[libc++] Do not derive __gnu_cxx::hash<T> from std::hash<T>.

[pcc]

Summary

Instead, define explicit specializations for the basic types listed in
the SGI documentation. This solves two problems:

1. Helps avoid silent ODR violations caused by the absence of a user-supplied __gnu_cxx::hash specialization in cases where a std::hash specialization exists (e.g. for std::string).
2. gnu_cxx::hash semantics are slightly different to those of std::hash (for example, the former may dereference a pointer argument) so it is inappropriate for gnu_cxx::hash to receive std::hash specializations by default.

[pcc]

Ping.

[pcc]

Add tests.

[pcc]

Ping.

[mclow.lists]

Other than the one test that I commented on, this looks ok to me.

test/extensions/hash/specializations.fail.cpp

16

Now about
assert(__gnu_cxx::hash<std::string>()(std::string()) == 0); // error

instead?
That way you can't get caught up with implicit conversions.
[ I don't think it will matter, but I've been caught out before by failing tests failing, but for the wrong reason. ]

[pcc]

Thanks, committing.

test/extensions/hash/specializations.fail.cpp

16

Done.

[pcc]

Closed by commit rL203070 (authored by @pcc).

include/ext/__hash

 #pragma GCC system_header
 
 #include <string>
 #include <cstring>
 
 namespace __gnu_cxx {
 using namespace std;
 
-template <typename _Tp> struct _LIBCPP_TYPE_VIS_ONLY hash : public std::hash<_Tp>
-    { };
+template <typename _Tp> struct _LIBCPP_TYPE_VIS_ONLY hash { };
 
 template <> struct _LIBCPP_TYPE_VIS_ONLY hash<const char*>
     : public unary_function<const char*, size_t>
 {
     _LIBCPP_INLINE_VISIBILITY
     size_t operator()(const char *__c) const _NOEXCEPT
     {
         return __do_string_hash(__c, __c + strlen(__c));
     }
 };
 
 template <> struct _LIBCPP_TYPE_VIS_ONLY hash<char *>
     : public unary_function<char*, size_t>
 {
     _LIBCPP_INLINE_VISIBILITY
     size_t operator()(char *__c) const _NOEXCEPT
     {
         return __do_string_hash<const char *>(__c, __c + strlen(__c));
     }
 };
+
+template <> struct _LIBCPP_TYPE_VIS_ONLY hash<char>
+    : public unary_function<char, size_t>
+{
+    _LIBCPP_INLINE_VISIBILITY
+    size_t operator()(char __c) const _NOEXCEPT
+    {
+        return __c;
+    }
+};
+
+template <> struct _LIBCPP_TYPE_VIS_ONLY hash<signed char>
+    : public unary_function<signed char, size_t>
+{
+    _LIBCPP_INLINE_VISIBILITY
+    size_t operator()(signed char __c) const _NOEXCEPT
+    {
+        return __c;
+    }
+};
+
+template <> struct _LIBCPP_TYPE_VIS_ONLY hash<unsigned char>
+    : public unary_function<unsigned char, size_t>
+{
+    _LIBCPP_INLINE_VISIBILITY
+    size_t operator()(unsigned char __c) const _NOEXCEPT
+    {
+        return __c;
+    }
+};
+
+template <> struct _LIBCPP_TYPE_VIS_ONLY hash<short>
+    : public unary_function<short, size_t>
+{
+    _LIBCPP_INLINE_VISIBILITY
+    size_t operator()(short __c) const _NOEXCEPT
+    {
+        return __c;
+    }
+};
+
+template <> struct _LIBCPP_TYPE_VIS_ONLY hash<unsigned short>
+    : public unary_function<unsigned short, size_t>
+{
+    _LIBCPP_INLINE_VISIBILITY
+    size_t operator()(unsigned short __c) const _NOEXCEPT
+    {
+        return __c;
+    }
+};
+
+template <> struct _LIBCPP_TYPE_VIS_ONLY hash<int>
+    : public unary_function<int, size_t>
+{
+    _LIBCPP_INLINE_VISIBILITY
+    size_t operator()(int __c) const _NOEXCEPT
+    {
+        return __c;
+    }
+};
+
+template <> struct _LIBCPP_TYPE_VIS_ONLY hash<unsigned int>
+    : public unary_function<unsigned int, size_t>
+{
+    _LIBCPP_INLINE_VISIBILITY
+    size_t operator()(unsigned int __c) const _NOEXCEPT
+    {
+        return __c;
+    }
+};
+
+template <> struct _LIBCPP_TYPE_VIS_ONLY hash<long>
+    : public unary_function<long, size_t>
+{
+    _LIBCPP_INLINE_VISIBILITY
+    size_t operator()(long __c) const _NOEXCEPT
+    {
+        return __c;
+    }
+};
+
+template <> struct _LIBCPP_TYPE_VIS_ONLY hash<unsigned long>
+    : public unary_function<unsigned long, size_t>
+{
+    _LIBCPP_INLINE_VISIBILITY
+    size_t operator()(unsigned long __c) const _NOEXCEPT
+    {
+        return __c;
+    }
+};
 }
 
 #endif  // _LIBCPP_EXT_HASH

test/extensions/hash/specializations.fail.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.
+//
+//===----------------------------------------------------------------------===//
+
+#include <assert.h>
+#include <ext/hash_map>
+#include <string>
+
+int main()
+{
+    assert(__gnu_cxx::hash<std::string>()("") == 0);  // error
+}

test/extensions/hash/specializations.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.
+//
+//===----------------------------------------------------------------------===//
+
+#include <assert.h>
+#include <ext/hash_map>
+#include <string>
+
+int main()
+{
+    char str[] = "test";
+    assert(__gnu_cxx::hash<const char *>()("test") ==
+           std::hash<std::string>()("test"));
+    assert(__gnu_cxx::hash<char *>()(str) == std::hash<std::string>()("test"));
+    assert(__gnu_cxx::hash<char>()(42) == 42);
+    assert(__gnu_cxx::hash<signed char>()(42) == 42);
+    assert(__gnu_cxx::hash<unsigned char>()(42) == 42);
+    assert(__gnu_cxx::hash<short>()(42) == 42);
+    assert(__gnu_cxx::hash<unsigned short>()(42) == 42);
+    assert(__gnu_cxx::hash<int>()(42) == 42);
+    assert(__gnu_cxx::hash<unsigned int>()(42) == 42);
+    assert(__gnu_cxx::hash<long>()(42) == 42);
+    assert(__gnu_cxx::hash<unsigned long>()(42) == 42);
+}

test/extensions/nothing_to_do.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.
+//
+//===----------------------------------------------------------------------===//
+
+int main()
+{
+}