memory/deleter: add deferred_delete and deferred_unique

memory/deleter.hpp

@@ -3,15 +3,59 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/.
  *
- * Copyright 2015 Danny Robson <danny@nerdcruft.net>
+ * Copyright 2015-2021 Danny Robson <danny@nerdcruft.net>
  */
 
-#ifndef __UTIL_MEMORY_DELETER_HPP
-#define __UTIL_MEMORY_DELETER_HPP
+#pragma once
 
 #include <functional>
+#include <memory>
 
 namespace cruft::memory {
+    ///////////////////////////////////////////////////////////////////////////
+    /// A deleter object that is deliberately left undefined.
+    ///
+    /// This allows it to be used in places where the definition of `ValueT`
+    /// would otherwise be required (eg, std::unique_ptr). This requires that
+    /// the body be explicitly defined by the user; see `DEFERRED_DELETE`.
+    template <typename ValueT>
+    struct deferred_delete {
+        void operator() (ValueT*) const;
+    };
+
+
+    ///------------------------------------------------------------------------
+    /// An implementation of deferred delete that just calls `delete` directly.
+    #define DEFERRED_DELETE(KLASS)  \
+    template <>                     \
+    void                            \
+    ::cruft::memory::deferred_delete<KLASS>::operator() (KLASS *ptr) const \
+    { delete ptr; }
+
+
+    ///////////////////////////////////////////////////////////////////////////
+    /// A convenience typedef for a unique_ptr that uses a deferred_delete
+    /// deleter object.
+    template <typename ValueT>
+    using deferred_unique = std::unique_ptr<ValueT, deferred_delete<ValueT>>;
+
+
+    ///------------------------------------------------------------------------
+    /// A convenience function for creation of deferred_unique objects
+    /// analogous to std::make_unique
+    template <typename ValueT, typename ...ArgsT>
+    deferred_unique<ValueT>
+    make_deferred_unique (ArgsT &&...args)
+    {
+        return deferred_unique<ValueT> (
+            new ValueT (
+                std::forward<ArgsT> (args)...
+            )
+        );
+    }
+
+
+    ///////////////////////////////////////////////////////////////////////////
     template <typename T>
     class func_deleter {
     public:
@@ -53,6 +97,4 @@ namespace cruft::memory {
     private:
         OwnerT& m_owner;
     };
-}
-
-#endif
+}