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.

gdb/ChangeLog

@@ -1,3 +1,16 @@
+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.
+
 2017-02-10  Martin Galvan  <martingalvan@sourceware.org>
 
 	* MAINTAINERS (Write After Approval): Update my e-mail address.

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 */

gdb/python/py-framefilter.c

@@ -31,6 +31,7 @@
 #include "mi/mi-cmds.h"
 #include "python-internal.h"
 #include "py-ref.h"
+#include "common/gdb_optional.h"
 
 enum mi_print_types
 {
@@ -365,7 +366,7 @@ py_print_single_arg (struct ui_out *out,
 
   TRY
     {
-      struct cleanup *cleanups = make_cleanup (null_cleanup, NULL);
+      gdb::optional<ui_out_emit_tuple> maybe_tuple;
 
       /*  MI has varying rules for tuples, but generally if there is only
       one element in each item in the list, do not start a tuple.  The
@@ -376,7 +377,7 @@ py_print_single_arg (struct ui_out *out,
       if (out->is_mi_like_p ())
 	{
 	  if (print_args_field || args_type != NO_VALUES)
-	    make_cleanup_ui_out_tuple_begin_end (out, NULL);
+	    maybe_tuple.emplace (out, nullptr);
 	}
 
       annotate_arg_begin ();
@@ -421,10 +422,7 @@ py_print_single_arg (struct ui_out *out,
       if (args_type == MI_PRINT_SIMPLE_VALUES && val != NULL)
 	{
 	  if (py_print_type (out, val) == EXT_LANG_BT_ERROR)
-	    {
+	    retval = EXT_LANG_BT_ERROR;
-	      retval = EXT_LANG_BT_ERROR;
-	      do_cleanups (cleanups);
-	    }
 	}
 
       if (retval != EXT_LANG_BT_ERROR)
@@ -454,8 +452,6 @@ py_print_single_arg (struct ui_out *out,
 		    retval = EXT_LANG_BT_ERROR;
 		}
 	    }
-
-	  do_cleanups (cleanups);
 	}
     }
   CATCH (except, RETURN_MASK_ERROR)
@@ -695,35 +691,24 @@ enumerate_locals (PyObject *iter,
       struct symbol *sym;
       struct block *sym_block;
       int local_indent = 8 + (8 * indent);
-      struct cleanup *locals_cleanups;
+      gdb::optional<ui_out_emit_tuple> tuple;
 
       gdbpy_ref item (PyIter_Next (iter));
       if (item == NULL)
 	break;
 
-      locals_cleanups = make_cleanup (null_cleanup, NULL);
-
       success = extract_sym (item.get (), &sym_name, &sym, &sym_block,
 			     &language);
       if (success == EXT_LANG_BT_ERROR)
-	{
+	return EXT_LANG_BT_ERROR;
-	  do_cleanups (locals_cleanups);
-	  goto error;
-	}
 
       success = extract_value (item.get (), &val);
       if (success == EXT_LANG_BT_ERROR)
-	{
+	return EXT_LANG_BT_ERROR;
-	  do_cleanups (locals_cleanups);
-	  goto error;
-	}
 
       if (sym != NULL && out->is_mi_like_p ()
 	  && ! mi_should_print (sym, MI_PRINT_LOCALS))
-	{
+	continue;
-	  do_cleanups (locals_cleanups);
-	  continue;
-	}
 
       /* If the object did not provide a value, read it.  */
       if (val == NULL)
@@ -735,8 +720,7 @@ enumerate_locals (PyObject *iter,
 	  CATCH (except, RETURN_MASK_ERROR)
 	    {
 	      gdbpy_convert_exception (except);
-	      do_cleanups (locals_cleanups);
+	      return EXT_LANG_BT_ERROR;
-	      goto error;
 	    }
 	  END_CATCH
 	}
@@ -747,7 +731,7 @@ enumerate_locals (PyObject *iter,
       if (out->is_mi_like_p ())
 	{
 	  if (print_args_field || args_type != NO_VALUES)
-	    make_cleanup_ui_out_tuple_begin_end (out, NULL);
+	    tuple.emplace (out, nullptr);
 	}
       TRY
 	{
@@ -765,18 +749,14 @@ enumerate_locals (PyObject *iter,
       CATCH (except, RETURN_MASK_ERROR)
 	{
 	  gdbpy_convert_exception (except);
-	  do_cleanups (locals_cleanups);
+	  return EXT_LANG_BT_ERROR;
-	  goto error;
 	}
       END_CATCH
 
       if (args_type == MI_PRINT_SIMPLE_VALUES)
 	{
 	  if (py_print_type (out, val) == EXT_LANG_BT_ERROR)
-	    {
+	    return EXT_LANG_BT_ERROR;
-	      do_cleanups (locals_cleanups);
-	      goto error;
-	    }
 	}
 
       /* CLI always prints values for locals.  MI uses the
@@ -787,10 +767,7 @@ enumerate_locals (PyObject *iter,
 
 	  if (py_print_value (out, val, &opts, val_indent, args_type,
 			      language) == EXT_LANG_BT_ERROR)
-	    {
+	    return EXT_LANG_BT_ERROR;
-	      do_cleanups (locals_cleanups);
-	      goto error;
-	    }
 	}
       else
 	{
@@ -798,15 +775,10 @@ enumerate_locals (PyObject *iter,
 	    {
 	      if (py_print_value (out, val, &opts, 0, args_type,
 				  language) == EXT_LANG_BT_ERROR)
-		{
+		return EXT_LANG_BT_ERROR;
-		  do_cleanups (locals_cleanups);
-		  goto error;
-		}
 	    }
 	}
 
-      do_cleanups (locals_cleanups);
-
       TRY
 	{
 	  out->text ("\n");
@@ -814,7 +786,7 @@ enumerate_locals (PyObject *iter,
       CATCH (except, RETURN_MASK_ERROR)
 	{
 	  gdbpy_convert_exception (except);
-	  goto error;
+	  return EXT_LANG_BT_ERROR;
 	}
       END_CATCH
     }
@@ -822,7 +794,6 @@ enumerate_locals (PyObject *iter,
   if (!PyErr_Occurred ())
     return EXT_LANG_BT_OK;
 
- error:
   return EXT_LANG_BT_ERROR;
 }
 
@@ -835,8 +806,6 @@ py_mi_print_variables (PyObject *filter, struct ui_out *out,
 		       enum ext_lang_frame_args args_type,
 		       struct frame_info *frame)
 {
-  struct cleanup *old_chain;
-
   gdbpy_ref args_iter (get_py_iter_from_func (filter, "frame_args"));
   if (args_iter == NULL)
     return EXT_LANG_BT_ERROR;
@@ -845,24 +814,19 @@ py_mi_print_variables (PyObject *filter, struct ui_out *out,
   if (locals_iter == NULL)
     return EXT_LANG_BT_ERROR;
 
-  old_chain = make_cleanup_ui_out_list_begin_end (out, "variables");
+  ui_out_emit_list list_emitter (out, "variables");
 
-  if (args_iter != Py_None)
+  if (args_iter != Py_None
-    if (enumerate_args (args_iter.get (), out, args_type, 1, frame)
+      && (enumerate_args (args_iter.get (), out, args_type, 1, frame)
-	== EXT_LANG_BT_ERROR)
+	  == EXT_LANG_BT_ERROR))
-      goto error;
+    return EXT_LANG_BT_ERROR;
 
-  if (locals_iter != Py_None)
+  if (locals_iter != Py_None
-    if (enumerate_locals (locals_iter.get (), out, 1, args_type, 1, frame)
+      && (enumerate_locals (locals_iter.get (), out, 1, args_type, 1, frame)
-	== EXT_LANG_BT_ERROR)
+	  == EXT_LANG_BT_ERROR))
-      goto error;
+    return EXT_LANG_BT_ERROR;
 
-  do_cleanups (old_chain);
   return EXT_LANG_BT_OK;
-
- error:
-  do_cleanups (old_chain);
-  return EXT_LANG_BT_ERROR;
 }
 
 /* Helper function for printing locals.  This function largely just
@@ -876,25 +840,18 @@ py_print_locals (PyObject *filter,
 		 int indent,
 		 struct frame_info *frame)
 {
-  struct cleanup *old_chain;
-
   gdbpy_ref locals_iter (get_py_iter_from_func (filter, "frame_locals"));
   if (locals_iter == NULL)
     return EXT_LANG_BT_ERROR;
 
-  old_chain = make_cleanup_ui_out_list_begin_end (out, "locals");
+  ui_out_emit_list list_emitter (out, "locals");
 
-  if (locals_iter != Py_None)
+  if (locals_iter != Py_None
-    if (enumerate_locals (locals_iter.get (), out, indent, args_type,
+      && (enumerate_locals (locals_iter.get (), out, indent, args_type,
-			  0, frame) == EXT_LANG_BT_ERROR)
+			    0, frame) == EXT_LANG_BT_ERROR))
-      goto locals_error;
+    return EXT_LANG_BT_ERROR;
 
-  do_cleanups (old_chain);
   return EXT_LANG_BT_OK;
-
- locals_error:
-  do_cleanups (old_chain);
-  return EXT_LANG_BT_ERROR;
 }
 
 /* Helper function for printing frame arguments.  This function
@@ -908,13 +865,11 @@ py_print_args (PyObject *filter,
 	       enum ext_lang_frame_args args_type,
 	       struct frame_info *frame)
 {
-  struct cleanup *old_chain;
-
   gdbpy_ref args_iter (get_py_iter_from_func (filter, "frame_args"));
   if (args_iter == NULL)
     return EXT_LANG_BT_ERROR;
 
-  old_chain = make_cleanup_ui_out_list_begin_end (out, "args");
+  ui_out_emit_list list_emitter (out, "args");
 
   TRY
     {
@@ -925,14 +880,14 @@ py_print_args (PyObject *filter,
   CATCH (except, RETURN_MASK_ALL)
     {
       gdbpy_convert_exception (except);
-      goto args_error;
+      return EXT_LANG_BT_ERROR;
     }
   END_CATCH
 
-  if (args_iter != Py_None)
+  if (args_iter != Py_None
-    if (enumerate_args (args_iter.get (), out, args_type, 0, frame)
+      && (enumerate_args (args_iter.get (), out, args_type, 0, frame)
-	== EXT_LANG_BT_ERROR)
+	  == EXT_LANG_BT_ERROR))
-      goto args_error;
+    return EXT_LANG_BT_ERROR;
 
   TRY
     {
@@ -942,16 +897,11 @@ py_print_args (PyObject *filter,
   CATCH (except, RETURN_MASK_ALL)
     {
       gdbpy_convert_exception (except);
-      goto args_error;
+      return EXT_LANG_BT_ERROR;
     }
   END_CATCH
 
-  do_cleanups (old_chain);
   return EXT_LANG_BT_OK;
-
- args_error:
-  do_cleanups (old_chain);
-  return EXT_LANG_BT_ERROR;
 }
 
 /*  Print a single frame to the designated output stream, detecting
@@ -978,7 +928,6 @@ py_print_frame (PyObject *filter, int flags,
   CORE_ADDR address = 0;
   struct gdbarch *gdbarch = NULL;
   struct frame_info *frame = NULL;
-  struct cleanup *cleanup_stack;
   struct value_print_options opts;
   int print_level, print_frame_info, print_args, print_locals;
   gdb::unique_xmalloc_ptr<char> function_to_free;
@@ -1022,12 +971,12 @@ py_print_frame (PyObject *filter, int flags,
       return EXT_LANG_BT_COMPLETED;
     }
 
-  cleanup_stack = make_cleanup (null_cleanup, NULL);
+  gdb::optional<ui_out_emit_tuple> tuple;
 
   /* -stack-list-locals does not require a
      wrapping frame attribute.  */
   if (print_frame_info || (print_args && ! print_locals))
-    make_cleanup_ui_out_tuple_begin_end (out, "frame");
+    tuple.emplace (out, "frame");
 
   if (print_frame_info)
     {
@@ -1042,7 +991,6 @@ py_print_frame (PyObject *filter, int flags,
 	  CATCH (except, RETURN_MASK_ERROR)
 	    {
 	      gdbpy_convert_exception (except);
-	      do_cleanups (cleanup_stack);
 	      return EXT_LANG_BT_ERROR;
 	    }
 	  END_CATCH
@@ -1055,18 +1003,12 @@ py_print_frame (PyObject *filter, int flags,
 	  gdbpy_ref paddr (PyObject_CallMethod (filter, "address", NULL));
 
 	  if (paddr == NULL)
-	    {
+	    return EXT_LANG_BT_ERROR;
-	      do_cleanups (cleanup_stack);
-	      return EXT_LANG_BT_ERROR;
-	    }
 
 	  if (paddr != Py_None)
 	    {
 	      if (get_addr_from_python (paddr.get (), &address) < 0)
-		{
+		return EXT_LANG_BT_ERROR;
-		  do_cleanups (cleanup_stack);
-		  return EXT_LANG_BT_ERROR;
-		}
 
 	      has_addr = 1;
 	    }
@@ -1105,7 +1047,6 @@ py_print_frame (PyObject *filter, int flags,
       CATCH (except, RETURN_MASK_ERROR)
 	{
 	  gdbpy_convert_exception (except);
-	  do_cleanups (cleanup_stack);
 	  return EXT_LANG_BT_ERROR;
 	}
       END_CATCH
@@ -1127,7 +1068,6 @@ py_print_frame (PyObject *filter, int flags,
 	  CATCH (except, RETURN_MASK_ERROR)
 	    {
 	      gdbpy_convert_exception (except);
-	      do_cleanups (cleanup_stack);
 	      return EXT_LANG_BT_ERROR;
 	    }
 	  END_CATCH
@@ -1140,20 +1080,14 @@ py_print_frame (PyObject *filter, int flags,
 	  const char *function = NULL;
 
 	  if (py_func == NULL)
-	    {
+	    return EXT_LANG_BT_ERROR;
-	      do_cleanups (cleanup_stack);
-	      return EXT_LANG_BT_ERROR;
-	    }
 
 	  if (gdbpy_is_string (py_func.get ()))
 	    {
 	      function_to_free = python_string_to_host_string (py_func.get ());
 
 	      if (function_to_free == NULL)
-		{
+		return EXT_LANG_BT_ERROR;
-		  do_cleanups (cleanup_stack);
-		  return EXT_LANG_BT_ERROR;
-		}
 
 	      function = function_to_free.get ();
 	    }
@@ -1163,10 +1097,7 @@ py_print_frame (PyObject *filter, int flags,
 	      struct bound_minimal_symbol msymbol;
 
 	      if (get_addr_from_python (py_func.get (), &addr) < 0)
-		{
+		return EXT_LANG_BT_ERROR;
-		  do_cleanups (cleanup_stack);
-		  return EXT_LANG_BT_ERROR;
-		}
 
 	      msymbol = lookup_minimal_symbol_by_pc (addr);
 	      if (msymbol.minsym != NULL)
@@ -1177,7 +1108,6 @@ py_print_frame (PyObject *filter, int flags,
 	      PyErr_SetString (PyExc_RuntimeError,
 			       _("FrameDecorator.function: expecting a " \
 				 "String, integer or None."));
-	      do_cleanups (cleanup_stack);
 	      return EXT_LANG_BT_ERROR;
 	    }
 
@@ -1192,7 +1122,6 @@ py_print_frame (PyObject *filter, int flags,
 	  CATCH (except, RETURN_MASK_ERROR)
 	    {
 	      gdbpy_convert_exception (except);
-	      do_cleanups (cleanup_stack);
 	      return EXT_LANG_BT_ERROR;
 	    }
 	  END_CATCH
@@ -1205,10 +1134,7 @@ py_print_frame (PyObject *filter, int flags,
   if (print_args)
     {
       if (py_print_args (filter, out, args_type, frame) == EXT_LANG_BT_ERROR)
-	{
+	return EXT_LANG_BT_ERROR;
-	  do_cleanups (cleanup_stack);
-	  return EXT_LANG_BT_ERROR;
-	}
     }
 
   /* File name/source/line number information.  */
@@ -1221,7 +1147,6 @@ py_print_frame (PyObject *filter, int flags,
       CATCH (except, RETURN_MASK_ERROR)
 	{
 	  gdbpy_convert_exception (except);
-	  do_cleanups (cleanup_stack);
 	  return EXT_LANG_BT_ERROR;
 	}
       END_CATCH
@@ -1231,10 +1156,7 @@ py_print_frame (PyObject *filter, int flags,
 	  gdbpy_ref py_fn (PyObject_CallMethod (filter, "filename", NULL));
 
 	  if (py_fn == NULL)
-	    {
+	    return EXT_LANG_BT_ERROR;
-	      do_cleanups (cleanup_stack);
-	      return EXT_LANG_BT_ERROR;
-	    }
 
 	  if (py_fn != Py_None)
 	    {
@@ -1242,10 +1164,7 @@ py_print_frame (PyObject *filter, int flags,
 		filename (python_string_to_host_string (py_fn.get ()));
 
 	      if (filename == NULL)
-		{
+		return EXT_LANG_BT_ERROR;
-		  do_cleanups (cleanup_stack);
-		  return EXT_LANG_BT_ERROR;
-		}
 
 	      TRY
 		{
@@ -1258,7 +1177,6 @@ py_print_frame (PyObject *filter, int flags,
 	      CATCH (except, RETURN_MASK_ERROR)
 		{
 		  gdbpy_convert_exception (except);
-		  do_cleanups (cleanup_stack);
 		  return EXT_LANG_BT_ERROR;
 		}
 	      END_CATCH
@@ -1271,19 +1189,13 @@ py_print_frame (PyObject *filter, int flags,
 	  int line;
 
 	  if (py_line == NULL)
-	    {
+	    return EXT_LANG_BT_ERROR;
-	      do_cleanups (cleanup_stack);
-	      return EXT_LANG_BT_ERROR;
-	    }
 
 	  if (py_line != Py_None)
 	    {
 	      line = PyLong_AsLong (py_line.get ());
 	      if (PyErr_Occurred ())
-		{
+		return EXT_LANG_BT_ERROR;
-		  do_cleanups (cleanup_stack);
-		  return EXT_LANG_BT_ERROR;
-		}
 
 	      TRY
 		{
@@ -1294,7 +1206,6 @@ py_print_frame (PyObject *filter, int flags,
 	      CATCH (except, RETURN_MASK_ERROR)
 		{
 		  gdbpy_convert_exception (except);
-		  do_cleanups (cleanup_stack);
 		  return EXT_LANG_BT_ERROR;
 		}
 	      END_CATCH
@@ -1314,7 +1225,6 @@ py_print_frame (PyObject *filter, int flags,
       CATCH (except, RETURN_MASK_ERROR)
 	{
 	  gdbpy_convert_exception (except);
-	  do_cleanups (cleanup_stack);
 	  return EXT_LANG_BT_ERROR;
 	}
       END_CATCH
@@ -1324,26 +1234,20 @@ py_print_frame (PyObject *filter, int flags,
     {
       if (py_print_locals (filter, out, args_type, indent,
 			   frame) == EXT_LANG_BT_ERROR)
-	{
+	return EXT_LANG_BT_ERROR;
-	  do_cleanups (cleanup_stack);
-	  return EXT_LANG_BT_ERROR;
-	}
     }
 
   {
     /* Finally recursively print elided frames, if any.  */
     gdbpy_ref elided (get_py_iter_from_func (filter, "elided"));
     if (elided == NULL)
-      {
+      return EXT_LANG_BT_ERROR;
-	do_cleanups (cleanup_stack);
-	return EXT_LANG_BT_ERROR;
-      }
 
     if (elided != Py_None)
       {
 	PyObject *item;
 
-	make_cleanup_ui_out_list_begin_end (out, "children");
+	ui_out_emit_list inner_list_emiter (out, "children");
 
 	if (! out->is_mi_like_p ())
 	  indent++;
@@ -1358,20 +1262,13 @@ py_print_frame (PyObject *filter, int flags,
 							      levels_printed);
 
 	    if (success == EXT_LANG_BT_ERROR)
-	      {
+	      return EXT_LANG_BT_ERROR;
-		do_cleanups (cleanup_stack);
-		return EXT_LANG_BT_ERROR;
-	      }
 	  }
 	if (item == NULL && PyErr_Occurred ())
-	  {
+	  return EXT_LANG_BT_ERROR;
-	    do_cleanups (cleanup_stack);
-	    return EXT_LANG_BT_ERROR;
-	  }
       }
   }
 
-  do_cleanups (cleanup_stack);
   return EXT_LANG_BT_COMPLETED;
 }
 

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 */