alloc/arena: Add an arena that contains an allocator.

alloc/arena.hpp

@@ -79,6 +79,43 @@ namespace cruft::alloc {
     private:
         T &m_store;
     };
-}
+
+
+    /// A simple allocator that contains a raw allocator and a forwarded
+    /// allocator.
+    ///
+    /// The raw allocator handles the memory allocation, the forwarded
+    /// allocator performs the initialisation, and we control the construction
+    /// of both.
+    ///
+    /// Ideally we wouldn't forward calls manually and instead do something
+    /// like inherit from arena<T>, but that makes it difficult to initialise
+    /// the raw allocator before we have to supply the reference to the arena.
+    template <typename AllocT>
+    class owned {
+    public:
+        template <typename ...ArgsT>
+        owned (ArgsT &&...args):
+            m_store (std::forward<ArgsT> (args)...),
+            m_arena {m_store}
+        { ; }
+
+        template <typename T, typename ...ArgsT>
+        decltype(auto) acquire (ArgsT &&...args)
+        { return m_arena.template acquire<T,ArgsT...> (std::forward<ArgsT> (args)...); }
+
+        template <typename ...ArgsT>
+        decltype(auto) release (ArgsT &&...args)
+        { return m_arena.release (std::forward<ArgsT> (args)...); }
+
+        template <typename T, typename ...ArgsT>
+        decltype(auto) unique (ArgsT &&...args)
+        { return m_arena.template unique<T,ArgsT...> (std::forward<ArgsT> (args)...); }
+
+    private:
+        AllocT m_store;
+        arena<AllocT> m_arena;
+    };
+};
 
 #endif