util/Callback: new utility class for callback functions

Makefile.am

@@ -394,6 +394,7 @@ endif
 libutil_a_SOURCES = \
 	src/util/RuntimeError.hxx \
 	src/util/Macros.hxx \
+	src/util/BoundMethod.hxx \
 	src/util/Cast.hxx \
 	src/util/Clamp.hxx \
 	src/util/DeleteDisposer.hxx \

src/util/BoundMethod.hxx

@@ -0,0 +1,62 @@
+/*
+ * Copyright (C) 2016 Max Kellermann <max@duempel.org>
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ *
+ * - Redistributions of source code must retain the above copyright
+ * notice, this list of conditions and the following disclaimer.
+ *
+ * - Redistributions in binary form must reproduce the above copyright
+ * notice, this list of conditions and the following disclaimer in the
+ * documentation and/or other materials provided with the
+ * distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+ * ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+ * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
+ * FOR A PARTICULAR PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL THE
+ * FOUNDATION OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
+ * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
+ * (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
+ * SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
+ * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT,
+ * STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
+ * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED
+ * OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+#ifndef BOUND_METHOD_HXX
+#define BOUND_METHOD_HXX
+
+#include <utility>
+
+/**
+ * This class can invoke a method of a given object.  To do this, it
+ * stores a pointer to the member function and a reference to the
+ * object.
+ *
+ * This is a utility to build callbacks.
+ *
+ * @param T the class whose method is going to be invoked
+ * @param R the method's return type
+ * @param Args argument types
+ */
+template<typename T, typename R, typename... Args>
+class BoundMethod final {
+	T &instance;
+	R (T::*method)(Args... args);
+
+public:
+	explicit constexpr BoundMethod(T &_instance,
+				       R (T::*_method)(Args... args))
+		:instance(_instance), method(_method) {}
+
+	template<typename... Args2>
+	R operator()(Args2&&... args) {
+		return (instance.*method)(std::forward<Args2>(args)...);
+	}
+};
+
+#endif