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

[Jonathan Wakely]

On Wed, 8 Nov 2023 at 20:00, François Dumont  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.

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

[François Dumont]

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
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
+struct _NodePtrGuard
+{
+  _HashtableAlloc& _M_h;
+  _NodePtr _M_ptr;
+
+  ~_NodePtrGuard()
+  {
+   if (_M_ptr)
+ _M_h._M_deallocate_node_ptr(_M_ptr);
+  }
+};
+
   template
 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;