File output using ofstream in C++ - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output in the file after running this code?

Consider the following C++ code that writes to a file using ofstream. What will be the content of the file output.txt after running this program?

C++

#include <fstream>
#include <string>

int main() {
    std::ofstream file("output.txt");
    file << "Hello" << std::endl;
    file << "World" << std::endl;
    file.close();
    return 0;
}

A "Hello\nWorld\n"

B "HelloWorld"

C "Hello World"

D "Hello\nWorld"

💡 Hint

Remember that std::endl inserts a newline character and flushes the stream.

The code writes "Hello" followed by a newline, then "World" followed by a newline. So the file contains two lines: "Hello" and "World".

❓ Predict Output

intermediate

What happens if the file cannot be opened?

What will be the output of this program if the file /root/secret.txt cannot be opened due to permission issues?

C++

#include <fstream>
#include <iostream>

int main() {
    std::ofstream file("/root/secret.txt");
    if (!file) {
        std::cout << "Failed to open file" << std::endl;
    } else {
        file << "Data" << std::endl;
    }
    return 0;
}

A Failed to open file

B Data

C Compilation error

D No output

💡 Hint

Check how the program tests if the file stream is valid.

If the file cannot be opened, the ofstream object evaluates to false. So the program prints "Failed to open file".

🔧 Debug

advanced

Why does this code not write anything to the file?

The following code is supposed to write "Test" to log.txt, but the file remains empty. What is the reason?

C++

#include <fstream>

int main() {
    std::ofstream file;
    file << "Test";
    file.close();
    return 0;
}

A The program lacks write permissions

B The file stream was not closed properly

C The file was never opened before writing

D The file path is incorrect

💡 Hint

Check how the ofstream object is created and used.

The ofstream object was declared but never opened with a filename. Writing to it does nothing because it is not connected to any file.

🧠 Conceptual

advanced

What is the effect of opening a file with std::ofstream in append mode?

What happens when you open a file with std::ofstream file("data.txt", std::ios::app); and write data to it?

A The existing file content is erased before writing

B The file is locked and cannot be written

C The file is opened in read-only mode

D New data is added at the end of the existing file content

💡 Hint

Think about what append mode means for files.

Opening a file with std::ios::app means all writes go to the end of the file, preserving existing content.

📝 Syntax

expert

Which option correctly opens a file for writing and checks for errors?

Choose the code snippet that correctly opens a file named report.txt for writing using ofstream and checks if the file was opened successfully.

A

#include <fstream>

int main() {
    std::ofstream file("report.txt");
    if (!file.is_open()) {
        return 1;
    }
    file << "Report data";
    file.close();
    return 0;
}

B

#include <fstream>

int main() {
    std::ofstream file;
    file.open("report.txt");
    if (!file) {
        return 1;
    }
    file << "Report data";
    file.close();
    return 0;
}

C

#include <fstream>

int main() {
    std::ofstream file;
    file.open("report.txt");
    if (file == nullptr) {
        return 1;
    }
    file << "Report data";
    file.close();
    return 0;
}

D

#include <fstream>

int main() {
    std::ofstream file("report.txt");
    if (file == NULL) {
        return 1;
    }
    file << "Report data";
    file.close();
    return 0;
}

💡 Hint

Check how to test if an ofstream is open and valid.

Option B correctly opens the file and checks the stream with if (!file) which tests if the stream is good. Options B and C use invalid comparisons. Option B uses is_open() which works but is less idiomatic than checking the stream state.