From ba8ebef509653127e74063097c1f7703b8a27185 Mon Sep 17 00:00:00 2001
From: Neil MacIntosh <neilmac@microsoft.com>
Date: Sun, 29 May 2016 17:06:29 -0700
Subject: [PATCH] Added span to object rep conversions.

---
 include/byte.h       | 33 ++++++++++++++++
 include/multi_span.h |  4 +-
 include/span.h       | 34 +++++++++++++----
 tests/span_tests.cpp | 91 ++++++++++++++++++++++++++++++--------------
 4 files changed, 124 insertions(+), 38 deletions(-)
 create mode 100644 include/byte.h

diff --git a/include/byte.h b/include/byte.h
new file mode 100644
index 0000000..c6449aa
--- /dev/null
+++ b/include/byte.h
@@ -0,0 +1,33 @@
+/////////////////////////////////////////////////////////////////////////////// 
+// 
+// Copyright (c) 2015 Microsoft Corporation. All rights reserved. 
+// 
+// This code is licensed under the MIT License (MIT). 
+// 
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR 
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, 
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE 
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER 
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, 
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN 
+// THE SOFTWARE. 
+// 
+///////////////////////////////////////////////////////////////////////////////
+
+#pragma once
+
+#ifndef GSL_BYTE_H
+#define GSL_BYTE_H
+
+namespace gsl
+{
+    // This is a simple definition for now that allows
+    // use of byte within span<> to be standards-compliant
+    //
+    // Ultimately a small language change would allow a more
+    // robust definition (see WG21 proposal P0257 for details).
+    using byte = unsigned char;
+    
+} // namespace gsl
+
+#endif // GSL_BYTE_H
\ No newline at end of file
diff --git a/include/multi_span.h b/include/multi_span.h
index 523f324..3d00c6e 100644
--- a/include/multi_span.h
+++ b/include/multi_span.h
@@ -21,6 +21,7 @@
 
 #include "gsl_assert.h"
 #include "gsl_util.h"
+#include "byte.h"
 #include <algorithm>
 #include <array>
 #include <cassert>
@@ -1053,9 +1054,6 @@ template <typename Span>
 class contiguous_span_iterator;
 template <typename Span>
 class general_span_iterator;
-enum class byte : std::uint8_t
-{
-};
 
 template <std::ptrdiff_t DimSize = dynamic_range>
 struct dim
diff --git a/include/span.h b/include/span.h
index 2760121..7152c2d 100644
--- a/include/span.h
+++ b/include/span.h
@@ -21,6 +21,7 @@
 
 #include "gsl_assert.h"
 #include "gsl_util.h"
+#include "byte.h"
 #include <array>
 #include <limits>
 #include <iterator>
@@ -126,13 +127,13 @@ struct is_allowed_element_type_conversion
 };
 
 template <class From>
-struct is_allowed_element_type_conversion<From, char>
+struct is_allowed_element_type_conversion<From, byte>
     : std::integral_constant<bool, !std::is_const<From>::value>
 {
 };
 
 template <class From>
-struct is_allowed_element_type_conversion<From, const char>
+struct is_allowed_element_type_conversion<From, const byte>
     : std::integral_constant<bool, true>
 {
 };
@@ -507,14 +508,33 @@ constexpr bool operator>=(const span<ElementType, Extent>& l, const span<Element
 { return !(l < r); }
 
 
-#if 0 // TODO
+namespace details
+{
+    // if we only supported compilers with good constexpr support then
+    // this pair of classes could collapse down to a constexpr function
+
+    // we should use a narrow_cast<> to go to size_t, but older compilers may not see it as constexpr
+    // and so will fail compilation of the template
+    template <class ElementType, ptrdiff_t Extent>
+    struct calculate_byte_size :
+        std::integral_constant<std::ptrdiff_t, static_cast<ptrdiff_t>(sizeof(ElementType) * static_cast<size_t>(Extent))>
+    {};
+
+    template <class ElementType>
+    struct calculate_byte_size<ElementType, dynamic_extent> :
+        std::integral_constant<std::ptrdiff_t, dynamic_extent>
+    {};
+}
+
+
 // [span.objectrep], views of object representation 
 template <class ElementType, ptrdiff_t Extent>
-constexpr span<const char, ((Extent == dynamic_extent) ? dynamic_extent : (sizeof(ElementType) * Extent))> as_bytes(span<ElementType, Extent> s) noexcept;
+constexpr span<const byte, details::calculate_byte_size<ElementType, Extent>::value> as_bytes(span<ElementType, Extent> s) noexcept
+{ return {reinterpret_cast<const byte*>(s.data()), s.size_bytes()}; }
 
-template <class ElementType, ptrdiff_t Extent>
-constexpr span<char, ((Extent == dynamic_extent) ? dynamic_extent : (sizeof(ElementType) * Extent))> as_writeable_bytes(span<ElementType, Extent>) noexcept;
-#endif
+template <class ElementType, ptrdiff_t Extent, class = std::enable_if_t<!std::is_const<ElementType>::value>>
+constexpr span<byte, details::calculate_byte_size<ElementType, Extent>::value> as_writeable_bytes(span<ElementType, Extent> s) noexcept
+{ return {reinterpret_cast<byte*>(s.data()), s.size_bytes()}; }
 
 } // namespace gsl
 
diff --git a/tests/span_tests.cpp b/tests/span_tests.cpp
index 694d9a2..ddae404 100644
--- a/tests/span_tests.cpp
+++ b/tests/span_tests.cpp
@@ -921,6 +921,69 @@ SUITE(span_tests)
         }
     }
 
+    TEST(as_bytes)
+    {
+        int a[] = {1, 2, 3, 4};
+
+        {
+            span<const int> s = a;
+            CHECK(s.length() == 4);
+            span<const byte> bs = as_bytes(s);
+            CHECK(static_cast<const void*>(bs.data()) == static_cast<const void*>(s.data()));
+            CHECK(bs.length() == s.length_bytes());
+        }
+
+        {
+            span<int> s;
+            auto bs = as_bytes(s);
+            CHECK(bs.length() == s.length());
+            CHECK(bs.length() == 0);
+            CHECK(bs.size_bytes() == 0);
+            CHECK(static_cast<const void*>(bs.data()) == static_cast<const void*>(s.data()));
+            CHECK(bs.data() == nullptr);
+        }
+
+        {
+            span<int> s = a;
+            auto bs = as_bytes(s);
+            CHECK(static_cast<const void*>(bs.data()) == static_cast<const void*>(s.data()));
+            CHECK(bs.length() == s.length_bytes());
+        }
+    }
+
+    TEST(as_writeable_bytes)
+    {
+        int a[] = {1, 2, 3, 4};
+
+        {
+#ifdef CONFIRM_COMPILATION_ERRORS
+            // you should not be able to get writeable bytes for const objects
+            span<const int> s = a;
+            CHECK(s.length() == 4);
+            span<const byte> bs = as_writeable_bytes(s);
+            CHECK(static_cast<void*>(bs.data()) == static_cast<void*>(s.data()));
+            CHECK(bs.length() == s.length_bytes());
+#endif
+        }
+
+        {
+            span<int> s;
+            auto bs = as_writeable_bytes(s);
+            CHECK(bs.length() == s.length());
+            CHECK(bs.length() == 0);
+            CHECK(bs.size_bytes() == 0);
+            CHECK(static_cast<void*>(bs.data()) == static_cast<void*>(s.data()));
+            CHECK(bs.data() == nullptr);
+        }
+
+        {
+            span<int> s = a;
+            auto bs = as_writeable_bytes(s);
+            CHECK(static_cast<void*>(bs.data()) == static_cast<void*>(s.data()));
+            CHECK(bs.length() == s.length_bytes());
+        }
+    }
+
 #if 0
     TEST(fixed_size_conversions)
     {
@@ -1018,34 +1081,6 @@ SUITE(span_tests)
         CHECK_THROW(f(), fail_fast);
     }
 
-    TEST(as_writeable_bytes)
-    {
-        int a[] = {1, 2, 3, 4};
-
-        {
-#ifdef CONFIRM_COMPILATION_ERRORS
-            // you should not be able to get writeable bytes for const objects
-            span<const int, dynamic_range> av = a;
-            auto wav = av.as_writeable_bytes();
-#endif
-        }
-
-        {
-            span<int, dynamic_range> av;
-            auto wav = as_writeable_bytes(av);
-            CHECK(wav.length() == av.length());
-            CHECK(wav.length() == 0);
-            CHECK(wav.size_bytes() == 0);
-        }
-
-        {
-            span<int, dynamic_range> av = a;
-            auto wav = as_writeable_bytes(av);
-            CHECK(wav.data() == (byte*) &a[0]);
-            CHECK(wav.length() == sizeof(a));
-        }
-    }
-
 #endif    
 }