Introduce basic_safe_range

This introduces the basic_safe_range class, which can be used to
create a basic_safe_iterator.  This also changes basic_safe_iterator
in two ways.

First, it simplifies the constructor.  This seemed unnecessarily
complicated to me, and keeping it this way would prevent the second
change...

... which is to add a second constructor for initializing the
one-past-the-end iterator that is stored in basic_safe_iterator.

gdb/ChangeLog
2019-12-12  Tom Tromey  <tom@tromey.com>

	* gdbsupport/safe-iterator.h (basic_safe_iterator): Simplify.  Add
	second constructor.
	(basic_safe_range): New class.

Change-Id: Ib351ef6fd435129a5053c64e5561877e1459ab37

gdb/ChangeLog

@@ -1,3 +1,9 @@
+2019-12-12  Tom Tromey  <tom@tromey.com>
+
+	* gdbsupport/safe-iterator.h (basic_safe_iterator): Simplify.  Add
+	second constructor.
+	(basic_safe_range): New class.
+
 2019-12-12  Tom Tromey  <tom@tromey.com>
 
 	* progspace.c (program_space::multi_objfile_p): New method.

gdb/gdbsupport/safe-iterator.h

@@ -48,17 +48,29 @@ public:
   typedef typename Iterator::iterator_category iterator_category;
   typedef typename Iterator::difference_type difference_type;
 
-  /* Construct by forwarding all arguments to the underlying
-     iterator.  */
-  template<typename... Args>
-  explicit basic_safe_iterator (Args &&...args)
-    : m_it (std::forward<Args> (args)...),
+  /* Construct using the given argument; the end iterator is default
+     constructed.  */
+  template<typename Arg>
+  explicit basic_safe_iterator (Arg &&arg)
+    : m_it (std::forward<Arg> (arg)),
       m_next (m_it)
   {
     if (m_it != m_end)
       ++m_next;
   }
 
+  /* Construct the iterator using the first argument, and construct
+     the end iterator using the second argument.  */
+  template<typename Arg>
+  explicit basic_safe_iterator (Arg &&arg, Arg &&arg2)
+    : m_it (std::forward<Arg> (arg)),
+      m_next (m_it),
+      m_end (std::forward<Arg> (arg2))
+  {
+    if (m_it != m_end)
+      ++m_next;
+  }
+
   /* Create a one-past-end iterator.  */
   basic_safe_iterator ()
   {}
@@ -90,4 +102,34 @@ private:
   Iterator m_end {};
 };
 
+/* A range adapter that wraps another range, and then returns safe
+   iterators wrapping the original range's iterators.  */
+
+template<typename Range>
+class basic_safe_range
+{
+public:
+
+  typedef basic_safe_iterator<typename Range::iterator> iterator;
+
+  explicit basic_safe_range (Range range)
+    : m_range (range)
+  {
+  }
+
+  iterator begin () const
+  {
+    return iterator (m_range.begin (), m_range.end ());
+  }
+
+  iterator end () const
+  {
+    return iterator (m_range.end (), m_range.end ());
+  }
+
+private:
+
+  Range m_range;
+};
+
 #endif /* COMMON_SAFE_ITERATOR_H */