What is the use of ios::trunc mode?

Correct option is C

The ios::trunc mode in C++ file handling is used when opening a file in output mode. If the file already exists, ios::trunc erases all its contents (truncates it to zero size) before writing new data.
Important Key Points:
1. Purpose of ios::trunc Mode:
· Used with ofstream (output file stream).
· If the file exists, it deletes all existing content before writing new data.
· If the file does not exist, it creates a new empty file.
2. Example Usage of ios::trunc in C++:
#include <fstream>
#include <iostream>
using namespace std;
int main() {
ofstream file("example.txt", ios::trunc); // Truncates the file to zero
file << "New content after truncation!";
file.close();
return 0;
}
· If example.txt already exists, its previous content is erased before writing "New content after truncation!"
· If example.txt does not exist, a new file is created.
Knowledge Booster:
· ios::app (append mode) is used when you want to add data without erasing the file content.
· ios::in (input mode) is used for reading from a file.
· Combining ios::trunc with ios::out ensures that the file is always cleared before writing.
· (a) To open a file in input mode → ❌ Incorrect: ios::trunc is only used with output mode, not input mode.
· (b) To open a file in output mode → ❌ Partially correct: ios::trunc is used along with output mode, but its specific function is to erase existing content, not just open the file.