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, 33 insertions, 0 deletions

diff --git a/gdb/ui-out.h b/gdb/ui-out.h
index d54843db7b..9278cabdaa 100644
--- a/gdb/ui-out.h
+++ b/gdb/ui-out.h
@@ -187,4 +187,37 @@ class ui_out
   ui_out_level *current_level () const;
 };
 
+/* This is similar to make_cleanup_ui_out_tuple_begin_end and
+   make_cleanup_ui_out_list_begin_end, but written as an RAII template
+   class.  It takes the ui_out_type as a template parameter.  Normally
+   this is used via the typedefs ui_out_emit_tuple and
+   ui_out_emit_list.  */
+template<ui_out_type Type>
+class ui_out_emit_type
+{
+public:
+
+  ui_out_emit_type (struct ui_out *uiout, const char *id)
+    : m_uiout (uiout)
+  {
+    uiout->begin (Type, id);
+  }
+
+  ~ui_out_emit_type ()
+  {
+    m_uiout->end (Type);
+  }
+
+  ui_out_emit_type (const ui_out_emit_type<Type> &) = delete;
+  ui_out_emit_type<Type> &operator= (const ui_out_emit_type<Type> &)
+      = delete;
+
+private:
+
+  struct ui_out *m_uiout;
+};
+
+typedef ui_out_emit_type<ui_out_type_tuple> ui_out_emit_tuple;
+typedef ui_out_emit_type<ui_out_type_list> ui_out_emit_list;
+
 #endif /* UI_OUT_H */