From 5415e29dfd912b953b8249e2d1b51dd18f57c4b5 Mon Sep 17 00:00:00 2001
From: yoh <kawasaki.liamg@gmail.com>
Date: Fri, 17 Feb 2023 14:13:27 +0900
Subject: [PATCH] =?UTF-8?q?functional/hash:=20=E9=80=8F=E9=81=8E=E7=9A=84?=
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C++20標準の`std::equal_to<std::string>`や`std::hash<std::string>`は、
透過的な計算動作を示すメンバ型`is_transparent`を持つことはない。
独自の文字列ハッシュ計算クラスと`std::equal_to<>`に置き換える。
---
 reference/functional/hash.md | 13 +++++++++++--
 1 file changed, 11 insertions(+), 2 deletions(-)

diff --git a/reference/functional/hash.md b/reference/functional/hash.md
index 09717118b1..a2df8a8dda 100644
--- a/reference/functional/hash.md
+++ b/reference/functional/hash.md
@@ -123,16 +123,25 @@ int main()
 #include <iostream>
 #include <unordered_map>
 #include <string>
+#include <string_view>
+
+struct string_hash {
+  using is_transparent = void;
+  // string/string_view/const char*共用ハッシュ計算
+  size_t operator()(std::string_view sv) const {
+    return std::hash<std::string_view>{}(sv);
+  }
+};
 
 int main()
 {
-  std::unordered_map<std::string, int> um = {
+  std::unordered_map<std::string, int, string_hash, std::equal_to<>> um = {
     {"Alice", 3},
     {"Bob", 1},
     {"Carol", 4}
   };
 
-  // std::equal_to<std::string>とstd::hash<std::string>がis_transparent型を持つ場合、
+  // string_hashおよびstd::equal_to<>はいずれもメンバ型にis_transparentを持つため、
   // find()などの検索関数に引数を渡す場合に、std::string一時オブジェクトが作られない
   auto it = um.find("Alice");
   if (it != um.end()) {