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_posts/2024-08-13-cpp-perf-repos.md

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+---
+title: C++ 性能优化代码 repo 收集
+date: 2024-08-13 +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++, github]      # TAG names should always be lowercase
+
+# 以下默认false
+math: true
+mermaid: true
+# pin: true
+---
+
+聚类算法资料收集
+
+- [github -- Allocator](https://github.com/moya-lang/Allocator)
+- [github -- poolSTL](https://github.com/alugowski/poolSTL)

_posts/2024-08-13-std-function.md

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+---
+title: 复习：std::function 用法笔记
+date: 2024-08-13 +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++]      # TAG names should always be lowercase
+
+# 以下默认false
+math: true
+mermaid: true
+# pin: true
+---
+
+`std::function` 可以将`函数`,`函数对象（仿函数）`,`lambda表达式`包装成一个`对象`。`std::function``对象`本身可以作为函数参数，并且是可复制的（复制构造、赋值）。
+
+## 1. 封装函数指针
+
+```cpp
+int add(int a, int b) { return a + b; }
+
+int main() {
+  std::function<int(int, int)> f = add;
+  int result = f(1, 2);
+  std::cout << result << std::endl;
+  return 0;
+}
+```
+
+## 2. 封装函数对象（仿函数）
+
+```cpp
+struct Adder {
+  int operator()(int a, int b) { return a + b; }
+};
+
+int main() {
+  Adder adder;
+  std::function<int(int, int)> f = adder;
+  int result = f(1, 2);
+  std::cout << result << std::endl;
+  return 0;
+}
+```
+
+## 3. 封装 lambda 表达式
+
+```cpp
+int main() {
+  // using func_t = int(int, int);
+  std::function<int(int, int)> f = [](int a, int b) { return a + b; };
+  int result = f(1, 2);
+  std::cout << result << std::endl;
+  return 0;
+}
+```
+
+## 4. 与 std::bind 结合使用
+
+```cpp
+int add(int a, int b) { return a + b; }
+
+int main() {
+  std::function<int(int)> f = std::bind(add, 1, std::placeholders::_1);
+  int result = f(2);
+  std::cout << result << std::endl;
+  return 0;
+}
+```
+
+## 5. lambda 表达式
+
+`lambda`表达式是一个匿名`函数对象`，即编译器会创建一个匿名对象，并将外部捕获的变量，添加到该匿名对象中。这些是`lambda`的额外开销。
+
+```cpp
+int main()
+{
+  // Lambda & auto
+  int member=10;
+  auto endGame = [=](int a, int b){ return a+b+member;};
+
+  endGame(4,5);
+
+  return 0;
+
+}
+```
+
+展开成：
+
+```cpp
+int main()
+{
+  int member = 10;
+
+  class __lambda_6_18
+  {
+    int member;
+    public: 
+    inline /*constexpr */ int operator()(int a, int b) const
+    {
+      return a + b + member;
+    }
+
+    public: __lambda_6_18(int _member)
+    : member{_member}
+    {}
+
+  };
+
+  __lambda_6_18 endGame = __lambda_6_18{member};
+  endGame.operator()(4, 5);
+
+  return 0;
+}
+```
+
+## 参考
+
+- [What is a lambda expression, and when should I use one?](https://stackoverflow.com/questions/7627098/what-is-a-lambda-expression-and-when-should-i-use-one)
+