diff --git a/README.md b/README.md
index 12b8932..b3ff383 100644
--- a/README.md
+++ b/README.md
@@ -7,3 +7,5 @@
 #### day2: const 和#define 的区别, 引用(reference) 【重要】, 内联函数, 函数的默认值参数, 函数的占位参数, 函数重载和 extern "c", 类与对象的概念, 面向对象程序设计案例
 
 #### day3: 对象的构造与析构, 构造函数的分类与调用, 拷贝构造函数的调用时机, 构造函数的调用规则, 深拷贝和浅拷贝, 多个对象的构造和析构(初始化列表, 类对象作为成员), explicit 关键字(禁止隐式转换), 动态创建对象(malloc/realloc/calloc 在堆中创建空间, new 关键字), 扩展 new 和 delete,
+
+#### day4: static 静态成员, 类的单例设计模式, 对象的存储（this 指针与链式编程, const 修饰成员函数与类对象）, 友元, 运算符重载
diff --git a/day4/d1.cpp b/day4/d1.cpp
new file mode 100644
index 0000000..ae4af56
--- /dev/null
+++ b/day4/d1.cpp
@@ -0,0 +1,26 @@
+// 类的静态成员变量的声明、初始化与访问
+#include <iostream>
+#include <cstring>
+#include <cstdlib>
+
+using namespace std;
+
+class A
+{
+public:
+    static int x;
+};
+
+int A::x = 20; // 静态变量初始化，静态成员变量初始化时不需要 static
+
+int main()
+{
+    A a1, a2;
+    // 访问类的静态成员: 1) A::x, 2) 对象名.x
+    cout << "A::x = " << A::x << endl; // 20
+    A::x = 30;
+    cout << "a1.x = " << a1.x << endl; // 30
+    cout << "a2.x = " << a2.x << endl; // 30
+
+    return 0;
+}
\ No newline at end of file
diff --git a/day4/d10.cpp b/day4/d10.cpp
new file mode 100644
index 0000000..2bf769e
--- /dev/null
+++ b/day4/d10.cpp
@@ -0,0 +1,32 @@
+// 重载 C++ 的 << 输出流运算符
+#include <iostream>
+#include <cstring>
+#include <cstdlib>
+
+using namespace std;
+
+class A
+{
+    friend ostream &operator<<(ostream &cout, A &a);
+
+private:
+    int x, y;
+
+public:
+    A(int x, int y) : x(x), y(y) {}
+};
+
+// << 输出运算符重载
+ostream &operator<<(ostream &cout, A &a)
+{
+    // x, y 是 A 类的私有成员
+    cout << "a.x : " << a.x << ", a.y : " << a.y << endl;
+    return cout;
+}
+
+int main()
+{
+    A a1(1, 2);
+    cout << a1;
+    return 0;
+}
diff --git a/day4/d11.cpp b/day4/d11.cpp
new file mode 100644
index 0000000..5c902b0
--- /dev/null
+++ b/day4/d11.cpp
@@ -0,0 +1,39 @@
+// 重载 C++ 的 >> 输入流运算符
+#include <iostream>
+#include <cstring>
+#include <cstdlib>
+
+using namespace std;
+
+class Worker
+{
+    friend istream &operator>>(istream &cin, Worker &obj);
+    friend ostream &operator<<(ostream &cout, Worker &obj);
+
+private:
+    string name;
+    int salary;
+};
+
+istream &operator>>(istream &cin, Worker &obj)
+{
+    cout << "Name: ";
+    cin >> obj.name;
+    cout << "salary: ";
+    cin >> obj.salary;
+    return cin;
+}
+
+ostream &operator<<(ostream &cout, Worker &obj)
+{
+    cout << "Worker name is " << obj.name << ", salary is " << obj.salary << endl;
+    return cout;
+}
+
+int main()
+{
+    Worker w1;
+    cin >> w1;
+    cout << w1;
+    return 0;
+}
\ No newline at end of file
diff --git a/day4/d12.cpp b/day4/d12.cpp
new file mode 100644
index 0000000..6829cc8
--- /dev/null
+++ b/day4/d12.cpp
@@ -0,0 +1,52 @@
+// 重载运算符
+#include <iostream>
+#include <cstring>
+#include <cstdlib>
+
+using namespace std;
+
+class Num
+{
+private:
+    int n;
+
+public:
+    explicit Num(int n) : n(n)
+    {
+    }
+    bool operator>(int other)
+    {
+        return this->n > other;
+    }
+    bool operator<(int other)
+    {
+        return this->n < other;
+    }
+    bool operator>(Num &other)
+    {
+        return this->n > other.n;
+    }
+    bool operator<(Num &other)
+    {
+        return this->n < other.n;
+    }
+};
+
+int main(int argc, char const *argv[])
+{
+    Num n1(atoi(argv[1])), n2(atoi(argv[2]));
+    if (n1 > 20)
+    {
+        cout << "n1 大于 20" << endl;
+
+        if (n1 < n2)
+        {
+            cout << "n1 小于 n2" << endl;
+        }
+    }
+    if (n1 > n2)
+    {
+        cout << "n1 大于 n2" << endl;
+    }
+    return 0;
+}
diff --git a/day4/d13.cpp b/day4/d13.cpp
new file mode 100644
index 0000000..27ddc2c
--- /dev/null
+++ b/day4/d13.cpp
@@ -0,0 +1,52 @@
+// 重载 * 和 ->
+// 注意: * 和 -> 的重载不能有参数
+#include <iostream>
+#include <cstring>
+#include <cstdlib>
+
+using namespace std;
+
+class A
+{
+private:
+    int x;
+
+public:
+    explicit A(int x) : x(x) {}
+    void printA()
+    {
+        cout << "x = " << x << endl;
+    }
+};
+
+class B
+{
+private:
+    A *mA;
+
+public:
+    B(int x) : mA(new A(x))
+    {
+    }
+
+    A *operator->()
+    {
+        return mA;
+    }
+    A &operator*()
+    {
+        return *mA;
+    }
+};
+
+int main()
+{
+    B b(20);
+    // b -> 执行 B 类的 -> 重载，返回 A 类的对象的指针
+    b->printA();
+
+    B b2(30);
+    // *b 执行 B 类的 * 重载函数，返回 A 类的对象
+    (*b2).printA();
+    return 0;
+}
diff --git a/day4/d1_2.cpp b/day4/d1_2.cpp
new file mode 100644
index 0000000..ca75fc8
--- /dev/null
+++ b/day4/d1_2.cpp
@@ -0,0 +1,30 @@
+// 静态成员函数， 内部只能访问静态成员变量
+#include <iostream>
+#include <cstring>
+#include <cstdlib>
+
+using namespace std;
+
+class A
+{
+public:
+    int m;
+    static int x;
+    static void add(int n)
+    {
+        // m = 10; // error, 静态成员函数不能访问非静态成员
+        x += n;
+        cout << "static x = " << x << endl;
+    }
+};
+
+int A::x = 20; // 静态变量的初始化
+
+int main()
+{
+    A a1;
+    A::add(10);                        // static x = 30
+    cout << "a1.x = " << a1.x << endl; // a1.x = 30
+
+    return 0;
+}
diff --git a/day4/d1_3.cpp b/day4/d1_3.cpp
new file mode 100644
index 0000000..a5234a5
--- /dev/null
+++ b/day4/d1_3.cpp
@@ -0,0 +1,27 @@
+// 静态成员变量不占类对象的空间
+#include <iostream>
+#include <cstring>
+#include <cstdlib>
+
+using namespace std;
+
+class A
+{
+public:
+    int m;
+    static int x;
+    A(int m)
+    {
+        this->m = m;
+    }
+};
+
+// int A::x = 20; // 静态变量初始化
+
+int main()
+{
+    A a1(3);
+    cout << "A size " << sizeof(A) << endl;   // 同下
+    cout << "a1 size " << sizeof(a1) << endl; // 4 Byte , 为 int m 的大小
+    return 0;
+}
diff --git a/day4/d2.cpp b/day4/d2.cpp
new file mode 100644
index 0000000..1f3dfca
--- /dev/null
+++ b/day4/d2.cpp
@@ -0,0 +1,53 @@
+// 类的单例设计模式
+// 私有化构造函数和拷贝函数， 提供一个public的静态函数返回类的指针对象， 声明一个静态的类
+// 的指针对象。
+#include <iostream>
+#include <cstring>
+#include <cstdlib>
+
+using namespace std;
+
+class A // 单例的类: 类的对象运行期间有且只有一个对象
+{
+private:
+    int x;
+    A(int x) { this->x = x; }
+    A(const A &other)
+    {
+        x = other.x;
+    }
+
+public:
+    static A *getInstance(int x = 0)
+    {
+        if (instance == NULL)
+        {
+            instance = new A(x);
+        }
+        return instance;
+    }
+    void show()
+    {
+        cout << "x = " << x << endl;
+    }
+
+public:
+    static A *instance;
+};
+
+A *A::instance = NULL; // 定义初始化，在静态全局区
+
+int main()
+{
+    A *a1 = A::getInstance(20);
+    a1->show();
+
+    A *a2 = A::getInstance();
+    a2->show();
+    cout << "a1 addr = " << a1 << endl;
+    cout << "a2 addr = " << a2 << endl;
+
+    delete A::instance; // 释放单例对象的空间
+
+    return 0;
+}
diff --git a/day4/d3.cpp b/day4/d3.cpp
new file mode 100644
index 0000000..f523416
--- /dev/null
+++ b/day4/d3.cpp
@@ -0,0 +1,81 @@
+// 对象的存储
+// this 指针与链式编程(构造器模式)
+#include <iostream>
+#include <cstring>
+#include <cstdlib>
+
+using namespace std;
+
+class A
+{
+private:
+    int a, b, c;
+
+public:
+    void setA(int a)
+    {
+        this->a = a;
+    }
+    void setB(int b)
+    {
+        this->b = b;
+    }
+    void setC(int c)
+    {
+        this->c = c;
+    }
+    void show()
+    {
+        cout << "a = " << a << ", b = " << b << ", c = " << c << endl;
+    }
+};
+
+class ABuilder
+{
+private:
+    A *instance;
+
+public:
+    ABuilder()
+    {
+        instance = new A();
+    }
+    ~ABuilder()
+    {
+        if (instance != NULL)
+        {
+            cout << "~ABuilder()" << endl;
+            delete instance;
+        }
+    }
+
+public:
+    ABuilder &a(int a)
+    {
+        instance->setA(a);
+        return *this;
+    }
+    ABuilder &b(int b)
+    {
+        instance->setB(b);
+        return *this;
+    }
+    ABuilder &c(int c)
+    {
+        instance->setC(c);
+        return *this;
+    }
+    A *build()
+    {
+        return instance;
+    }
+};
+
+int main()
+{
+    // 构建器设计模式: 构造产品，需要构造器一步一步的完成构造，最后 build() 出产品
+    ABuilder builder;
+    A *a1 = builder.a(1).b(3).c(8).build();
+    a1->show();
+    return 0;
+}
diff --git a/day4/d4.cpp b/day4/d4.cpp
new file mode 100644
index 0000000..4ddc297
--- /dev/null
+++ b/day4/d4.cpp
@@ -0,0 +1,38 @@
+// 当const修饰成员函数时，函数内不能修改普通成员变量，但可以修改mutable修饰的成员变量。 当const修饰类对象时，只能读成员变量，不能修改成员变量的值。可以调用const修饰的成员函数，不
+// 能调用普通成员函数
+#include <iostream>
+#include <cstring>
+#include <cstdlib>
+
+using namespace std;
+
+class A
+{
+public:
+    int x;
+    mutable int y;
+
+public:
+    void setXY(int x, int y) const
+    {
+        // this->x = x; // error
+        this->y = y;
+    }
+    void show()
+    {
+        cout << x << ", " << y << endl;
+    }
+};
+
+int main()
+{
+    A a1;
+    a1.setXY(1, 2); // const 函数，内部不修改非 mutable 的成员变量
+    a1.show();
+
+    // const A a2; // 报错，const 变量必须存在初始值（右值）
+    const A a2 = A();
+    a2.setXY(0, 30); // const 对象可以访问 const 的成员函数
+    // a2.show();       // const 对象不能访问非 const 成员函数
+    return 0;
+}
diff --git a/day4/d5.cpp b/day4/d5.cpp
new file mode 100644
index 0000000..47414cd
--- /dev/null
+++ b/day4/d5.cpp
@@ -0,0 +1,38 @@
+// 友元
+// 全局友元函数
+#include <iostream>
+#include <cstring>
+#include <cstdlib>
+
+using namespace std;
+
+class B
+{
+    // 声明全局函数的友元
+    friend void show(B &b); // 将全局的 show() 函数设置为当前类的友元函数
+
+private:
+    int x;
+
+public:
+    B(int x) : x(x) {}
+    void show()
+    {
+        cout << "成员函数 x = " << x << endl;
+    }
+};
+
+// 全局的友元函数
+void show(B &b)
+{
+    // b 的私有成员
+    cout << "全局友元函数 b.x = " << b.x << endl;
+}
+
+int main()
+{
+    B b(100);
+    b.show(); // 成员函数 x = 100
+    show(b);  // 全局友元函数 b.x = 100
+    return 0;
+}
diff --git a/day4/d6.cpp b/day4/d6.cpp
new file mode 100644
index 0000000..85765b9
--- /dev/null
+++ b/day4/d6.cpp
@@ -0,0 +1,48 @@
+// 友元
+// 友元的类成员函数
+#include <iostream>
+#include <cstring>
+#include <cstdlib>
+
+using namespace std;
+
+class B;
+
+class C
+{
+public:
+    // 在此函数内，要访问 B 类中的所有成员，将此函数在 B 类中声明友元
+    // 先声明，不能实现
+    void showB(B &b);
+};
+
+class B
+{
+    // 声明友元的成员函数
+    friend void C::showB(B &b);
+
+private:
+    int x;
+
+public:
+    B(int x) : x(x) {}
+    void show()
+    {
+        cout << "成员函数 x = " << x << endl;
+    }
+};
+
+// 定义或实现友元的成员函数
+void C::showB(B &b)
+{
+    cout << "友元的成员函数 b.x = " << b.x << endl;
+}
+
+int main()
+{
+    B b(100);
+    b.show();
+    C c;
+    c.showB(b);
+    return 0;
+}
diff --git a/day4/d6_2.cpp b/day4/d6_2.cpp
new file mode 100644
index 0000000..49f8cdf
--- /dev/null
+++ b/day4/d6_2.cpp
@@ -0,0 +1,43 @@
+// 友元类
+#include <iostream>
+#include <cstring>
+#include <cstdlib>
+
+using namespace std;
+
+class B;
+
+class C
+{
+public:
+    void showB(B &b);
+};
+
+class B
+{
+    friend class C;
+
+private:
+    int x;
+
+public:
+    B(int x) : x(x) {}
+    void show()
+    {
+        cout << "成员函数 x = " << x << endl;
+    }
+};
+
+void C::showB(B &b)
+{
+    cout << "友元类 b.x = " << b.x << endl;
+}
+
+int main()
+{
+    B b(100);
+    b.show();
+    C c;
+    c.showB(b);
+    return 0;
+}
diff --git a/day4/d8.cpp b/day4/d8.cpp
new file mode 100644
index 0000000..0aa8e67
--- /dev/null
+++ b/day4/d8.cpp
@@ -0,0 +1,41 @@
+// 运算符重载
+// + 运算符
+#include <iostream>
+#include <cstring>
+#include <cstdlib>
+
+using namespace std;
+
+class A
+{
+private:
+    int x;
+
+public:
+    explicit A(int x)
+    {
+        this->x = x;
+    }
+    void show()
+    {
+        cout << x << endl;
+    }
+    // + 运算符重载，类成员函数，双目运算符，只需要一个参数
+    A *operator+(A &other);
+};
+
+A *A::operator+(A &other)
+{
+    A *tmp = new A(this->x + other.x);
+    return tmp;
+}
+
+int main()
+{
+    A a1(5), a2(6);
+    a1.show();
+    A *a3 = a1 + a2;
+    a3->show();
+    delete a3;
+    return 0;
+}
diff --git a/day4/d9.cpp b/day4/d9.cpp
new file mode 100644
index 0000000..70bed10
--- /dev/null
+++ b/day4/d9.cpp
@@ -0,0 +1,75 @@
+// 运算符重载
+// ++、-- 运算符
+#include <iostream>
+#include <cstring>
+#include <cstdlib>
+
+using namespace std;
+
+class A
+{
+private:
+    int x;
+
+public:
+    explicit A(int x)
+    {
+        this->x = x;
+    }
+    void show()
+    {
+        cout << x << endl;
+    }
+    // ++ 运算符重载，类成员函数，单目运算符，++ 在前时不要参数，++ 在后需要一个占位参数
+    A &operator++(int);
+    A &operator++();
+    A *operator--(int);
+    A *operator--();
+};
+
+A &A::operator++(int) // ++ 在后需要有占位参数
+{
+    this->x++;
+    return *this;
+}
+
+A &A::operator++()
+{
+    ++this->x;
+    return *this;
+}
+
+A *A::operator--(int)
+{
+    A *tmp = new A(this->x--);
+    return tmp;
+}
+
+A *A::operator--()
+{
+    A *tmp = new A(--this->x);
+    return tmp;
+}
+
+int main()
+{
+    A a1(5);
+    a1.show();
+    a1++;
+    a1.show();
+    ++a1;
+    a1--;
+    a1.show();
+    --a1;
+    a1.show();
+
+    A a2(8);
+    a2--;
+    a2--;
+    a2++;
+    ++a2;
+    --a2;
+    a2.show();
+
+    return 0;
+}
diff --git a/day4/homework/h6_2.cpp b/day4/homework/h6_2.cpp
index e1008ab..9324bed 100644
--- a/day4/homework/h6_2.cpp
+++ b/day4/homework/h6_2.cpp
@@ -1,136 +1,9 @@
 #include <iostream>
 
+using namespace std;
+
 class ArrayList
 {
 private:
-    int *elements; // 存储元素的数组
-    int capacity;  // 数组的容量
-    int count;     // 当前元素的数量
-
-public:
-    ArrayList()
-    {
-        capacity = 10; // 初始容量为10
-        elements = new int[capacity];
-        count = 0;
-    }
-
-    ~ArrayList()
-    {
-        delete[] elements;
-    }
-
-    void add(int value)
-    {
-        if (count == capacity)
-        {
-            // 如果数组已满，扩展容量为原来的两倍
-            int newCapacity = capacity * 2;
-            int *newElements = new int[newCapacity];
-            for (int i = 0; i < count; i++)
-            {
-                newElements[i] = elements[i];
-            }
-            delete[] elements;
-            elements = newElements;
-            capacity = newCapacity;
-        }
-        elements[count] = value;
-        count++;
-    }
-
-    void remove(int index)
-    {
-        if (index >= 0 && index < count)
-        {
-            for (int i = index; i < count - 1; i++)
-            {
-                elements[i] = elements[i + 1];
-            }
-            count--;
-        }
-    }
-
-    int get(int index)
-    {
-        if (index >= 0 && index < count)
-        {
-            return elements[index];
-        }
-        return -1; // 返回一个特殊值表示索引无效
-    }
-
-    void print()
-    {
-        for (int i = 0; i < count; i++)
-        {
-            std::cout << elements[i] << " ";
-        }
-        std::cout << std::endl;
-    }
-
-    int size()
-    {
-        return count;
-    }
-
-    void sort(bool reversed = false)
-    {
-        for (int i = 0; i < count - 1; i++)
-        {
-            for (int j = 0; j < count - i - 1; j++)
-            {
-                if (reversed)
-                {
-                    if (elements[j] < elements[j + 1])
-                    {
-                        int temp = elements[j];
-                        elements[j] = elements[j + 1];
-                        elements[j + 1] = temp;
-                    }
-                }
-                else
-                {
-                    if (elements[j] > elements[j + 1])
-                    {
-                        int temp = elements[j];
-                        elements[j] = elements[j + 1];
-                        elements[j + 1] = temp;
-                    }
-                }
-            }
-        }
-    }
-};
-
-int main()
-{
-    ArrayList list;
-
-    list.add(5);
-    list.add(2);
-    list.add(8);
-    list.add(1);
-
-    std::cout << "Original list: ";
-    list.print();
-
-    std::cout << "Size: " << list.size() << std::endl;
-
-    std::cout << "Element at index 2: " << list.get(2) << std::endl;
-
-    list.remove(1);
-
-    std::cout << "After removing element at index 1: ";
-    list.print();
-
-    std::cout << "Sorted list in ascending order: ";
-    list.sort();
-    list.print();
-
-    std::cout << "Sorted list in descending order: ";
-    list.sort(true);
-    list.print();
-
-    return 0;
-}
+    int
+};
\ No newline at end of file