fstream读写创建可能不存在的文件

fstream file("b.txt", fstream::in|fstream::out|fstream::app); 

ate 并不会导致create属性，

app可造成out属性

in 表示只读属性

out表可写属性+create shuxing

in+out 表只读+可写属性（没有创建属性）

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 only writes at the end, but that 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.

void open (const char * filename, openmode mode);

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:

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

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 :

ofstream file;
file.open ("example.bin", ios::out | ios::app | ios::binary);

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:

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();
}