static_cast<目标类型>(表达式)

int num = 2;
double result =static_cast<double>(num);

class Person
{
public:
    void  print()
    {
        cout << "Person " << endl;
    }
};

class Son : public Person
{
public:
    void print()
    {
        cout << "Son " << endl;
    }
};

void print1(Person *p)
{
    p->print();
}

int main()
{
    Son s;
    print1(static_cast<Person *>(&s));
    return 0;
}

class Student
{
private:
int roll;
public:
Student(int r) :roll(r) {}
void fun() const
{
(const_cast<Student*> (this))->roll = 5;
}
int getRoll() { return roll; }
};

int main() {
    Student student(3);
    std::cout << "Old roll number: " << student.getRoll() << std::endl;
    student.fun();
    std::cout << "New roll number: " << student.getRoll() << std::endl;

    // const_cast只能调节类型限定符，不能更改基础类型
    int a1 = 40;
    //const int* b1 = &a1;
    //char* c1 = const_cast <char*> (b1); // 编译程序时出错

    const volatile int* d1 = &a1;
    std::cout << "typeid of d1 " << typeid(d1).name() << '\n'; // int const volatile *
    int* e1 = const_cast <int*> (d1);
    std::cout << "typeid of e1 " << typeid(e1).name() << '\n'; // int *
    
    return 0;
}

int a=20;
const int *p=&a;
char *c1=const_cast<char*> (p);//编译时程序出错

data_type *var_name = reinterpret_cast <data_type *>(pointer_variable);

class A {
public:
    int a;
    A(int i) :a(i) {}
    void fun_a()
    {
        std::cout << "In class A\n";
    }
};

class B {
public:
    int b;
    B(int i) :b(i) {}
    void fun_b()
    {
        std::cout << "In class B\n";
    }
};

void testReinterpretCast() {
    B *x = new B(5);
    A* y = reinterpret_cast<A*>(x);
    y->fun_a();                      // In class A
    std::cout << y->a << std::endl;  // 5
}

dynamic_cast <type-id> (expression)

class AA {
public:
    virtual void print() {
        cout << "in class AA" << endl;
    };
};

class BB :public AA {
public:
    void print() {
        cout << "in class BB" << endl;
    };
};

void testDynamicCast() {
    AA* a1 = new BB; // a1是A类型的指针指向一个B类型的对象
    AA* a2 = new AA; // a2是A类型的指针指向一个A类型的对象

    BB* b1, * b2, * b3, * b4;

    b1 = dynamic_cast<BB*>(a1);// not null，向下转换成功，a1 之前指向的就是 B 类型的对象，所以可以转换成 B 类型的指针。
    if (b1 == nullptr) 
        cout << "b1 is null" << endl;
    else               
        cout << "b1 is not null" << endl;

    b2 = dynamic_cast<BB*>(a2);// null，向下转换失败
    if (b2 == nullptr) 
        cout << "b2 is null" << endl;
    else               
        cout << "b2 is not null" << endl;

    // 用 static_cast，Resharper C++ 会提示修改为 dynamic_cast
    b3 = static_cast<BB*>(a1);// not null
    if (b3 == nullptr) 
        cout << "b3 is null" << endl;
    else               
        cout << "b3 is not null" << endl;

    b4 = static_cast<BB*>(a2);// not null
    if (b4 == nullptr) 
        cout << "b4 is null" << endl;
    else               
        cout << "b4 is not null" << endl;

    a1->print();// in class BB
    a2->print();// in class AA

    b1->print();// in class BB
    //b2->print();    // null 引发异常
    b3->print();// in class BB
    b4->print();// in class AA
}

// 完整语法
[capture list] (params list) mutable(optional) constexpr(optional)(c++17) exception attribute -> return type { function body };

// 可选的简化语法
[capture list] (params list) -> return type {function body};  //1
[capture list] (params list) {function body};//2
[capture list] {function body};//3

vector<int> vec{1,0,9,5,3,3,7,8,2};

sort(lbvec.begin(), lbvec.end(), [](int a, int b) -> bool { return a < b; });  

int a = 123;
auto f = [a] { cout << a << endl; }; 
f(); // 输出：123
a = 321;
f(); // 输出：123

auto add_x = [x](int a) mutable { x *= 2; return a + x; };  // 复制捕捉x
    
cout << add_x(10) << endl; // 输出：30

int a = 123;
auto f = [&a] { cout << a << endl; }; 
a = 321;
f(); // 输出：321

int a = 123, b=321;
auto df = [=] { cout << a << b << endl; };    // 值捕获
auto rf = [&] { cout << a << b << endl; };    // 引用捕获

std::function<int(int)> add_x(int x)
{
return [&](int a) {return x+a;};
}

std::function<int(int)> add_x(int x)
{
    return [=](int a) { return x + a; };
}

// 正确，单一return语句
transform(vi.begin(),vi.end(),vi.begin(), 
    [] (int i) { 
    return i<0? -i; i;
    }
);
// 错误。不能推断返回类型
transform(vi.begin(),vi.end(),vi.begin(), 
    [] (int i) { 
        if (I<0) return -i;
        else return i;
    }
);
// 正确，尾置返回类型
transform(vi.begin(),vi.end(),vi.begin(), 
    [] (int i) ->int{ 
        if (I<0) 
            return -i;
        else 
            return i;
    }
);

int x =8,y=9;
auto add = [](int a,int b){return a+b;};
std::function<int(int,int)> Add = [=] (int a,int b){return a+b};

auto a = [] { cout << "A" << endl; };
auto b = [] { cout << "B" << endl; };

a = b;   // 非法，lambda无法赋值
auto c = a;   // 合法，生成一个副本

ClosureType& operator=(const ClosureType&) = delete;

// 利用表达式捕获，可以更灵活地处理作用域内的变量
int x = 4;
auto y = [&r = x, x = x + 1] { r += 2; return x * x; }();
// 此时 x 更新为6，y 为25

// 直接用字面值初始化变量
auto z = [str = "string"]{ return str; }();
// 此时z是const char* 类型，存储字符串 string

auto myPi = std::make_unique<double>(3.1415);

auto circle_area = [pi = std::move(myPi)](double r) { return *pi * r * r; };
cout << circle_area(1.0) << endl; // 3.1415

template <typename T>
T sum(T arg){
    return arg;
}

template<typename T,typename...Args>
T sum(T first ,Args...res){
    return first+sum(rest...);
}

auto add=[](int x,int y)->int {
    return x+y;
}
auto add=[](int x,int y)->auto
{
    return x+y;
}
//函数模板
template<typename T, typename U>
auto multiply(T x, U y) -> decltype(x * y) {
    return x * y;
}

cppCopy codetemplate< class InputIt, class T >
T accumulate( InputIt first, InputIt last, T init );

decltype(expression)

#include <iostream>
#include <type_traits>

int add(int a, int b) {
    return a + b;
}

int main() {
    // 获取 add 函数的返回类型
    typedef std::result_of<decltype(add)&(int, int)>::type result_type;

    // 输出返回类型
    std::cout << "Result type: " << typeid(result_type).name() << std::endl;

    return 0;
}

#include <iostream>
#include <thread>

using namespace std;

int main() {
    auto func = []() {
        for (int i = 0; i < 10; ++i) {
            cout << i << " ";
        }
        cout << endl;
    };
    std::thread t(func);
    if (t.joinable()) {
        t.detach();
    }
    auto func1 = [](int k) {
        for (int i = 0; i < k; ++i) {
            cout << i << " ";
        }
        cout << endl;
    };
    std::thread tt(func1, 20);
    if (tt.joinable()) { // 检查线程可否被join
        tt.join();
    }
    return 0;
}

class ThreadGuard
{
public:
    enum class DeAction{join,detach};
    ThreadGuard(std::thread&& t,DeAction action): m_t(move(t)) , m_action(action){};
    ~ThreadGuard()
    {
        if(m_t.joinable())
        {
            if(m_action==DeAction::join)
            {
                m_t.join();
            }
            else
            {
                m_t.detach();
            }
        }
    }
    ThreadGuard(ThreadGuard&&)=default;
    ThreadGuard& operator=(ThreadGuard&&)=default;
private:
    std::thread m_t;
    DeAction m_action;
};

std::thread t(func);
cout << "当前线程ID " << t.get_id() << endl;
cout << "当前cpu个数 " << std::thread::hardware_concurrency() << endl;
auto handle = t.native_handle();// handle可用于pthread相关操作
std::this_thread::sleep_for(std::chrono::seconds(1));

#include <iostream>
#include <mutex>
#include <thread>

using namespace std;
std::mutex mutex_;

int main() {
    auto func1 = [](int k) {
        mutex_.lock();
        for (int i = 0; i < k; ++i) {
            cout << i << " ";
        }
        cout << endl;
        mutex_.unlock();
    };
    std::thread threads[5];
    for (int i = 0; i < 5; ++i) {
        threads[i] = std::thread(func1, 200);
    }
    for (auto& th : threads) {
        th.join();
    }
    return 0;
}

#include <iostream>
#include <mutex>
#include <thread>
#include <chrono>

using namespace std;
std::timed_mutex timed_mutex_;

int main() {
    auto func1 = [](int k) {
        timed_mutex_.try_lock_for(std::chrono::milliseconds(200));
        for (int i = 0; i < k; ++i) {
            cout << i << " ";
        }
        cout << endl;
        timed_mutex_.unlock();
    };
    std::thread threads[5];
    for (int i = 0; i < 5; ++i) {
        threads[i] = std::thread(func1, 200);
    }
    for (auto& th : threads) {
        th.join();
    }
    return 0;
}

#include <mutex>

template <class Mutex>
class lock_guard;

#include <mutex>

template <class Mutex>
class unique_lock;

struct OriginCounter { // 普通的计数器
    int count;
    std::mutex mutex_;
    void add() {
        std::lock_guard<std::mutex> lock(mutex_);
        ++count;
    }

    void sub() {
        std::lock_guard<std::mutex> lock(mutex_);
        --count;
    }

    int get() {
        std::lock_guard<std::mutex> lock(mutex_);
        return count;
    }
};

struct NewCounter { // 使用原子变量的计数器
    std::atomic<int> count;
    void add() {
        ++count;
        // count.store(++count);这种方式也可以
    }

    void sub() {
        --count;
        // count.store(--count);
    }

    int get() {
        return count.load();
    }
};

std::once_flag onceflag;

void CallOnce() {
    std::call_once(onceflag, []() {
        cout << "call once" << endl;
    });
}

int main() {
    std::thread threads[5];
    for (int i = 0; i < 5; ++i) {
        threads[i] = std::thread(CallOnce);
    }
    for (auto& th : threads) {
        th.join();
    }
    return 0;
}

int *p = xxx;
int a = *p;
int b = *p;

class CountDownLatch {
   public:
    explicit CountDownLatch(uint32_t count) : count_(count);

    void CountDown() {
        std::unique_lock<std::mutex> lock(mutex_);
        --count_;
        if (count_ == 0) {
            cv_.notify_all();
        }
    }

    void Await(uint32_t time_ms = 0) {
        std::unique_lock<std::mutex> lock(mutex_);
        while (count_ > 0) {
            if (time_ms > 0) {
                cv_.wait_for(lock, std::chrono::milliseconds(time_ms));
            } else {
                cv_.wait(lock);
            }
        }
    }

    uint32_t GetCount() const {
        std::unique_lock<std::mutex> lock(mutex_);
          return count_; 
    }

   private:
    std::condition_variable cv_;
    mutable std::mutex mutex_;
    uint32_t count_ = 0;
};

#include <functional>
#include <future>
#include <iostream>
#include <thread>

using namespace std;

void func(std::future<int>& fut) {
    int x = fut.get();
    cout << "value: " << x << endl;
}

int main() {
    std::promise<int> prom;
    std::future<int> fut = prom.get_future();
    std::thread t(func, std::ref(fut));
    prom.set_value(144);
    t.join();
    return 0;
}

#include <functional>
#include <future>
#include <iostream>
#include <thread>

using namespace std;

int func(int in) {
    return in + 1;
}

int main() {
    std::packaged_task<int(int)> task(func);
    std::future<int> fut = task.get_future();
    std::thread(std::move(task), 5).detach();
    cout << "result " << fut.get() << endl;
    return 0;
}

#include <functional>
#include <future>
#include <iostream>
#include <thread>

using namespace std;

int func(int in) { return in + 1; }

int main() {
    auto res = std::async(func, 5);
    // res.wait();
    cout << res.get() << endl; // 阻塞直到函数返回
    return 0;
}

std::async(std::launch::async, []{ f(); }); // 临时量的析构函数等待 f()
std::async(std::launch::async, []{ g(); }); // f() 完成前不开始

template <typename F, typename... Args>
inline auto ReallyAsync(F&& f, Args&&... params) {
    return std::async(std::launch::async, std::forward<F>(f), std::forward<Args>(params)...);
}

int tcgetattr(int fd, struct termios *termios_p);

int tcsetattr(int fd, int optional_actions, const struct termios *termios_p);

char* getlogin(void)
//返回一个指向当前登陆用户的用户名的字符串指针

size_t strscpn(const char *str1,const char *str2);
//str1：要搜索的字符串
//str2：要查找的字符集合
//计算字符串中连续不包含指定字符集合的最大前缀长度。

size_t strftime(char *str, size_t maxsize, const char *format, const struct tm *timeptr);

chrono::system_clock::now();
//是c++<chrono>头文件中的一个函数调用，用于获取当前的系统时间点
chrono::system_clock::to_time_t
//是一个函数模板，将time_point对象转换为time_t的类型

char* getenv(const char* name);

const char* path = s[1].c_str();
//将string对象转换为c风格的字符串.

int setenv(const char* name, const char* value, int overwrite);

std::istream& getline(std::istream& is, std::string& str, char delim);

int pipe(int pipefd[2]);

#include <unistd.h>

int dup2(int oldfd, int newfd);

#include "shell.hpp"

void Shell::print()
{   cout << "----------------------------------------------------------------------------" << endl;
    cout << "           ___                                                     ___" << endl;
    cout << "  ______  /  /___       ______  _____  ___   ___       ____  _____/  /___" << endl;
    cout << " /  __  \\/  __   \\     /  ___ - ___  \\/  /  /  /     /_   / /  __/  __   \\" << endl;
    cout << "/  /_/  /  /  /  /    /  /  /  /  /  /  /__/  /       /  /_(__  )  /  /  /" << endl;
    cout << "\\______/__/  /__/    /__/  /__/  /__/\\____,  /       /____/____/__/  /__/" << endl;
    cout << "                                    /_______/" << endl;
    cout << "----------------------------------------------------------------------------" << endl;
    char cwd [1024];
    auto now = chrono::system_clock::now();//返回当前的时间
    time_t time = chrono::system_clock::to_time_t(now);//将time_t point转换为time_t类型
    tm* timeinfo = std::localtime(&time);//将time_t转换为本地时间年月日,并存储在timeinfo中
    char timeStr[9];
    strftime(timeStr, sizeof(timeStr), "%H:%M:%S", timeinfo);
    //std::strftime()将std::tm结构的时间信息格式化为字符串，并将结果存储在指定的字符数组中。这里使用"%H:%M:%S"作为格式字符串
    //，它表示小时、分钟和秒的格式。timeStr是目标字符数组，sizeof(timeStr)用于指定数组的大小，
    //timeinfo是包含时间信息的std::tm结构的指针。
    cout << "# future @ future-arch in " << getcwd(cwd,1024) << " [" << timeStr << "]" << endl;
    cout << "$ " ;
    fflush(stdout);
}

void Shell ::setlinebuf()
{
    getline(cin,this->linebuf);
}

string Shell::retlinebuf ()
{
    return this->linebuf;
}

void Shell:: setsignal()
{
    signal(SIGINT, SIG_IGN);//忽略ctrl+c的操作
}

bool Shell ::isempty() 
{
    if(this->linebuf.empty())
    {
        return true;
    }
    return false;
}

void Shell::addHistory(const string& line)
{
    for(vector<string>::iterator it = history.begin(); it != history.end();it++)
    {
        if((*it).compare(line) == 0)
        return ;
    }
    history.push_back(line);
}

void Shell ::setEOF()
{
    struct termios term;//存储终端的相关信息
    tcgetattr(STDIN_FILENO, &term);//获取相关信息，并存入结构体
    term.c_cc[VEOF] = _POSIX_VDISABLE;//禁止使用EOF
    tcsetattr(STDIN_FILENO, TCSANOW, &term);//将设置存入终端
}

void Shell::printHistory()
{
    for(vector<string>::iterator it = this->history.begin(); it != this->history.end();it++)
    {
        cout << *it << endl;
    }
    cout << "~~~~~~~~~~~~~~~~~~~~~~~~~~~"<< endl;
}

void Shell::searchAndReplay(const string& keyword)
{
    for (const string& line : history)
    {
        if (line.find(keyword) != string::npos)
        {
            cout << "Replaying: " << line << endl;
            linebuf = line;
            break;
        }
    }
}

void Shell::parse(string& s)
{
    vector<vector<string>> tokens;
    istringstream iss(s);//可方便的从字符串中读取数据
    string root;
    while (getline(iss, root, '|'))//将s以|分割开
    {
        istringstream root_iss(root);
        vector<string> root_tokens;
        string token;
        while (root_iss >> token)//将单个命令以空格分隔开
        {
            root_tokens.push_back(token);
        }

        tokens.push_back(root_tokens);
    }
    if (tokens.size() == 1)
    {
        vector<string>& command = tokens[0];
        if (command[0] == "cd")
        {
            mycd(command);
        }
        else
        {
            executeCommand(command);
        }
    }
    else if (tokens.size() > 1)
    {
        int numPipes = tokens.size() - 1;
        vector<int> pipefds(2 * numPipes, 0); // 用于存储管道的文件描述符
        for (int i = 0; i < numPipes; ++i)
        {
            if (pipe(pipefds.data() + i * 2) < 0)
            {
                perror("pipe()");
                exit(1);
            }
        }
        int commandIndex = 0;
        for (auto& command : tokens)
        {
            pid_t pid = fork();
            if (pid < 0)
            {
                perror("fork()");
                exit(1);
            }
            if (pid == 0)
            {
                // 子进程
                if (commandIndex != 0) // 不是第一个命令，读取前一个管道的输出
                {
                    if (dup2(pipefds[(commandIndex - 1) * 2], STDIN_FILENO) < 0)
                    {
                        perror("dup2()");
                        exit(1);
                    }
                }
                if (commandIndex != numPipes) // 不是最后一个命令，将输出写入下一个管道
                {
                    if (dup2(pipefds[commandIndex * 2 + 1], STDOUT_FILENO) < 0)
                    {
                        perror("dup2()");
                        exit(1);
                    }
                }
                // 关闭所有管道的文件描述符
                for (int i = 0; i < 2 * numPipes; ++i)
                {
                    close(pipefds[i]);
                }
                if (command[0] == "cd")
                {
                    mycd(command);
                }
                else
                {
                    executeCommand(command);
                }

                exit(0);
            }
            else
            {
                // 父进程
                ++commandIndex;
            }
        }
        // 关闭所有管道的文件描述符
        for (int i = 0; i < 2 * numPipes; ++i)
        {
            close(pipefds[i]);
        }

        // 等待所有子进程结束
        for (size_t i = 0; i < tokens.size(); ++i)
        {
            wait(NULL);
        }
    }
}

void Shell::mycd(vector<string>& s)
{
    if (s.size() > 2)
    {
        cout << "cd: 函数调用参数过多" << endl;
        return;
    }
    else if (s.size() == 1 || s[1].compare("~") == 0)
    {
        const char* homeDir = getenv("HOME");
        if (homeDir)
        {
            if (chdir(homeDir) == -1)
            {
                perror("chdir()");
            }
        }
        else
        {
            cout << "cd: 无法获取家目录路径" << endl;
            return;
        }
    }
    else if (s[1].compare("-") == 0)
    {
        const char* lastDir = getenv("OLDPWD");
        if (lastDir)
        {
            if (chdir(lastDir) == -1)
            {
                perror("chdir()");
            }
        }
        else
        {
            cout << "cd: 无法找到上一个路径" << endl;
        }
    }
    else
    {
        const char* path = s[1].c_str();
        if (chdir(path) == -1)
        {
            perror("chdir()");
        }
    }
    // 更新环境变量 OLDPWD
    char* currentDir = get_current_dir_name();//获取当前工作目录的路径
    if (currentDir)
    {
        if (setenv("OLDPWD", currentDir, 1) == -1)//设置环境变量
        {
            perror("setenv()");
        }
        free(currentDir);
    }
    else
    {
        perror("get_current_dir_name()");
    }
}

void Shell::executeCommand(const vector<string>& command)
{
    pid_t pid;
    fflush(NULL);
    pid = fork();
    if (pid < 0)
    {
        perror("fork()");
    }
    if (pid == 0)
    {
        vector<char*> args;
        vector<int> outputFds; // 存储输出文件描述符的容器

        for (size_t i = 0; i < command.size(); ++i)
        {
            if (command[i] == ">") // 输出重定向：覆盖写入文件
            {
                int fd = open(command[i + 1].c_str(), O_WRONLY | O_CREAT | O_TRUNC, 0644);
                if (fd == -1)
                {
                    perror("open()");
                    exit(1);
                }
                outputFds.push_back(fd); // 将新的文件描述符添加到容器中
                dup2(fd, STDOUT_FILENO);
                close(fd);
                i++;
            }
            else if (command[i] == ">>") // 输出重定向：追加写入文件
            {
                int fd = open(command[i + 1].c_str(), O_WRONLY | O_CREAT | O_APPEND, 0644);
                if (fd == -1)
                {
                    perror("open()");
                    exit(1);
                }
                outputFds.push_back(fd); // 将新的文件描述符添加到容器中
                dup2(fd, STDOUT_FILENO);
                close(fd);
                i++;
            }
            else if (command[i] == "<") // 输入重定向
            {
                int fd = open(command[i + 1].c_str(), O_RDONLY);
                if (fd == -1)
                {
                    perror("open()");
                    exit(1);
                }
                dup2(fd, STDIN_FILENO);
                close(fd);
                i++;
            }
            else if (command[i] == "<<") // Here文档
            {
                string delimiter = command[i + 1];
                string inputText;

                while (getline(cin, inputText))
                {
                    if (inputText == delimiter)
                        break;
                    inputText += "\n";
                    write(STDIN_FILENO, inputText.c_str(), inputText.size());
                }

                i++;
            }
            else
            {
                args.push_back(const_cast<char*>(command[i].c_str()));
            }
        }

        args.push_back(nullptr); // execvp 需要以 nullptr 结尾的参数数组
        execvp(args[0], args.data());

        // execvp 执行失败时才会执行到这里
        perror("execvp()");
        exit(1);
    }
    else
    {
        wait(NULL);
    }
}

#include "func.cpp"

int main()
{
    int num = 1;
    Shell mysh;
    mysh.setsignal();   
    mysh.setEOF();
    while(num)
    {
        mysh.print();
        mysh.setlinebuf();
        string t = mysh.retlinebuf();
        if(t.empty())
        {
            cout << "No input" << endl;
        }
        else if(t.rfind("exit")!=string::npos)
        {
            break;
        }
        else if(t.find("!")==0)
        {
            string keyword = t.substr(1); //获取去除感叹号的关键字
            mysh.searchAndReplay(keyword);
        }
        else if(t.find("clear")==0)
        {
            mysh.addHistory(t);
            // cout << "\033[2J\033[1;1H";//通过转义序列清除终端，但只能清楚当次的命令
            system("clear");
            num++;
        }
        else
        {
            mysh.addHistory(t);
            num++;
        }
        mysh.parse(t);
        if(num%3==0)
        {
            cout << "~~~~~~~~~~~~~~~~~~~~~~~~~~~"<< endl;
            cout << " History Order :" << endl;
            mysh.printHistory();
        }
    }
    return 0;
}

//公共权限  public
//保护权限  protected
//私有权限  private
//私有权限与保护权限在类内可以访问，在类外不可以访问
class Num
{
public:
    int m_A;
protected:
    int m_B;
private:
    int m_C;
public:
    void func()
    {
m_A=10;
        m_B=20;
        m_C=30;
    }
}

int main()
{
Num m;
    m.m_A=10;
    //m.m_B=20;//类外不可以访问
    //m.m_C=30;
}

class Num
{
public:
    Num(int a): m_A(a)
    {
cout << "gz" << endl;
    }
    int m_A;
};

class P
{
public:
    P()
    {

    }
    P(int a)
    {
        m_A = a;
    }
    explicit P(P& a)
    {
        m_A = a.m_A;
    }
    int m_A;
};

int main()
{
    P p;
    p.m_A = 1;
    //P p1 = p;//会报错，因为声明不可以隐式类型转换
    P p1=P(1);
    cout << p1.m_A << endl;
    return 0;
}

class Num
{
public:
    Num()
    {
        cout << "gz" << endl;
    }
    Num(const Num &other)
    {
        m_A=other.m_A;
        m_B=new int (*other.m_B);
        cout << "kb" << endl;
    }
    ~Num()
    {
        if(this->m_B)
        {
            delete this->m_B;
            this->m_B=nullptr;
        }
    }
    int m_A;
    int *m_B;
};

int main()
{
    Num a;
    a.m_A=1;
    a.m_B=new int(20);
    Num b(a);
    cout << b.m_A << endl;
    cout << *b.m_B << endl;
    cout << a.m_B << endl;
    return 0;
}

int sum1 = 0;
int& sum(int &a,int &b) 
{
    sum1 = a + b;
    return sum1;
}

int main()
{
    int a=10;
    int b=10;
    sum(a,b)=30;
    cout << sum1 << endl;    
    return 0;
}

void func(int &ref)
{
    ref=100;
}

int main()
{
int a=10;
    //自动转换为int *const ref = &a;指针常量是指针指向不可改，也说明为什么引用不可以改
    int &ref=a;
    ref=20;
    
    func(a);
    return 0;
}