Remove some ui_out-related cleanups from Python

This patch introduces a bit of infrastructure -- namely, a minimal
std::optional analogue called gdb::optional, and an RAII template
class that works like make_cleanup_ui_out_tuple_begin_end or
make_cleanup_ui_out_list_begin_end -- and then uses these in the
Python code.  This removes a number of cleanups and generally
simplifies this code.

std::optional is only available in C++17.  Normally I would have had
this code check __cplusplus, but my gcc apparently isn't new enough to
find <optional>, even with -std=c++1z; so, because I could not test
it, the patch does not do this.

gdb/ChangeLog
2017-02-10  Tom Tromey  <tom@tromey.com>

	* ui-out.h (ui_out_emit_type): New class.
	(ui_out_emit_tuple, ui_out_emit_list): New typedefs.
	* python/py-framefilter.c (py_print_single_arg): Use gdb::optional
	and ui_out_emit_tuple.
	(enumerate_locals): Likewise.
	(py_mi_print_variables, py_print_locals, py_print_args): Use
	ui_out_emit_list.
	(py_print_frame): Use gdb::optional, ui_out_emit_tuple,
	ui_out_emit_list.
	* common/gdb_optional.h: New file.

1 files changed, 87 insertions, 0 deletions

diff --git a/gdb/common/gdb_optional.h b/gdb/common/gdb_optional.h
new file mode 100644
index 00000000000..d991da123cf
--- /dev/null
+++ b/gdb/common/gdb_optional.h
@@ -0,0 +1,87 @@
+/* An optional object.
+
+   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/>.  */
+
+#ifndef GDB_OPTIONAL_H
+#define GDB_OPTIONAL_H
+
+namespace gdb
+{
+
+/* This class attempts to be a compatible subset of std::optional,
+   which is slated to be available in C++17.  This class optionally
+   holds an object of some type -- by default it is constructed not
+   holding an object, but later the object can be "emplaced".  This is
+   similar to using std::unique_ptr, but in-object allocation is
+   guaranteed.  */
+template<typename T>
+class optional
+{
+public:
+
+  optional ()
+    : m_instantiated (false)
+  {
+  }
+
+  ~optional ()
+  {
+    if (m_instantiated)
+      destroy ();
+  }
+
+  /* These aren't deleted in std::optional, but it was simpler to
+     delete them here, because currently the users of this class don't
+     need them, and making them depend on the definition of T is
+     somewhat complicated.  */
+  optional (const optional &other) = delete;
+  optional<T> &operator= (const optional &other) = delete;
+
+  template<typename... Args>
+  void emplace (Args &&... args)
+  {
+    if (m_instantiated)
+      destroy ();
+    new (&m_item) T (std::forward<Args>(args)...);
+    m_instantiated = true;
+  }
+
+private:
+
+  /* Destroy the object.  */
+  void destroy ()
+  {
+    gdb_assert (m_instantiated);
+    m_instantiated = false;
+    m_item.~T ();
+  }
+
+  /* The object.  */
+  union
+  {
+    struct { } m_dummy;
+    T m_item;
+  };
+
+  /* True if the object was ever emplaced.  */
+  bool m_instantiated;
+};
+
+}
+
+#endif /* GDB_OPTIONAL_H */