From dea65112cb473e4885db0c4859ff91e2fd45f4db Mon Sep 17 00:00:00 2001
From: "A. Jiang" <de34@live.cn>
Date: Wed, 24 Jul 2024 09:25:10 +0800
Subject: [PATCH] Synchronize `identity` with C++20

Making it not a template.
---
 MyTinySTL/functional.h | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/MyTinySTL/functional.h b/MyTinySTL/functional.h
index b445017..165ac1a 100644
--- a/MyTinySTL/functional.h
+++ b/MyTinySTL/functional.h
@@ -139,10 +139,10 @@ struct logical_not :public unarg_function<T, bool>
 };
 
 // 证同函数：不会改变元素，返回本身
-template <class T>
-struct identity :public unarg_function<T, bool>
+struct identity
 {
-  const T& operator()(const T& x) const { return x; }
+  template <class T>
+  constexpr T&& operator()(T&& x) const noexcept { return static_cast<T&&>(x); }
 };
 
 // 选择函数：接受一个 pair，返回第一个元素