Implement P0463R1: "Endian just Endian"

[mclow.lists]

Summary

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.

[lebedev.ri]

include/type_traits

4740

(Apologies for double commenting, did not notice that it was in phab until after replying)

I'm probably wrong, but if this is a C++2a proposal, shouldn't this

#if _LIBCPP_STD_VER > 14

be

#if _LIBCPP_STD_VER > 17

?

[mclow.lists]

include/type_traits

4740

Yes it should. Thanks!

[majnemer]

test/std/utilities/meta/meta.type.synop/endian.pass.cpp

40–43

This is undefined behavior as-is because you are reading from a union member other than the active union member.

I'd recommend a sequence like:

uint32_t i = 0x01020304;
char c[4];
memcpy(c, &i, 4);

[mclow.lists]

Fixed test and #ifdef guard

[EricWF]

include/type_traits

4746

These macros have been changed to be #ifdef style.

4750

Does this else case ever actually happen?

[mclow.lists]

Update macro checks.

[lebedev.ri]

test/std/utilities/meta/meta.type.synop/endian.pass.cpp

10

c++17, not c++1z
At least grep says there are no c++1z left in the libc++ tests.

[mclow.lists]

Committed as revision 323296

include/type_traits

 
 #endif
 
+#if _LIBCPP_STD_VER > 17
+enum class endian
+{
+    little = 0xDEAD,
+    big    = 0xFACE,
+#if defined(_LIBCPP_LITTLE_ENDIAN)
+    native = little
+#elif defined(_LIBCPP_BIG_ENDIAN)
+    native = big
+#else
+    native = 0xCAFE
+#endif
+};
+#endif
+
 _LIBCPP_END_NAMESPACE_STD
 
 #if _LIBCPP_STD_VER > 14

test/std/utilities/meta/meta.type.synop/endian.pass.cpp

+//===----------------------------------------------------------------------===//
+//
+//                     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++11, c++14, c++1z
+
+// enum class endian;
+
+#include <type_traits>
+#include <cstring>
+#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
+    {
+    uint32_t i = 0x01020304;
+    char c[4];
+    static_assert(sizeof(i) == sizeof(c));
+    std::memcpy(c, &i, sizeof(c));
+
+    assert ((c[0] == 1) == (std::endian::native == std::endian::big));
+    }
+}