C++ fstream

In this page:

What is fstream?
Reading and Writing Sequentially
Appending with fstream
Pointer Manipulation (seekg & seekp)
Telling Positions (tellg & tellp)

What is fstream?

std::fstream combines the capabilities of ifstream and ofstream into a single bidirectional stream object, letting you both read and write through the same handle without needing two separate stream objects for one file.

Example: What is fstream?

cpp

#include <iostream>
#include <fstream>

int main() {
	std::fstream file("data.txt", std::ios::out);
	file << "Hello";
	file.close();
	std::cout << "Written with a single bidirectional fstream" << std::endl;
	return 0;
}

Reading and Writing Sequentially

A stream tracks its read/write position internally, so after writing to a file you can't immediately read back what you wrote without first moving the cursor -- call seekg() to reposition the read pointer (commonly back to the start with seekg(0)) before reading.

Example: Reading and Writing Sequentially

cpp

#include <iostream>
#include <fstream>

int main() {
	std::fstream file("data.txt", std::ios::in | std::ios::out | std::ios::trunc);
	file << "42";
	file.seekg(0);
	int value;
	file >> value;
	std::cout << value << std::endl;
	return 0;
}

Appending with fstream

Opening an fstream with the ios::app flag appends new writes to the end of the file while still permitting reads elsewhere in the file through the same open stream -- useful for programs that need to both log new entries and inspect older ones without reopening the file.

Example: Appending with fstream

cpp

#include <iostream>
#include <fstream>

int main() {
	std::fstream file("data.txt", std::ios::app | std::ios::in);
	file << "Appended";
	std::cout << "Appended while still allowing reads" << std::endl;
	return 0;
}

Pointer Manipulation (seekg & seekp)

fstream actually maintains two separate position indicators: seekg() moves the "get" (read) cursor, and seekp() moves the "put" (write) cursor. You can pass an absolute byte offset or a relative offset from the beginning, current position, or end of the file to either.

Example: Pointer Manipulation (seekg & seekp)

cpp

#include <iostream>
#include <fstream>

int main() {
	std::fstream file("data.txt", std::ios::in | std::ios::out | std::ios::trunc);
	file << "0123456789";
	file.seekg(3);
	char c;
	file.get(c);
	std::cout << c << std::endl;
	return 0;
}

Telling Positions (tellg & tellp)

tellg() and tellp() report the current byte offset of the read and write cursors respectively, without moving them. A common trick to measure a file's total size is to seekg to the end and then call tellg() to read off that final position.

Example: Telling Positions (tellg & tellp)

cpp

#include <iostream>
#include <fstream>

int main() {
	std::fstream file("data.txt", std::ios::in | std::ios::out | std::ios::trunc);
	file << "Hello";
	file.seekg(0, std::ios::end);
	std::cout << "Size: " << file.tellg() << std::endl;
	return 0;
}

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