std::fstream
- C++ course
- Intermediate
- Lesson 90/116
- 5 min
Learn to both read and write with std::fstream.
std::fstream in one class
std::fstream is used for both reading and writing. You specify the mode with std::ios::in and std::ios::out flags.
Example: std::fstream fs("data.txt", std::ios::out); opens the file for writing. After writing and closing, fs.open("data.txt", std::ios::in); reopens the same object for reading. So a single fs object can both write and read. std::ios::trunc erases content, std::ios::app appends. You can combine flags with |.
Write and read with the same object
You can use a std::fstream object first for writing, then for reading. A close() and open() call in between switches the mode.
Example: fs.open("data.txt", std::ios::out); to write, fs.close(); to close. Then fs.open("data.txt", std::ios::in); to read. This is practical for programs that write data to a file and then process it. Using two separate objects (std::ifstream and std::ofstream) is also possible; std::fstream combines them under one roof.
Practice Write and read with std::fstream
Use a single std::fstream object. First open it in std::ios::out mode, write "Hello" and close it. Then reopen in std::ios::in mode, read one word with >> and print it with std::cout.
Starter code
#include <iostream>
#include <fstream>
#include <string>
int main() {
// Write with std::fstream, close, then read
return 0;
}Sample solution
#include <iostream>
#include <fstream>
#include <string>
int main() {
std::fstream fs("data.txt", std::ios::out);
fs << "Hello";
fs.close();
fs.open("data.txt", std::ios::in);
std::string w;
fs >> w;
std::cout << w;
return 0;
}Expected output
Hello
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Match each mode to its role
| Term | Meaning |
|---|---|
std::ios::in |
Reading |
std::ios::out |
Writing |
std::ios::app |
Appending |
std::ios::trunc |
Erasing content |
2 Specify the write mode
std::fstream fs("data.txt", std::ios::____);std::fstream fs("data.txt", std::ios::out);Explanation: std::ios::out opens in write mode.
3 What is std::fstream used for?
std::fstream fs("data.txt");- Reading only
- Writing only
- Deleting files only
- Both reading and writing Correct answer
Explanation: std::fstream is used for both reading and writing.