Introduce gdb::function_view

This commit adds a new function_view type.  This type holds a
non-owning reference to a callable.  It is meant to be used as
callback type of functions, instead of using the C-style pair of
function pointer and 'void *data' arguments.  function_view allows
passing references to stateful function objects / lambdas with
captures as callbacks efficiently, while function pointer + 'void *'
does not.

See the intro in the new function-view.h header for more.

Unit tests included, put into a new gdb/unittests/ subdir.

gdb/ChangeLog:
2017-02-23  Pedro Alves  <palves@redhat.com>

	* Makefile.in (SUBDIR_UNITTESTS_SRCS, SUBDIR_UNITTESTS_OBS): New.
	(%.o) <unittests/%.c>: New pattern.
	* configure.ac ($development): Add $(SUBDIR_UNITTESTS_OBS) to
	CONFIG_OBS, and $(SUBDIR_UNITTESTS_SRCS) to CONFIG_SRCS.
	* common/function-view.h: New file.
	* unittests/function-view-selftests.c: New file.
	* configure: Regenerate.

gdb/unittests/function-view-selftests.c

@@ -0,0 +1,178 @@
+/* Self tests for function_view for GDB, the GNU debugger.
+
+   Copyright (C) 2017 Free Software Foundation, Inc.
+
+   This file is part of GDB.
+
+   This program 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.
+
+   This program 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 this program.  If not, see <http://www.gnu.org/licenses/>.  */
+
+#include "defs.h"
+#include "selftest.h"
+#include "common/function-view.h"
+
+namespace selftests {
+namespace function_view {
+
+static int
+plus_one_fn_int (int val)
+{
+  return ++val;
+}
+
+static short
+plus_one_fn_short (short val)
+{
+  return ++val;
+}
+
+static int
+call_callback_int (int val, gdb::function_view <int (int)> callback)
+{
+  return callback (val);
+}
+
+static void
+call_callback_void (int val, gdb::function_view <void (int)> callback)
+{
+  callback (val);
+}
+
+struct plus_one_int_func_obj
+{
+  int operator () (int val)
+  {
+    ++call_count;
+    return ++val;
+  }
+
+  /* Number of times called.  */
+  int call_count = 0;
+};
+
+static void
+run_tests ()
+{
+  /* A simple lambda.  */
+  auto plus_one_lambda = [] (int val) { return ++val; };
+
+  /* A function_view that references the lambda.  */
+  gdb::function_view<int (int)> plus_one_func_view (plus_one_lambda);
+
+  /* Check calling the lambda directly.  */
+  SELF_CHECK (plus_one_lambda (0) == 1);
+  SELF_CHECK (plus_one_lambda (1) == 2);
+
+  /* Check calling lambda via the view.  */
+  SELF_CHECK (plus_one_func_view (2) == 3);
+  SELF_CHECK (plus_one_func_view (3) == 4);
+
+  /* Check calling a function that takes a function_view as argument,
+     by value.  Pass a lambda, making sure a function_view is properly
+     constructed implicitly.  */
+  SELF_CHECK (call_callback_int (1, [] (int val)
+    {
+      return val + 2;
+    }) == 3);
+
+  /* Same, passing a named/lvalue lambda.  */
+  SELF_CHECK (call_callback_int (1, plus_one_lambda) == 2);
+  /* Same, passing a named/lvalue function_view (should copy).  */
+  SELF_CHECK (call_callback_int (1, plus_one_func_view) == 2);
+
+  /* Check constructing a function view over a function-object
+     callable, and calling it.  */
+  plus_one_int_func_obj func_obj;
+  SELF_CHECK (func_obj (0) == 1);
+  SELF_CHECK (call_callback_int (1, func_obj) == 2);
+  /* Check that the callable was referenced, not copied.  */
+  SELF_CHECK (func_obj.call_count == 2);
+
+  /* Check constructing a function_view over a free-function callable,
+     and calling it.  */
+  SELF_CHECK (call_callback_int (1, plus_one_fn_int) == 2);
+
+  /* Check calling a function with a
+     compatible-but-not-exactly-the-same prototype.  */
+  SELF_CHECK (call_callback_int (1, [] (short val) -> short
+    {
+      return val + 2;
+    }) == 3);
+  /* Same, but passing a function pointer.  */
+  SELF_CHECK (call_callback_int (1, plus_one_fn_short) == 2);
+
+  /* Like std::function, a function_view that expects a void return
+     can reference callables with non-void return type.  The result is
+     simply discarded.  Check a lambda, function object and a function
+     pointer.  */
+  call_callback_void (1, [] (int val) -> int
+    {
+      return val + 2;
+    });
+  call_callback_void (1, func_obj);
+  call_callback_void (1, plus_one_fn_int);
+
+  /* Check that the main ctor doesn't hijack the copy ctor.  */
+  auto plus_one_func_view2 (plus_one_func_view);
+  auto plus_one_func_view3 (plus_one_func_view2);
+  static_assert (std::is_same<decltype (plus_one_func_view),
+		 decltype (plus_one_func_view2)>::value, "");
+  static_assert (std::is_same<decltype (plus_one_func_view),
+		 decltype (plus_one_func_view3)>::value, "");
+
+  SELF_CHECK (plus_one_func_view3 (1) == 2);
+
+  /* Likewise, but propagate a NULL callable.  If this calls the main
+     function_view ctor instead of the copy ctor by mistake, then
+     null_func_2 ends up non-NULL (because it'd instead reference
+     null_func_1 as just another callable).  */
+  constexpr gdb::function_view<int (int)> null_func_view_1 = nullptr;
+  constexpr auto null_func_view_2 (null_func_view_1);
+
+  /* While at it, check whether the function_view is bound using
+     various forms, op==, op!= and op bool.  */
+
+  /* op== */
+  static_assert (null_func_view_2 == nullptr, "");
+  static_assert (nullptr == null_func_view_2, "");
+  static_assert (null_func_view_2 == NULL, "");
+  static_assert (NULL == null_func_view_2, "");
+
+  /* op!= */
+  static_assert (!(null_func_view_2 != nullptr), "");
+  static_assert (!(nullptr != null_func_view_2), "");
+  static_assert (!(null_func_view_2 != NULL), "");
+  static_assert (!(NULL != null_func_view_2), "");
+
+  /* op bool */
+  static_assert (!null_func_view_2, "");
+
+  /* Check the nullptr_t ctor.  */
+  constexpr gdb::function_view<int (int)> check_ctor_nullptr (nullptr);
+  static_assert (!check_ctor_nullptr, "");
+
+  /* Check the nullptr_t op= */
+  gdb::function_view<int (int)> check_op_eq_null (plus_one_fn_int);
+  SELF_CHECK (check_op_eq_null);
+  check_op_eq_null = nullptr;
+  SELF_CHECK (!check_op_eq_null);
+}
+
+} /* namespace function_view */
+} /* namespace selftests */
+
+void
+_initialize_function_view_selftests ()
+{
+  register_self_test (selftests::function_view::run_tests);
+}