Szerkesztő:LinguisticMystic/cpp/Files

✅ #include <fstream>

This header provides the C++ file stream classes:

• std::ifstream – for reading from files.
• std::ofstream – for writing to files.
• std::fstream – for both reading and writing.

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✅ Writing to a file

Code:

#include <iostream>
#include <fstream>

int main() {
    std::ofstream outputFile;
    std::string filename = "SomeText.txt";

    outputFile.open(filename);
    outputFile << "Hi, I'm just a text you're trying to write to a file.";
    outputFile.close();

    return 0;
}

Explanation:

• std::ofstream outputFile; – Declares an output file stream object.
• outputFile.open(filename); – Opens or creates the file SomeText.txt.
• outputFile << ... – Writes text to the file.
• outputFile.close(); – Closes the file. Always good practice.

📝 The file is created in the same directory where your program’s executable runs.

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✅ Reading from a file

Code:

#include <iostream>
#include <fstream>

int main() {
    std::ifstream inputFile;
    inputFile.open("SomeText.txt");

    std::string line;
    getline(inputFile, line);

    inputFile.close();

    std::cout << "Data from file: " << line << std::endl;

    return 0;
}

Explanation:

• std::ifstream – Used for reading.
• getline() – Reads one line from the file.
• inputFile.close() – Closes the file.
• Output shows the content of the file on the console.

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✅ File Modes (std::ios::...)

These are flags you can pass when opening files to control how they behave:

Mode	Meaning
std::ios::in	Open for reading
std::ios::out	Open for writing (clears existing file)
std::ios::app	Append at end
std::ios::ate	Go to end immediately after opening
std::ios::trunc	Truncate existing file (default with out)
std::ios::binary	Binary mode (no newline translation)

🛠 Combine modes using | (bitwise OR), e.g.:

std::ofstream file("file.txt", std::ios::app | std::ios::binary);

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✅ Examples of file modes

Write at the end of the file:

std::ofstream file("example.txt", std::ios::ate | std::ios::out);

Truncate file (erase content first):

std::ofstream file("example.txt", std::ios::trunc | std::ios::out);

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✅ File manipulation with <cstdio>

Rename (move) a file:

const char* oldFileName = "file.txt";
const char* newFileName = "../file.txt";
std::rename(oldFileName, newFileName);

Rename (just change the name):

std::rename("old.txt", "new.txt");

Delete a file:

std::remove("new.txt");

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✅ Copy a file (binary-safe):

std::ifstream src("source.txt", std::ios::binary);
std::ofstream dst("dest.txt", std::ios::binary);
dst << src.rdbuf();  // Copy raw bytes

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✅ Seek to a specific position in file

std::ifstream inputFile("data.txt");
inputFile.seekg(10);  // Jump to 11th byte
char ch;
inputFile.get(ch);    // Read that byte

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✅ Working with Directories – <filesystem> (C++17+)

Create a directory:

std::filesystem::create_directory("my_folder");

Rename or move a directory:

std::filesystem::rename("old_folder", "new_folder");

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✅ Error handling

File open check:

std::ifstream file("nonexistent.txt");
if (!file.is_open()) {
    std::cerr << "Failed to open file!" << std::endl;
}

Detect end-of-file while reading:

std::ifstream inputFile("data.txt");
std::string line;
while (getline(inputFile, line)) {
    if (!inputFile.eof()) {
        // Process line
    }
}

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✅ Summary

Task	Class/Function
Read a file	std::ifstream
Write to a file	std::ofstream
Read/write both	std::fstream
Check file state	is_open(), eof()
Manipulate files	std::rename(), std::remove()
Work with folders	<filesystem> (C++17+)