2021-10-25 文件读写C++

C++提供文件读写类
fstream
#include

ofstream out("…", ios::out);
ifstream in("…", ios::in);
fstream foi("…", ios::in|ios::out);

#include
using namespace std;

int main ()
{
ifstream fr;
ofstream fw;
char word[200]， line[200];

fw.open("write.txt");
fr.open("read.txt");

fr  >> word;    //读取文件，一个单词
fr.getline (line, 100);        //读取一行内容

fw << "write file test" << endl;

fw.close();
fr.close();

return 0;

}

2. int v, w, weight;
   ifstream infile; //输入流
   ofstream outfile; //输出流
   infile.open(“G:C++ projectReaddata.txt”, ios::in);
   if(!infile.is_open ())
   cout << “Open file failure” << endl;
   while (!infile.eof()) // 若未到文件结束一直循环
   {
   infile >> v >> w >> weight;
   cost[v][w] = weight;
   cost[w][v] = weight;
   }
   infile.close(); //关闭文件

   outfile.open(“G:C++ projectReadresult.txt”, ios::app); //每次写都定位的文件结尾，不会丢失原来的内容，用out则会丢失原来的内容
   if(!outfile.is_open ())
   cout << “Open file failure” << endl;
   for (int i = 0; i != 10; ++i)
   {
   for (int j = 0; j != 10; ++j)
   {
   outfile << i << “t” << j << “t” << cost[i][j] << endl; //在result.txt中写入结果
   }

   }
   outfile.close();

3. 设置偏移量
   f.seekg（）是对输入文件定位，它有两个参数：第一个参数是偏移量，第二个参数是基地址。
   f.seekp（）是对输出文件定位，它有两个参数：第一个参数是偏移量，第二个参数是基地址
   seekg,seekp,到达指定偏移量
   tellg,tellp; 当前get/put流的偏移量
   file.seekg ( off_type offset, seekdir direction );
   file.seekp ( off_type offset, seekdir direction );
   // obtaining file size
   #include
   #include

   const char * filename = “example.txt”;

   int main () {
   long l,m;
   ifstream file (filename, ios::in|ios::binary);
   l = file.tellg();
   file.seekg (0, ios::end);
   m = file.tellg();
   file.close();
   cout << “size of " << filename;
   cout << " is " << (m-l) << " bytes.n”;
   return 0;
   }