First part of P1024: Usability Enhancements for std::span. Remove operator() for indexing, and add 'front' and 'back' calls.

llvm-svn: 354801

1 files changed, 21 insertions, 7 deletions

diff --git a/libcxx/include/span b/libcxx/include/span
index 774f69823bd..b212f8722be 100644
--- a/libcxx/include/span
+++ b/libcxx/include/span
@@ -88,7 +88,8 @@ public:
 
     // [span.elem], span element access
     constexpr reference operator[](index_type idx) const;
-    constexpr reference operator()(index_type idx) const;
+    constexpr reference front() const;
+    constexpr reference back() const;
     constexpr pointer data() const noexcept;
 
     // [span.iterators], span iterator support
@@ -319,10 +320,16 @@ public:
         return __data[__idx];
     }
 
-    _LIBCPP_INLINE_VISIBILITY constexpr reference operator()(index_type __idx) const noexcept
+    _LIBCPP_INLINE_VISIBILITY constexpr reference front() const noexcept
     {
-        _LIBCPP_ASSERT(__idx >= 0 && __idx < size(), "span<T,N>() index out of bounds");
-        return __data[__idx];
+        static_assert(_Extent > 0, "span<T,N>[].front() on empty span");
+        return __data[0];
+    }
+
+    _LIBCPP_INLINE_VISIBILITY constexpr reference back() const noexcept
+    {
+        static_assert(_Extent > 0, "span<T,N>[].back() on empty span");
+        return __data[size()-1];
     }
 
     _LIBCPP_INLINE_VISIBILITY constexpr pointer data()                         const noexcept { return __data; }
@@ -484,12 +491,19 @@ public:
         return __data[__idx];
     }
 
-    _LIBCPP_INLINE_VISIBILITY constexpr reference operator()(index_type __idx) const noexcept
+    _LIBCPP_INLINE_VISIBILITY constexpr reference front() const noexcept
     {
-        _LIBCPP_ASSERT(__idx >= 0 && __idx < size(), "span<T>() index out of bounds");
-        return __data[__idx];
+        _LIBCPP_ASSERT(!empty(), "span<T>[].front() on empty span");
+        return __data[0];
     }
 
+    _LIBCPP_INLINE_VISIBILITY constexpr reference back() const noexcept
+    {
+        _LIBCPP_ASSERT(!empty(), "span<T>[].back() on empty span");
+        return __data[size()-1];
+    }
+
+
     _LIBCPP_INLINE_VISIBILITY constexpr pointer data()                         const noexcept { return __data; }
 
 // [span.iter], span iterator support