Add sentinel support to IteratorRange

As of C++20, range based for supports that the end iterator has a different type. This allows to implement so called sentinels tagging the end. This can make it simpler and more efficient to implement ranges where it's not straight forward to get the end in O(1) or where comparing real iterators is expensive. To support this approach, this adds support for using a different type for the end iterator to Dune::IteratorRange.

dune/common/test/rangeutilitiestest.cc

@@ -92,7 +92,7 @@ auto checkSameRange(R1&& r1, BeginIt2&& it2, EndIt2&& end2)
 {
   auto it1 = r1.begin();
   auto end1 = r1.end();
-  for(; (it1 < end1) and (it2 < end2); ++it1, ++it2)
+  for(; (it1 != end1) and (it2 != end2); ++it1, ++it2)
     if (*it1 != *it2)
       return false;
   if ((it1 != end1) or (it2 != end2))
@@ -387,11 +387,64 @@ auto testSparseRange()
 
 
 
+// An empty class tagging the end of a range
+class SentinelIterator
+{};
+
+// Classical use case for sentinels: An iterator for
+// a null-terminated string. Using a sentinel allows
+// to implement a range without a linear scan to find
+// the end or storing an additional isEnd flag in the
+// iterator.
+class NullTerminatedStringIterator
+{
+public:
+  NullTerminatedStringIterator(char* p)
+    : p_(p)
+  {}
+
+  NullTerminatedStringIterator& operator++()
+  {
+    ++p_;
+    return *this;
+  }
+
+  char& operator*()
+  {
+    return *p_;
+  }
+
+  friend bool operator==(const NullTerminatedStringIterator& it, const SentinelIterator& end)
+  {
+    return (*it.p_) == 0;
+  }
+
+private:
+  char* p_;
+};
+
+
+
+Dune::TestSuite testIteratorRange()
+{
+  Dune::TestSuite suite("Check IteratorRange with sentinel");
+
+  // Check sentinel terminated range for null-terminated string
+  char testStr[] = "FooBar";
+  auto range_sentinel = Dune::IteratorRange(NullTerminatedStringIterator(testStr), SentinelIterator());
+  suite.check(checkSameRange(range_sentinel, std::string_view(testStr)));
+
+  return suite;
+}
+
+
+
 int main()
 {
   // Check IsIterable<> for https://gitlab.dune-project.org/core/dune-common/issues/58
   static_assert(Dune::IsIterable< std::array<int, 3> >::value, "std::array<int> must be a range");
   static_assert(Dune::IsIterable< Dune::IteratorRange<int*> >::value, "IteratorRange must be a range");
+//  static_assert(Dune::IsIterable< Dune::IteratorRange<int*, void*> >::value, "IteratorRange must be a range");
   static_assert(!Dune::IsIterable< int >::value, "int must not be a range");
 
   Dune::TestSuite suite;
@@ -505,6 +558,8 @@ int main()
 
   suite.subTest(testSparseRange());
 
+  suite.subTest(testIteratorRange());
+
   return suite.exit();
 
 }