Introduce ui_file_as_string

ui_file_as_string is a variant of ui_file_xstrdup that returns a
std::string instead of a xmalloc'ed char *.  The idea is using the new
function to eliminate "make_cleanup (xfree, ...)"  cleanups
throughout.

Following patches will make use of this.

gdb/ChangeLog:
2016-11-08  Pedro Alves  <palves@redhat.com>

	* ui-file.c (do_ui_file_as_string, ui_file_as_string): New
	functions.
	* ui-file.h: Include <string>.
	(ui_file_as_string): New declaration.

gdb/ChangeLog

@@ -1,3 +1,10 @@
+2016-11-08  Pedro Alves  <palves@redhat.com>
+
+	* ui-file.c (do_ui_file_as_string, ui_file_as_string): New
+	functions.
+	* ui-file.h: Include <string>.
+	(ui_file_as_string): New declaration.
+
 2016-11-08  Pedro Alves  <palves@redhat.com>
 
 	* ada-lang.c (ada_read_renaming_var_value): Use expression_up.

gdb/ui-file.c

@@ -356,6 +356,28 @@ ui_file_xstrdup (struct ui_file *file, long *length)
   return acc.buffer;
 }
 
+/* ui_file utility function for converting a ``struct ui_file'' into a
+   std:string.  */
+
+static void
+do_ui_file_as_string (void *context, const char *buffer, long length)
+{
+  std::string *str = (std::string *) context;
+
+  *str = std::string (buffer, length);
+}
+
+/* See ui-file.h.  */
+
+std::string
+ui_file_as_string (struct ui_file *file)
+{
+  std::string str;
+
+  ui_file_put (file, do_ui_file_as_string, &str);
+  return str;
+}
+
 static void
 do_ui_file_obsavestring (void *context, const char *buffer, long length)
 {

gdb/ui-file.h

@@ -22,6 +22,8 @@
 struct obstack;
 struct ui_file;
 
+#include <string>
+
 /* Create a generic ui_file object with null methods.  */
 
 extern struct ui_file *ui_file_new (void);
@@ -117,6 +119,10 @@ extern void ui_file_put (struct ui_file *src,
    minus that appended NUL.  */
 extern char *ui_file_xstrdup (struct ui_file *file, long *length);
 
+/* Returns a std::string containing the entire contents of FILE (as
+   determined by ui_file_put()).  */
+extern std::string ui_file_as_string (struct ui_file *file);
+
 /* Similar to ui_file_xstrdup, but return a new string allocated on
    OBSTACK.  */
 extern char *ui_file_obsavestring (struct ui_file *file,