+/*
+ This file is part of LilyPond, the GNU music typesetter.
+
+ Copyright (C) 2016 David Kastrup <dak@gnu.org>
+
+ LilyPond is free software: you can redistribute it and/or modify
+ it under the terms of the GNU General Public License as published by
+ the Free Software Foundation, either version 3 of the License, or
+ (at your option) any later version.
+
+ LilyPond is distributed in the hope that it will be useful,
+ but WITHOUT ANY WARRANTY; without even the implied warranty of
+ MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+ GNU General Public License for more details.
+
+ You should have received a copy of the GNU General Public License
+ along with LilyPond. If not, see <http://www.gnu.org/licenses/>.
+*/
+
+#ifndef CALLBACK_HH
+#define CALLBACK_HH
+
+// A callback wrapper creates a Scheme-callable version of a fixed C++
+// function. It is generally used for calling template-generated
+// trampoline functions leading to calling a particular member
+// function on a given Smob.
+//
+// The class itself is not templated in order not to explode the
+// number of smob types: each class can support a particular call
+// signature.
+//
+// Check the GET_LISTENER call for a typical use case.
+
+#include "smobs.hh"
+
+class Callback_wrapper : public Simple_smob<Callback_wrapper>
+{
+ // We use an ordinary function pointer pointing to a trampoline
+ // function (templated on the callback in question) instead of
+ // storing a member function pointer to a common base class like
+ // Smob_core. The additional code for the trampolines is negligible
+ // and the performance implications of using member function
+ // pointers in connection with inheritance are somewhat opaque as
+ // this involves an adjustment of the this pointer from Smob_core to
+ // the scope containing the callback.
+ SCM (*trampoline_) (SCM, SCM);
+ Callback_wrapper (SCM (*trampoline) (SCM, SCM)) : trampoline_ (trampoline)
+ { } // Private constructor, use only in make_smob
+public:
+ static const char * const type_p_name_; // = 0
+ LY_DECLARE_SMOB_PROC (&Callback_wrapper::call, 2, 0, 0)
+ SCM call (SCM target, SCM arg)
+ {
+ return trampoline_ (target, arg);
+ }
+ // Callback wrappers are for an unchanging entity, so we do the Lisp
+ // creation just once on the first call of make_smob. So we only
+ // get a single Callback_wrapper instance for each differently
+ // templated make_smob call.
+ template <SCM (*trampoline) (SCM, SCM)>
+ static SCM make_smob ()
+ {
+ static SCM res =
+ scm_permanent_object (Callback_wrapper (trampoline).smobbed_copy ());
+ return res;
+ }
+};
+
+
+#endif