support bazel complie this project and format code.

practical_exercises/10_day_practice/day4/friend/BUILD

@@ -0,0 +1,13 @@
+# please run `bazel run //practical_exercises/10_day_practice/day4/friend:class`
+# please run `bazel run //practical_exercises/10_day_practice/day4/friend:func`
+
+load("@rules_cc//cc:defs.bzl", "cc_binary")
+
+cc_binary(
+    name = "class",
+    srcs = ["class.cpp"],
+)
+cc_binary(
+    name = "func",
+    srcs = ["func.cpp"],
+)

practical_exercises/10_day_practice/day4/friend/class.cpp

@@ -0,0 +1,43 @@
+#include <iostream>
+using namespace std;
+/*
+若一个类为另一个类的友元，则此类的所有成员都能访问对方类的私有成员。
+声明语法：将友元类名在另一个类中使用friend修饰说明。
+*/
+
+/*
+如果声明B类是A类的友元，B类的成员函数就可以访问A类的私有和保护数据，
+但A类的成员函数却不能访问B类的私有、保护数据。
+*/
+class A {
+  friend class B;
+
+public:
+  void Display() { cout << x << endl; }
+
+private:
+  int x;
+};
+class B {
+public:
+  void Set(int i);
+  void Display();
+
+private:
+  A a;
+};
+void B::Set(int i) { a.x = i; }
+void B::Display() { a.Display(); }
+
+int main(int argc, char const *argv[]) {
+  B b;
+  b.Set(10);
+  b.Display();
+
+  
+  return 0;
+}
+
+/*
+如果声明B类是A类的友元，B类的成员函数就可以访问A类的私有和保护数据，但A类的成员函数却不能访问B类的私有、保护数据
+*/

practical_exercises/10_day_practice/day4/friend/func.cpp

@@ -0,0 +1,27 @@
+//使用友元函数计算两点间距离
+#include <cmath>
+#include <iostream>
+using namespace std;
+class Point {
+public:
+  Point(int x = 0, int y = 0) : X(x), Y(y) {}
+  int GetX() { return X; }
+  int GetY() { return Y; }
+  friend float Distance(Point &a, Point &b);
+
+private:
+  int X, Y; //私有数据成员
+};
+//通过将一个模块声明为另一个模块的友元，一个模块能够引用到另一个模块中本是被隐藏的信息。
+float Distance(Point &a, Point &b) {
+  double dx = a.X - b.X;
+  double dy = a.Y - b.Y;
+  return sqrt(dx * dx + dy * dy);
+}
+
+int main() {
+  Point p1(3.0, 5.0), p2(4.0, 6.0);
+  cout << "两点距离为：" << Distance(p1, p2) << endl;
+  
+  return 0;
+}

practical_exercises/10_day_practice/day4/friend/readme.md

@@ -0,0 +1,9 @@
+# <20><>Ԫ<EFBFBD><D4AA><EFBFBD>
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+# <20><><EFBFBD><EFBFBD>ʹ<EFBFBD>ã<EFBFBD>
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