c++ fstream用法(2)

一>

 #include "stdafx.h"
 #include<iostream>
 #include<string>
 #include<fstream>
 using namespace std;

 class Rectangle
 {
 private:
     double length, width;
 public:
     Rectangle(double l, double w);
 };

 Rectangle::Rectangle(double l, double w) :length(l), width(w)
 {}

 int main()
 {
     Rectangle rect(20.0, 10.3);
     ofstream out("a.dat", ios_base::binary);     //文件的输出
     if (out.is_open())                           //如果打开成功
     {
         out.write((char*)&rect, sizeof(Rectangle));//把Rectangle类的rect成员写入
         out.close();                             //注意:此步一定不能省
     }
     char* buf = new char[sizeof(Rectangle)];     //缓冲区
     ifstream in("a.dat", ios_base::binary);      //文件的写入
     if (in.is_open())                            //如果文件打开成功
     {
         in.read(buf, sizeof(Rectangle));         //从in指定的已打开文件中读取Rectangle字节到buf中
         in.close();                              //注意:此步一定不能省
     }
     return ;
 }

结果(注意:.dat文件要用WinHex打开，网上有下载):

二>

 #include "stdafx.h"
 #include<iostream>
 #include<string>
 #include<fstream>
 using namespace std;

 class Rectangle
 {
 private:
     double length, width;
 public:
     Rectangle(double l, double w);
     void displayArea() { cout << length * width << endl; }
 };

 Rectangle::Rectangle(double l, double w) :length(l), width(w)
 {}

 int main()
 {
     int a[] = { ,,, };
     Rectangle rect(20.0, 10.3);
     ofstream out("a.dat", ios_base::binary);
     if (out.is_open())
     {
         out.write((char*)&rect, sizeof(Rectangle));
         out.write((char*)a,  * sizeof(int));  //对数组来说，数组有n个元素那么后面就是n*sizeof(int)
         out.close();
     }
     char* buf = new char[sizeof(Rectangle)];
     char* buf2 = new char[ * sizeof(int)];
     ifstream in("a.dat", ios_base::binary);
     if (in.is_open())
     {
         in.read(buf, sizeof(Rectangle));
         in.read(buf2, sizeof(int) * );
         int *a = new int[];
         memcpy(a, buf2,  * sizeof(int));
         cout << a[] << endl;
         delete[] a;
         in.close();

         Rectangle* pr = (Rectangle*)buf;
         pr->displayArea();
     }
     delete[] buf;
     return ;
 }

结果:

三>

 #include "stdafx.h"
 #include<iostream>
 #include<string>
 #include<fstream>
 using namespace std;

 class Rectangle
 {
 private:
     double length, width;
 public:
     Rectangle(double l, double w);
     void displayArea() { cout << length * width << endl; }
 };

 Rectangle::Rectangle(double l, double w) :length(l), width(w)
 {}

 int main()
 {
     fstream file("a.txt", ios_base::out | ios_base::app);
     if (file.is_open())
     {
         file << "Hello world!";
         file.close();

     }
     file.open("a.txt", ios_base::in);
     if (file.is_open())
     {
         while (file.good())
         {
             string words;
             file >> words;
             cout << words << " ";
         }
         file.close();
         cout << endl;
     }
     return ;
 }

结果:(文件中与命令提示框输出得一样哦)