Initial callback template class. Untested.

src/callback.h

@@ -0,0 +1,74 @@
+///\file
+
+/******************************************************************************
+The MIT License(MIT)
+
+Embedded Template Library.
+https://github.com/ETLCPP/etl
+http://www.etlcpp.com
+
+Copyright(c) 2015 jwellbelove
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files(the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and / or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions :
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+SOFTWARE.
+******************************************************************************/
+
+#ifndef __ETL_CALLBACK__
+#define __ETL_CALLBACK__
+
+namespace etl
+{
+  template <typename TParameter, const int ID>
+  class callback
+  {
+  private:
+
+    // Creates a parameter type unique to this ID.
+    template <typename T, const int I>
+    struct parameter
+    {
+        parameter(T value)
+            : value(value)
+        {
+        }
+
+        typedef T value_type;
+
+        T value;
+
+    private:
+
+        parameter();
+    };
+
+    // Specialisation for void.
+    template <const int I>
+    struct parameter<void, I>
+    {
+        typedef void value_type;
+    };
+
+  public:
+
+    typedef parameter<TParameter, ID> type;
+
+    virtual void etl_callback(type p = type()) = 0;
+  };
+}
+
+#endif