support bazel complie this project and format code.

practical_exercises/10_day_practice/day5/virtual/BUILD

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+# please run `bazel run //practical_exercises/10_day_practice/day5/virtual:example2`
+# please run `bazel run //practical_exercises/10_day_practice/day5/virtual:init`
+# please run `bazel run //practical_exercises/10_day_practice/day5/virtual:seq`
+# please run `bazel run //practical_exercises/10_day_practice/day5/virtual:example1`
+
+load("@rules_cc//cc:defs.bzl", "cc_binary")
+
+cc_binary(
+    name = "example2",
+    srcs = ["example2.cpp"],
+)
+cc_binary(
+    name = "init",
+    srcs = ["init.cpp"],
+)
+cc_binary(
+    name = "seq",
+    srcs = ["seq.cpp"],
+)
+cc_binary(
+    name = "example1",
+    srcs = ["example1.cpp"],
+)

practical_exercises/10_day_practice/day5/virtual/example1.cpp

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+/* 例1.cpp */
+#include <iostream>
+using namespace std;
+class A {
+public:
+  void vf() { cout << "I come from class A" << endl; }
+};
+class B : public A {};
+class C : public A {};
+class D : public B, public C {};
+
+int main() {
+  D d;
+  d.vf(); // error
+  
+  return 0;
+}

practical_exercises/10_day_practice/day5/virtual/example2.cpp

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+/* 例2.cpp */
+#include <iostream>
+using namespace std;
+class A {
+public:
+  void vf() { cout << "I come from class A" << endl; }
+};
+class B : virtual public A {};
+class C : virtual public A {};
+class D : public B, public C {};
+
+int main() {
+  D d;
+  d.vf(); // okay
+  
+  return 0;
+}

practical_exercises/10_day_practice/day5/virtual/init.cpp

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+/* 派生类初始化.cpp */
+#include <iostream>
+using namespace std;
+class A {
+  int a;
+
+public:
+  A(int x) {
+    a = x;
+    cout << "Virtual Bass A..." << endl;
+  }
+};
+class B : virtual public A {
+public:
+  B(int i) : A(i) { cout << "Virtual Bass B..." << endl; }
+};
+class C : virtual public A {
+  int x;
+
+public:
+  C(int i) : A(i) {
+    cout << "Constructing C..." << endl;
+    x = i;
+  }
+};
+class ABC : public C, public B {
+public:
+  //虚基类由最终派生类初始化
+  ABC(int i, int j, int k)
+      : C(i), B(j), A(i) // L1，这里必须对A进行初始化
+  {
+    cout << "Constructing ABC..." << endl;
+  }
+};
+int main() {
+  ABC obj(1, 2, 3);
+  
+  return 0;
+}

practical_exercises/10_day_practice/day5/virtual/readme.md

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+多继承下的二义性：在多继承方式下，派生类继承了多个基类的成员，当两个不同基类拥有同名成员时，容易产生名字冲突问题。
+
+虚拟继承引入的原因：重复基类，派生类间接继承同一基类使得间接基类（Person）在派生类中有多份拷贝，引发二义性。
+
+1、虚拟继承virtual inheritance的定义
+语法
+class derived_class : virtual […] base_class
+虚基类virtual base class
+被虚拟继承的基类
+在其所有的派生类中，仅出现一次
+
+2、虚拟继承的构造次序
+  虚基类的初始化与一般的多重继承的初始化在语法上是一样的，但构造函数的调用顺序不同；
+  若基类由虚基类派生而来，则派生类必须提供对间接基类的构造（即在构造函数初始列表中构造虚基类，无论此虚基类是直接还是间接基类）
+  调用顺序的规定：
+先调用虚基类的构造函数，再调用非虚基类的构造函数
+若同一层次中包含多个虚基类,这些虚基类的构造函数按它们的说明的次序调用
+若虚基类由非基类派生而来,则仍然先调用基类构造函数,再调用派生类构造函数
+3、虚基类由最终派生类初始化 
+在没有虚拟继承的情况下，每个派生类的构造函数只负责其直接基类的初始化。但在虚拟继承方式下，虚基类则由最终派生类的构造函数负责初始化。
+在虚拟继承方式下，若最终派生类的构造函数没有明确调用虚基类的构造函数，编译器就会尝试调用虚基类不需要参数的构造函数（包括缺省、无参和缺省参数的构造函数），如果没找到就会产生编译错误。

practical_exercises/10_day_practice/day5/virtual/seq.cpp

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+/* 虚基类调用次序(重要).cpp */
+//重要!!!
+#include <iostream>
+using namespace std;
+class A {
+  int a;
+
+public:
+  A() { cout << "Constructing A" << endl; }
+};
+class B {
+public:
+  B() { cout << "Constructing B" << endl; }
+};
+class B1 : virtual public B, virtual public A {
+public:
+  B1(int i) { cout << "Constructing B1" << endl; }
+};
+class B2 : public A, virtual public B {
+public:
+  B2(int j) { cout << "Constructing B2" << endl; }
+};
+class D : public B1, public B2 {
+public:
+  D(int m, int n) : B1(m), B2(n) { cout << "Constructing D" << endl; }
+  A a;
+};
+
+int main() {
+  D d(1, 2);
+  
+  return 0;
+}