Opening a file to read and write, create it if it doesn't exist

Question

I was trying to create a file in both read and write modes, but it doesn't create the file, what can be the problem?

This is code:

fstream file("NameFile.txt", ios::out| ios::in);

The program will start, but it will not create any files.

Does this answer your question? [std::fstream doesn't create file](https://stackoverflow.com/questions/8835888/stdfstream-doesnt-create-file) — ChrisMM, Apr 15 '21 at 08:48
That way it doesn't even create anything. `fstream file;` `file.open("test.txt",ios::out | ios::in)` — , Apr 15 '21 at 08:54

anastaciu · Accepted Answer · 2023-07-20T21:47:21.147

When you open the file using fstream:

To read, the file is required to exist.
To write you need to specify a write mode using the needed flags:

Replaces the contents of the file when you write (ofstream default mode):

  std::fstream file("NameFile.txt", std::ios::out | std::ios::in | std::ios::trunc);
                                                                   ^^^^^^^^^^^^^^^

Creates the file if it doesn't exist.

Appends to the existing data in the file when you write:

  std::fstream file("NameFile.txt", std::ios::out | std::ios::in | std::ios::app);
                                                                   ^^^^^^^^^^^^^

Also creates the file if it doesn't yet exist.

Having a file opened both to write and read needs some extra attention. For example, say you want to read what you previously wrote, you'll need to set the offset position in the file. Here's some code to illustrate this:

std::string s = "my string";
std::string in;

file << s; 
file >> in;

file >> in will not read anything, the position indicator is at the end of the file after file << s, you'll need to reset it if you want to read previously written data, for example using seekg:

file << s; 
file.seekg(0);
file >> in;

This resets the read position indicator to the beginning of the file, before the file is read from.

Basically you need take care of the indicator's position both to not read from and to not write in the wrong places.

References:

https://en.cppreference.com/w/cpp/io/basic_fstream

score 0 · Answer 2 · answered Apr 15 '21 at 08:45

0

well, you initialized an object, now to create a file use

file.open();

see fstream won't create a file

answered Apr 15 '21 at 08:45

helloworlder

1
2

2

The constructor should create the file. No need to `open` – bolov Apr 15 '21 at 08:49
That way it doesn't even create anything. `fstream file;` `file.open("test.txt",ios::out | ios::in)` – Apr 15 '21 at 08:55

Opening a file to read and write, create it if it doesn't exist

2 Answers2