base: Add 'construct_at' utility

This utility allows for the manual placement of objects without the need
to have a global placement new operation nor the need for type-specific
new operators.

Issue #989

base/include/util/construct_at.h

@@ -0,0 +1,84 @@
+/*
+ * \brief  Utility for the manual placement of objects
+ * \author Norman Feske
+ * \date   2014-02-07
+ */
+
+/*
+ * Copyright (C) 2014 Genode Labs GmbH
+ *
+ * This file is part of the Genode OS framework, which is distributed
+ * under the terms of the GNU General Public License version 2.
+ */
+
+#ifndef _INCLUDE__UTIL__CONSTRUCT_AT_H_
+#define _INCLUDE__UTIL__CONSTRUCT_AT_H_
+
+#include <base/stdint.h>
+#include <base/printf.h>
+
+namespace Genode {
+	template <typename T, typename... ARGS>
+	static inline T *construct_at(void *, ARGS &&...);
+}
+
+
+/**
+ * Construct object of given type at a specific location
+ *
+ * \param T     object type
+ * \param at    desired object location
+ * \param args  list of arguments for the object constructor
+ *
+ * \return  typed object pointer
+ *
+ * We use move semantics (ARGS &&) because otherwise the compiler would create
+ * a temporary copy of all arguments that have a reference type and use a
+ * reference to this copy instead of the original within this function.
+ *
+ * There is a slight difference between the object that is constructed by this
+ * function and a common object of the given type. If the destructor of the
+ * given type or of any base of the given type is virtual, the vtable of the
+ * returned object references an empty delete(void *) operator for that
+ * destructor. However, this shouldn't be a problem as an object constructed by
+ * this function should never get destructed implicitely or through a delete
+ * expression.
+ */
+template <typename T, typename... ARGS>
+static inline T * Genode::construct_at(void *at, ARGS &&... args)
+{
+	/**
+	 * Utility to equip an existing type 'T' with a placement new operator
+	 */
+	struct Placeable : T
+	{
+		Placeable(ARGS &&... args) : T(args...) { }
+
+		void * operator new (size_t, void *ptr) { return ptr; }
+		void   operator delete (void *, void *) { }
+
+		/**
+		 * Standard delete operator
+		 *
+		 * As we explicitely define one version of the delete operator, the
+		 * compiler won't implicitely define any delete version for this class.
+		 * But if type T has a virtual destructor, the compiler implicitely
+		 * defines a 'virtual ~Placeable()' which needs the following operator.
+		 */
+		void  operator delete (void *)
+		{
+			PERR("cxx: Placeable::operator delete (void *) not supported.");
+		}
+	};
+
+	/*
+	 * If the args input to this function contains rvalues, the compiler would
+	 * use the according rvalue references as lvalues at the following call if
+	 * we don't cast them back to rvalue references explicitely. We can not use
+	 * lvalues here because the compiler can not bind them to rvalue references
+	 * as expected by Placeable.
+	 */
+	return new (at) Placeable(static_cast<ARGS &&>(args)...);
+}
+
+#endif /* _INCLUDE__UTIL__CONSTRUCT_AT_H_ */

os/include/util/volatile_object.h

@@ -14,6 +14,7 @@
 #ifndef _INCLUDE__UTIL__VOLATILE_OBJECT_H_
 #define _INCLUDE__UTIL__VOLATILE_OBJECT_H_
 
+#include <util/construct_at.h>
 #include <base/printf.h>
 #include <base/stdint.h>
 
@@ -39,21 +40,6 @@ class Genode::Volatile_object
 {
 	private:
 
-		/**
-		 * Utility to equip an existing type 'T' with a placement new operator
-		 */
-		template <typename T>
-		struct Placeable : T
-		{
-			template <typename... ARGS>
-			Placeable(ARGS &&... args)
-			:
-				T(args...)
-			{ }
-
-			void *operator new (size_t, void *ptr) { return ptr; }
-		};
-
 		/**
 		 * Static reservation of memory for the embedded object
 		 */
@@ -66,7 +52,7 @@ class Genode::Volatile_object
 
 		template <typename... ARGS> void _do_construct(ARGS &&... args)
 		{
-			new (_space) Placeable<MT>(args...);
+			construct_at<MT>(_space, args...);
 			_constructed = true;
 		}
 

os/run/volatile_object.run

@@ -59,6 +59,11 @@ compare_output_to {
 [init -> test-volatile_object] destruct Member_with_reference
 [init -> test-volatile_object] destruct Object 2
 [init -> test-volatile_object] destruct Object 1
+[init -> test-volatile_object] -- construct Throwing object
+[init -> test-volatile_object] construct Throwing -> don't throw
+[init -> test-volatile_object] destruct Throwing
+[init -> test-volatile_object] construct Throwing -> throw exception
+[init -> test-volatile_object] -- catched exception as expected
 [init -> test-volatile_object] --- test-volatile_object finished ---
 }
 

os/src/test/volatile_object/main.cc

@@ -75,6 +75,35 @@ struct Compound
 };
 
 
+struct Bool
+{
+	bool b;
+
+	Bool(Bool const &) = delete;
+
+	Bool(bool const &b) : b(b) { }
+};
+
+
+struct Throwing
+{
+	Throwing(Bool const &throws)
+	{
+		if (throws.b) {
+			PLOG("construct Throwing -> throw exception");
+			throw -1;
+		} else {
+			PLOG("construct Throwing -> don't throw");
+		}
+	}
+
+	virtual ~Throwing()
+	{
+		PLOG("destruct Throwing");
+	}
+};
+
+
 static void call_const_method(Compound const &compound)
 {
 	compound.member->reference.const_method();
@@ -125,6 +154,17 @@ int main(int, char **)
 		printf("-- destruct Compound and Objects 1 and 2 --\n");
 	}
 
+	try {
+		printf("-- construct Throwing object\n");
+		Bool const b_false(false), b_true(true);
+
+		Volatile_object<Throwing> inst(b_false);
+		inst.construct(b_true);
+		PERR("expected contructor to throw");
+	} catch (int i) {
+		printf("-- catched exception as expected\n");
+	}
+
 	printf("--- test-volatile_object finished ---\n");
 
 	return 0;