[PATCH] D35472: Implement P0463R1: "Endian just Endian"

[Marshall Clow via Phabricator via cfe-commits]

mclow.lists created this revision.

Implement the C++2a feature "A compile time endian-ness detection idiom"
Howard's suggested implementation is:

enum class endian
	{
	#ifdef _WIN32

  		little = 0,
  		big    = 1,
  		native = little

#else

  		little = __ORDER_LITTLE_ENDIAN__,
  		big    = __ORDER_BIG_ENDIAN__
  		native = __BYTE_ORDER__,

#endif
	};

but libc++ has it's own macros that have done the work to detect this.

The other option would be to rip out `_LIBCPP_LITTLE_ENDIAN` and `_LIBCPP_BIG_ENDIAN`, which is tempting, but that would entail changes in code that has to compile for earlier C++ language versions.


https://reviews.llvm.org/D35472

Files:
  include/type_traits
  test/std/utilities/meta/meta.type.synop/endian.pass.cpp


Index: test/std/utilities/meta/meta.type.synop/endian.pass.cpp
===================================================================
--- test/std/utilities/meta/meta.type.synop/endian.pass.cpp
+++ test/std/utilities/meta/meta.type.synop/endian.pass.cpp
@@ -0,0 +1,46 @@
+//===----------------------------------------------------------------------===//
+//
+//                     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.
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++98, c++03, c++14, c++17
+
+// enum class endian;
+
+#include <type_traits>
+#include <cassert>
+
+#include "test_macros.h"
+
+int main() {
+    typedef std::endian E;
+    static_assert(std::is_enum<std::endian>::value, "");
+
+// Check that E is a scoped enum by checking for conversions.
+    typedef std::underlying_type<std::endian>::type UT;
+    static_assert(!std::is_convertible<std::endian, UT>::value, "");
+
+// test that the enumeration values exist
+    static_assert( std::endian::little == std::endian::little );
+    static_assert( std::endian::big    == std::endian::big );
+    static_assert( std::endian::native == std::endian::native );
+    static_assert( std::endian::little != std::endian::big );
+
+//  Technically not required, but true on all existing machines
+    static_assert( std::endian::native == std::endian::little || 
+                   std::endian::native == std::endian::big );
+    
+//  Try to check at runtime
+    {
+    union {
+        uint32_t i;
+        char c[4];
+    } u = {0x01020304};
+
+    assert ((u.c[0] == 1) == (std::endian::native == std::endian::big));
+    }
+}
Index: include/type_traits
===================================================================
--- include/type_traits
+++ include/type_traits
@@ -4737,6 +4737,21 @@
 
 #endif
 
+#if _LIBCPP_STD_VER > 14
+enum class endian
+{
+    little = 0xDEAD,
+    big    = 0xFACE,
+#if _LIBCPP_LITTLE_ENDIAN
+    native = little
+#elif _LIBCPP_BIG_ENDIAN
+    native = big
+#else
+    native = 0xCAFE
+#endif
+};
+#endif
+
 _LIBCPP_END_NAMESPACE_STD
 
 #if _LIBCPP_STD_VER > 14


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