C++ 文件操作

　　用于文件操作主要有两个类ifstream,ofstream,fstream,例子如下

1. 读操作

#include <ifstream>
using name space std;

int main()
{
　　　　cCar  car;    // initialize a car object
       ifstream in;
       in.open("test.txt", ios::in); //open file
       if (!in)
       {
            cout << "open file error" << endl;
            return 0;
       }
       while (in.read((char*) &car, sizeof(car))){ // read sizeof(car) bytes from file and write into memory &car
            cout << car.color << endl;
　　　　}
       in.close();
}

2. 写操作

#include <ofstream>
using namespace std;

int main()
{
　　 cCar  car;    // initialize a car object
       ofstream out;
       out.open("test.txt", ios::out); //open file
       if (!out)
       {
            cout << "open file error" << endl;
            return 0;
       }
       out.write((char*) &car, sizeof(car)) // write sizeof(car) bytes from memory &car and write them into file
       out.close(); // opened file object must be closed at last.
}

3. 上面两个例子只是介绍了基本的读写操作，而且是逐字节顺序地操作文件。但是如果我想从文件中的中间一行读或写一个字节数字，可以做到不？

其实fstream, ofstream,ifstream里实现了很多方便实用的成员函数，完成刚才提到的问题只需要额外地操作一下读写指针就可轻松搞定。

何谓读写指针，就是指向当前字节流位置的指针，因此，只要我们控制好读写指针的位置，想在哪写入或读出都非常容易。

读写操作例子

#include <fstream>
using namespace std;

int main()
{
　　 cCar  car;    // initialize a car object
       fstream f;
       f.open("test.txt", ios::in|ios::out); //open file, it can be written and read at the same time.
       if (!f)
       {
            cout << "open file error" << endl;
            return 0;
       }
       f.seekp(3*sizeof(int), ios::beg) ; // let read/write point jump to the 4th int offset from beginning
       f.read((char*) &car, sizeof(car));
       f.seekp(10);  // location the 10th byte
       f.seekp(1, ios::cur); // jump to 1 bype from current point address
       f.seekp(1, ios::end); // jump to 1 byte from the end of file
       long loc = f.tellp(); // get the location which pointer point to
       f.write("hellp", 6);
       f.close();
}