[PATCH][_Hahstable] Use RAII to guard node pointer while constructing

[PATCH][_Hahstable] Use RAII to guard node pointer while constructing

[François Dumont]

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Another proposal to use RAII rather than __try/__catch block.

libstdc++: [_Hashtable] Use RAII type to guard node while constructing value

libstdc++-v3/ChangeLog:

         * include/bits/hashtable_policy.h
         (struct _NodePtrGuard<_HashtableAlloc, _NodePtr>): New.
         (_ReuseAllocNode::operator()(_Args&&...)): Use latter to guard 
allocated node
         pointer while constructing in place the value_type instance.

Tested under Linux x64, ok to commit ?

François

[-- Attachment #2: raii_reuse_alloc_nodeptr.txt --]
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diff --git a/libstdc++-v3/include/bits/hashtable_policy.h b/libstdc++-v3/include/bits/hashtable_policy.h
index cd8943d8d05..c67eebd3b2b 100644
--- a/libstdc++-v3/include/bits/hashtable_policy.h
+++ b/libstdc++-v3/include/bits/hashtable_policy.h
@@ -173,6 +173,19 @@ namespace __detail
 	{ return __node_gen(std::forward<_Kt>(__k)); }
     };
 
+  template<typename _HashtableAlloc, typename _NodePtr>
+    struct _NodePtrGuard
+    {
+      _HashtableAlloc& _M_h;
+      _NodePtr _M_ptr;
+
+      ~_NodePtrGuard()
+      {
+	if (_M_ptr)
+	  _M_h._M_deallocate_node_ptr(_M_ptr);
+      }
+    };
+
   template<typename _NodeAlloc>
     struct _Hashtable_alloc;
 
@@ -201,27 +214,20 @@ namespace __detail
 	__node_ptr
 	operator()(_Args&&... __args) const
 	{
-	  if (_M_nodes)
-	    {
+	  if (!_M_nodes)
+	    return _M_h._M_allocate_node(std::forward<_Args>(__args)...);
+
 	  __node_ptr __node = _M_nodes;
 	  _M_nodes = _M_nodes->_M_next();
 	  __node->_M_nxt = nullptr;
 	  auto& __a = _M_h._M_node_allocator();
 	  __node_alloc_traits::destroy(__a, __node->_M_valptr());
-	      __try
-		{
+	  _NodePtrGuard<__hashtable_alloc, __node_ptr> __guard { _M_h, __node };
 	  __node_alloc_traits::construct(__a, __node->_M_valptr(),
 					 std::forward<_Args>(__args)...);
-		}
-	      __catch(...)
-		{
-		  _M_h._M_deallocate_node_ptr(__node);
-		  __throw_exception_again;
-		}
+	  __guard._M_ptr = nullptr;
 	  return __node;
 	}
-	  return _M_h._M_allocate_node(std::forward<_Args>(__args)...);
-	}
 
     private:
       mutable __node_ptr _M_nodes;

Re: [PATCH][_Hahstable] Use RAII to guard node pointer while constructing

[Jonathan Wakely]

On Wed, 8 Nov 2023 at 20:00, François Dumont <frs.dumont@gmail.com> wrote:
>
> Another proposal to use RAII rather than __try/__catch block.
>
> libstdc++: [_Hashtable] Use RAII type to guard node while constructing value
>
> libstdc++-v3/ChangeLog:
>
>          * include/bits/hashtable_policy.h
>          (struct _NodePtrGuard<_HashtableAlloc, _NodePtr>): New.
>          (_ReuseAllocNode::operator()(_Args&&...)): Use latter to guard
> allocated node
>          pointer while constructing in place the value_type instance.
>
> Tested under Linux x64, ok to commit ?

Looks good, OK.