File open and close operations in C++ - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this file operation code?

Consider the following C++ code that opens a file, writes to it, and then closes it. What will be the output printed on the console?

C++

#include <iostream>
#include <fstream>

int main() {
    std::ofstream file("test.txt");
    if (!file.is_open()) {
        std::cout << "Failed to open file" << std::endl;
        return 1;
    }
    file << "Hello, file!" << std::endl;
    file.close();
    if (file.is_open()) {
        std::cout << "File is still open" << std::endl;
    } else {
        std::cout << "File closed successfully" << std::endl;
    }
    return 0;
}

A File closed successfully

B Failed to open file

C File is still open

D No output

💡 Hint

Check what file.is_open() returns after closing the file.

After calling file.close(), the file stream is closed, so file.is_open() returns false. Therefore, the program prints "File closed successfully".

🧠 Conceptual

intermediate

Which statement about file closing is true?

In C++, what happens if you do not explicitly call close() on an ofstream object before the program ends?

A The file remains open and data is lost

B The program will crash due to an unclosed file

C The file is automatically closed when the <code>ofstream</code> object is destroyed

D The file is deleted from the disk

💡 Hint

Think about what happens to objects when they go out of scope.

The ofstream destructor automatically closes the file if it is still open, so explicit close() is not always required.

🔧 Debug

advanced

Why does this code fail to write to the file?

Examine the following code snippet. It attempts to write "Hello" to a file but the file remains empty. What is the cause?

C++

#include <fstream>

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

A The file is closed before opening

B The file is not opened at all

C The file is opened in read mode only

D The file is opened after writing, so data is not saved

💡 Hint

Check the order of operations: when is the file opened vs when is data written?

The code writes to the file stream before opening the file, so the data is not sent to any file. The file is opened only after writing, so the file remains empty.

📝 Syntax

advanced

Which option will cause a compilation error?

Which of the following code snippets will cause a compilation error when trying to open a file for writing?

A std::ofstream file("data.txt");

B std::ifstream file("data.txt"); file << "Hello";

C std::ofstream file; file.open("data.txt");

D std::ofstream file; file.open("data.txt", std::ios::out);

💡 Hint

Consider which stream type supports writing with the << operator.

std::ifstream is for reading files and does not support writing with <<. Attempting to write causes a compilation error.

🚀 Application

expert

How many times is the file opened in this code?

Analyze the following code. How many times is the file "log.txt" opened during execution?

C++

#include <fstream>

void writeLog() {
    std::ofstream file("log.txt", std::ios::app);
    file << "Log entry\n";
}

int main() {
    for (int i = 0; i < 3; ++i) {
        writeLog();
    }
    return 0;
}

A 3

B 0

C 1

D 6

💡 Hint

Consider how many times the function writeLog() is called and what happens inside it.

The function writeLog() opens the file each time it is called. Since it is called 3 times in the loop, the file is opened 3 times.