fstream讀寫建立可能不存在的檔案
fstream file("b.txt", fstream::in|fstream::out|fstream::app);
ate 並不會導致create屬性，
app可造成out屬性
in 表示只讀屬性
out表可寫屬性+create shuxing
in+out 表只讀+可寫屬性（沒有建立屬性）
dd.out.txt:
s
{
cout<<"---------------------------------------\n";
fstream file("dd.out.txt", fstream::out); // make dd.out.txt empty
//fstream file("dd.out.txt", fstream::out|fstream::in);
string s;
file>>s;
cout<<s;
file.close();
}
You're specifying std::fstream::in in
your call to fstream::open().
This is known to
force it to require an existing file.
If you look at e.g. this reference, you will see:
app seek to the end of stream before each write and
ate seek to the end of stream immediately after open
This means that ios::app
ios::ate reads
and writes at the end by default. However, with ios::ate you
can seek freely in the file, but with ios::app you
will alwayswrite at the end, no matter what position you set for the writing pointer.
where filename is a string of characters representing the name of the file to be opened and mode is a combination of the following flags:void open (const char * filename, openmode mode);
These flags can be combined using bitwise operator OR: |. For example, if we want to open the file "example.bin" in binary mode to add data we could do it by the following call to function-member open:
ios::in Open file for reading ios::out Open file for writing ios::ate Initial position: end of file ios::app Every output is appended at the end of file ios::trunc If the file already existed it is erased ios::binary Binary mode
ofstream file;All of the member functions open of classes ofstream, ifstream and fstream include a default mode when opening files that varies from one to the other:
file.open ("example.bin", ios::out | ios::app | ios::binary);
class default mode to parameter ofstream ios::out | ios::trunc ifstream ios::in fstream ios::in | ios::out
//fstream file("b1.txt", fstream::in|fstream::out); do NOT create if no exist
fstream file("b1.txt", fstream::out); // create
file<<"s";
cout<<"s";
file.close();
}
{
fstream file("b.txt", fstream::in|fstream::out|fstream::app); // create
file<<"s";
cout<<"s";
file.close();
}