default constructor for the local iterators.
-      _Hash_code_base() = default;
-
     protected:
       typedef void* 					__hash_code;
       typedef _Hash_node<_Value, false>			__node_type;
 
-    protected:
+      // We need the default constructor for the local iterators.
+      _Hash_code_base() = default;
+
       _Hash_code_base(const _ExtractKey& __ex, const _H1&, const _H2&,
 		      const _Hash& __h)
       : __ebo_extract_key(__ex), __ebo_hash(__h) { }
@@ -1004,13 +1002,13 @@
       hash_function() const
       { return _M_h1(); }
 
-      // We need the default constructor for the local iterators.
-      _Hash_code_base() = default;
-
     protected:
       typedef std::size_t 				__hash_code;
       typedef _Hash_node<_Value, false>			__node_type;
 
+      // We need the default constructor for the local iterators.
+      _Hash_code_base() = default;
+
       _Hash_code_base(const _ExtractKey& __ex,
 		      const _H1& __h1, const _H2& __h2,
 		      const _Default_ranged_hash&)
Index: include/bits/hashtable.h
===================================================================
--- include/bits/hashtable.h	(revision 195955)
+++ include/bits/hashtable.h	(working copy)
@@ -266,7 +266,10 @@
       // __hash_code_base above to compute node bucket index so it has to be
       // default constructible.
       static_assert(__if_hash_not_cached<
-		      is_default_constructible<__hash_code_base>>::value,
+		    is_default_constructible<
+		      // We use _Hashtable_ebo_helper to access the protected
+		      // default constructor.
+		      __detail::_Hashtable_ebo_helper<0, __hash_code_base>>>::value,
 		    "Cache the hash code or make functors involved in hash code"
 		    " and bucket index computation default constructible");