add note for CRTP

_posts/2024-09-02-CRTP-introduce.md

@@ -0,0 +1,59 @@
+---
+title: CRTP：使用笔记
+date: 2024-09-02 +0800 # 2022-01-01 13:14:15 +0800 只写日期也行；不写秒也行；这样也行 2022-03-09T00:55:42+08:00
+categories: [c++]
+tags: [c++, template]      # TAG names should always be lowercase
+
+# 以下默认false
+math: true
+mermaid: true
+# pin: true
+---
+
+## 1. 动态继承运行时时间损耗
+
+* 每个 virtual 方法，都需要通过指针查找到虚函数入口（间接寻址），且可能引起`I-Cache` `cache miss`；
+* virtual 方法，不能被优化为`inline`，针对一些短小的函数，时间损耗较大；
+
+## 2. CRTP 使用举例：子类也是 template 模板类
+
+```c++
+template <typename DerivedT>
+class HoleDetectorBase {
+ public:
+  using underlying_type = HoleDetectorBase<DerivedT>;
+  //...
+};
+
+template <typename condPairFuncT, typename condGHIPairFuncT>
+class HoleDetector2 : public HoleDetectorBase<HoleDetector2<condPairFuncT, condGHIPairFuncT>> {
+ public:
+  using base_t = HoleDetectorBase<HoleDetector2<condPairFuncT, condGHIPairFuncT>>;
+  using typename base_t::underlying_type;
+
+  //...
+};
+```
+
+* 继承写法为：`HoleDetector2<condPairFuncT, condGHIPairFuncT>`，即带上template参数。
+
+### 2.1 添加在基类中封装子类的函数
+
+```c++
+template <typename DerivedT>
+class HoleDetectorBase {
+//...
+
+DerivedT& underlying() { return static_cast<DerivedT&>(*this); }
+DerivedT const& underlying() const { return static_cast<DerivedT const&>(*this); }
+
+//...
+};
+```
+
+## 3. 参考资料
+
+* [unique_ptr 与抽象类的多态](https://hedzr.com/c++/algorithm/unique_ptr-and-abstract-class/)
+* [An Implementation Helper For The Curiously Recurring Template Pattern](https://www.fluentcpp.com/2017/05/19/crtp-helper/)
+* [The Curiously Recurring Template Pattern in C++](https://eli.thegreenplace.net/2011/05/17/the-curiously-recurring-template-pattern-in-c/#id3)
+* [The cost of dynamic (virtual calls) vs. static (CRTP) dispatch in C++](https://eli.thegreenplace.net/2013/12/05/the-cost-of-dynamic-virtual-calls-vs-static-crtp-dispatch-in-c)