void Test ()
{
  const int a = 2;
  int* p = const_cast< int*>(&a );
  *p = 3;
  cout<<a <<endl;
}

3.4 dynamic_cast

class A
{
public :
virtual void f(){}
};
class B : public A
{};
void fun (A* pa)
{
// dynamic_cast会先检查是否能转换成功，能成功则转换，不能则返回
B* pb1 = static_cast<B*>(pa);
B* pb2 = dynamic_cast<B*>(pa);
cout<<"pb1:" <<pb1<< endl;
cout<<"pb2:" <<pb2<< endl;
}
int main ()
{
  A a;
  B b;
  fun(&a);
  fun(&b);
  return 0;
}

4.C++中的IO流

4.1C语言的输入和输出

4.2 流是什么

istream& operator>> (int& val);
explicit operator bool() const;

class Data
{

	friend istream& operator >> (istream& in,  Data& d);
	friend ostream& operator << (ostream& out, const Data& d);
public:
	Data(int year = 0,int month = 0,int day = 0)
		:_year(year)
		,_months(month)
		,_day(day)
	{ }

	string TO_string()
	{
		string ret;
		ret += to_string(_year);
		ret += ' ';
		ret += to_string(_months);
		ret += ' ';
		ret += to_string(_day);
		return ret;
	}
	
private:
	int _year;
	int _months;
	int _day;
};

ostream& operator << (ostream& out, const Data& d)
{
	out << d._year << " " << d._months << " " << d._day;

	return out;
}

istream& operator >> (istream& in, Data& d)
{
	in >> d._year >> d._months >> d._day;

	return in;
}


struct ServerInfo//定义一个结构体
{
 char _address[32];
 int _port;
 Date _date;
};

struct ConfigManager
{
public:
 ConfigManager(const char* filename)//初始化传入文件名称
 :_filename(filename)
 {}

 void WriteBin(const ServerInfo& info)//二进制写入
 {
 ofstream ofs(_filename, ios_base::out | ios_base::binary);
 ofs.write((const char*)&info, sizeof(info));
 }

 void ReadBin(ServerInfo& info)//二进制读取
 {
 ifstream ifs(_filename, ios_base::in | ios_base::binary);
 ifs.read((char*)&info, sizeof(info));
 }

    // C++文件流的优势就是可以对内置类型和自定义类型，都使用
    // 一样的方式，去流插入和流提取数据
    // 当然这里自定义类型Date需要重载>> 和 <<
    // istream& operator >> (istream& in, Date& d)
// ostream& operator << (ostream& out, const Date& d)

//文本的写入
 void WriteText(const ServerInfo& info)
 {
 ofstream ofs(_filename);
 ofs << info._address << " " << info._port<< " "<<info._date;
 }

//文本的读取
 void ReadText(ServerInfo& info)
 {
 ifstream ifs(_filename);
 ifs >> info._address >> info._port>>info._date;
 }

private:
 string _filename; // 配置文件
};

int main()
{
 ServerInfo winfo = { "192.0.0.1", 80, { 2022, 4, 10 } };
 
    // 二进制读写
 ConfigManager cf_bin("test.bin");

 cf_bin.WriteBin(winfo);

 ServerInfo rbinfo;

 cf_bin.ReadBin(rbinfo);

 cout << rbinfo._address << " " << rbinfo._port <<" ";

<<rbinfo._date << endl;

    // 文本读写
 ConfigManager cf_text("test.text");

 cf_text.WriteText(winfo);

 ServerInfo rtinfo;

 cf_text.ReadText(rtinfo);

 cout << rtinfo._address << " " << rtinfo._port << " " <<

rtinfo._date << endl;

 return 0;
}

4.4stringstream的简单介绍

#include<sstream>
int main()
{
 int a = 12345678;
 string sa;
 // 将一个整形变量转化为字符串，存储到string类对象中
 stringstream s;
 s << a;
 s >> sa;
    // clear()
    // 注意多次转换时，必须使用clear将上次转换状态清空掉
    // stringstreams在转换结尾时(即最后一个转换后),会将其内部状态设置为badbit
    // 因此下一次转换是必须调用clear()将状态重置为goodbit才可以转换
    // 但是clear()不会将stringstreams底层字符串清空掉
    
    // s.str("");
 // 将stringstream底层管理string对象设置成"", 
 // 否则多次转换时，会将结果全部累积在底层string对象中
    
 s.str("");
 s.clear();   // 清空s, 不清空会转化失败
 double d = 12.34;
 s << d;
 s >> sa;
 string sValue;
 sValue = s.str();   // str()方法：返回stringsteam中管理的string类型
 cout << sValue << endl; 
 return 0;
}