【C++】右值引用和移动语义 | 新的类功能 | 可变参数模板（下）

class Person
{
public:
  Person(const char* name = "", int age = 0)
    :_name(name)
    , _age(age)
  {}
  // 会默认生成拷贝构造和移动构造
  /*Person(const Person& p)
  :_name(p._name)
  ,_age(p._age)
  {}*/
  /*Person& operator=(const Person& p)
  {
  if(this != &p)
  {
  _name = p._name;
  _age = p._age;
  }
  return *this;
  }*/
  /*~Person()
  {}*/
private:
  Joy::string _name;
  int _age;
};
int main()
{
  Person s1("张三", 18);  
  Person s2 = s1; // 拷贝构造
  Person s3 = std::move(s1);  // 移动构造
  Person s4;
  s4 = std::move(s2); // 移动赋值
  return 0;
}

// 只能在堆上创建对象
class HeapOnly
{
public:
  void Destroy()
  {
    delete[] _str;
    // delete this 也要调用析构函数
    operator delete(this);
  }
  HeapOnly()
  {
    _str = new char[10];
  }
  ~HeapOnly() = delete;
private:
  char* _str;
};
int main()
{
  //HeapOnly hp1;
  //static HeapOnly hp2;
  HeapOnly* ptr = new HeapOnly;
  ptr->Destroy();
  //delete ptr; // 编译报错
  return 0;
}

// Args是一个模板参数包，args是一个函数形参参数包
// 声明一个参数包Args...args，这个参数包中可以包含0到任意个模板参数。
template <class ...Args>
void ShowList(Args... args)
{}

// 可变参数的模板
template <class ...Args>
void ShowList(Args... args)
{
  cout << sizeof...(args) << endl;
}
int main()
{
  string str("hello");
  ShowList();
  ShowList(1);
  ShowList(1, 'A');
  ShowList(1, 'A', str);
  return 0;
}

// 模板参数包不支持以下玩法
for (size_t i = 0; i < sizeof...(args); ++i)
{
  cout << args[i] << " ";
}
cout << endl;

// 递归终止函数
void ShowList()
{
  cout << "ShowList()" << endl;
}
// 展开函数,参数包args包含N个参数(N>=0)
template <class T, class ...Args>
void ShowList(const T& val, Args... args)
{
  cout << "ShowList(" << val << ", 参数包args有 " << sizeof...(args) << " 个参数)" << endl;
  ShowList(args...);  // 递归调用
}
int main()
{
  string str("hello");
  ShowList(1, 'A', str);
  return 0;
}

template <class T>
void PrintArg(const T& val)
{
  cout << val << " ";
}
//展开函数
template <class ...Args>
void ShowList(Args... args)
{
  int arr[] = { (PrintArg(args), 0)... };
  cout << endl;
  for (size_t i = 0; i < sizeof...(args); ++i)
  {
    cout << arr[i] << ' ';
  }
  cout << endl;
}
int main()
{
  string str("hello");
  ShowList(1, 'A', str);
  return 0;
}

template <class T>
int PrintArg(const T& val)
{
  cout << val << " ";
  return 0;
}
//展开函数
template <class ...Args>
void ShowList(Args... args)
{
  int arr[] = { PrintArg(args)... };
  cout << endl;
  for (size_t i = 0; i < sizeof...(args); ++i)
  {
    cout << arr[i] << ' ';
  }
  cout << endl;
}
int main()
{
  string str("hello");
  ShowList(1, 'A', str);
  return 0;
}

class Date
{
public:
  Date(int year = 1, int month = 1, int day = 1)
    :_year(year)
    , _month(month)
    , _day(day)
  {
    cout << "Date(int year = 1, int month = 1, int day = 1)" << endl;
  }
  Date(const Date& d)
    :_year(d._year)
    , _month(d._month)
    , _day(d._day)
  {
    cout << "Date(const Date& d)" << endl;
  }
  Date& operator=(const Date& d)
  {
    cout << "Date& operator=(const Date& d))" << endl;
    return *this;
  }
private:
  int _year;
  int _month;
  int _day;
};
int main()
{
  // 没有区别
  vector<int> v1;
  v1.push_back(1);
  v1.emplace_back(2);
  // 有区别
  list<Date> lt1;
  lt1.push_back(Date(2022, 11, 16));
  cout << "---------------------------------" << endl;
  lt1.emplace_back(2022, 11, 16);
  return 0;
}