vc++ ios::in ios::out 含义

fstream fs(“fname”, ios_base::in);// attempt open for readif (!fs){  // file doesn't exist; don't create a new one}else //ok, file exists. close and reopen in write mode{  fs.close();  fs.open(“fname”, ios_base::out); // reopen for write}You can just do the opposite for ios::noreplace:fstream fs(“fname”, ios_base::in);// attempt open for readif (!fs){ // file doesn't exist; create a new one fs.open(“fname”, ios_base::out); }else //ok, file exists; close and reopen in write mode{ fs.close() fs.open(“fname”, ios_base::out); // reopen for write}可以用这种方法判断文件是否存在