[libcxx-commits] [libcxx] [libc++] Refactor key extraction for __hash_table and __tree (PR #154512)

[Louis Dionne via libcxx-commits]

================
@@ -0,0 +1,109 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef _LIBCPP___UTILITY_TRY_EXTRACT_KEY_H
+#define _LIBCPP___UTILITY_TRY_EXTRACT_KEY_H
+
+#include <__config>
+#include <__fwd/pair.h>
+#include <__fwd/tuple.h>
+#include <__type_traits/enable_if.h>
+#include <__type_traits/is_same.h>
+#include <__type_traits/remove_const.h>
+#include <__type_traits/remove_const_ref.h>
+#include <__utility/declval.h>
+#include <__utility/forward.h>
+#include <__utility/piecewise_construct.h>
+#include <__utility/priority_tag.h>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#  pragma GCC system_header
+#endif
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+template <class _KeyT, class _Ret, class _WithKey, class _WithoutKey, class... _Args>
+_LIBCPP_HIDE_FROM_ABI _Ret
+__try_key_extraction_impl(__priority_tag<0>, _WithKey, _WithoutKey __without_key, _Args&&... __args) {
+  return __without_key(std::forward<_Args>(__args)...);
+}
+
+template <class _KeyT,
+          class _Ret,
+          class _WithKey,
+          class _WithoutKey,
+          class _Arg,
+          __enable_if_t<is_same<_KeyT, __remove_const_ref_t<_Arg> >::value, int> = 0>
+_LIBCPP_HIDE_FROM_ABI _Ret
+__try_key_extraction_impl(__priority_tag<1>, _WithKey __with_key, _WithoutKey, _Arg&& __arg) {
+  return __with_key(__arg, std::forward<_Arg>(__arg));
+}
+
+template <class _KeyT,
+          class _Ret,
+          class _WithKey,
+          class _WithoutKey,
+          class _Arg,
+          __enable_if_t<__is_pair_v<__remove_const_ref_t<_Arg> > &&
+                            is_same<__remove_const_t<typename __remove_const_ref_t<_Arg>::first_type>, _KeyT>::value,
+                        int> = 0>
+_LIBCPP_HIDE_FROM_ABI _Ret
+__try_key_extraction_impl(__priority_tag<1>, _WithKey __with_key, _WithoutKey, _Arg&& __arg) {
+  return __with_key(__arg.first, std::forward<_Arg>(__arg));
+}
+
+template <class _KeyT,
+          class _Ret,
+          class _WithKey,
+          class _WithoutKey,
+          class _Arg1,
+          class _Arg2,
+          __enable_if_t<is_same<_KeyT, __remove_const_ref_t<_Arg1> >::value, int> = 0>
+_LIBCPP_HIDE_FROM_ABI _Ret
+__try_key_extraction_impl(__priority_tag<1>, _WithKey __with_key, _WithoutKey, _Arg1&& __arg1, _Arg2&& __arg2) {
+  return __with_key(__arg1, std::forward<_Arg1>(__arg1), std::forward<_Arg2>(__arg2));
+}
+
+#ifndef _LIBCPP_CXX03_LANG
+template <class _KeyT,
+          class _Ret,
+          class _WithKey,
+          class _WithoutKey,
+          class _PiecewiseConstruct,
+          class _Tuple1,
+          class _Tuple2,
+          __enable_if_t<is_same<__remove_const_ref_t<_PiecewiseConstruct>, piecewise_construct_t>::value &&
+                            __is_tuple_v<_Tuple1> && tuple_size<_Tuple1>::value == 1 &&
+                            is_same<__remove_const_ref_t<typename tuple_element<0, _Tuple1>::type>, _KeyT>::value,
+                        int> = 0>
+_LIBCPP_HIDE_FROM_ABI _Ret __try_key_extraction_impl(
+    __priority_tag<1>,
+    _WithKey __with_key,
+    _WithoutKey,
+    _PiecewiseConstruct&& __pc,
+    _Tuple1&& __tuple1,
+    _Tuple2&& __tuple2) {
+  return __with_key(
+      std::get<0>(__tuple1),
+      std::forward<_PiecewiseConstruct>(__pc),
+      std::forward<_Tuple1>(__tuple1),
+      std::forward<_Tuple2>(__tuple2));
+}
+#endif // _LIBCPP_CXX03_LANG
+
+template <class _KeyT, class _WithKey, class _WithoutKey, class... _Args>
----------------
ldionne wrote:

Please add a comment explaining what this function does. Something like

```
// This function tries extracting the given _KeyT from _Args...
// If it succeeds to extract the key, it calls the `__with_key` function with the extracted key and all of the arguments.
// If it fails to extract the key, it calls the `__without_key` function with all of the arguments.
//
// Both `__with_key` and `__without_key` must take their arguments by reference.
```

https://github.com/llvm/llvm-project/pull/154512