Implement string_view from the library fundamentals TS (n4023). Also works in C++11 and 03, with reduced functionality (mostly in the area of constexpr)

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@210659 91177308-0d34-0410-b5e6-96231b3b80d8

test/experimental/string.view/string.view.iterators/rbegin.pass.cpp

@@ -0,0 +1,59 @@
+//===----------------------------------------------------------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is dual licensed under the MIT and the University of Illinois Open
+// Source Licenses. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+// <string_view>
+
+// const_iterator rbegin() const;
+
+#include <experimental/string_view>
+#include <cassert>
+
+template <class S>
+void
+test(S s)
+{
+    const S& cs = s;
+    typename S::reverse_iterator b = s.rbegin();
+    typename S::const_reverse_iterator cb1 = cs.rbegin();
+    typename S::const_reverse_iterator cb2 = s.crbegin();
+    if (!s.empty())
+    {
+        const size_t last = s.size() - 1;
+        assert(   *b ==  s[last]);
+        assert(  &*b == &s[last]);
+        assert( *cb1 ==  s[last]);
+        assert(&*cb1 == &s[last]);
+        assert( *cb2 ==  s[last]);
+        assert(&*cb2 == &s[last]);
+        
+    }
+    assert(  b == cb1);
+    assert(  b == cb2);
+    assert(cb1 == cb2);
+}
+
+
+int main()
+{
+    using string_view    = std::experimental::string_view;
+    using u16string_view = std::experimental::u16string_view;
+    using u32string_view = std::experimental::u32string_view;
+    using wstring_view   = std::experimental::wstring_view;
+
+    test(string_view   ());
+    test(u16string_view());
+    test(u32string_view());
+    test(wstring_view  ());
+    test(string_view   ( "123"));
+    test(wstring_view  (L"123"));
+#if __cplusplus >= 201103L
+    test(u16string_view{u"123"});
+    test(u32string_view{U"123"});
+#endif
+}