Как удалить n-ую строку txt файла

// C++ Program to delete the given
// line number from a file
#include <bits/stdc++.h>
using namespace std;
  
// Delete n-th line from given file
void delete_line(const char *file_name, int n)
{
    // open file in read mode or in mode
    ifstream is(file_name);
  
    // open file in write mode or out mode
    ofstream ofs;
    ofs.open("temp.txt", ofstream::out);
  
    // loop getting single characters
    char c;
    int line_no = 1;
    while (is.get(c))
    {
        // if a newline character
        if (c == '\n')
        line_no++;
  
        // file content not to be deleted
        if (line_no != n)
            ofs << c;
    }
  
    // closing output file
    ofs.close();
  
    // closing input file
    is.close();
  
    // remove the original file
    remove(file_name);
  
    // rename the file
    rename("temp.txt", file_name);
}
  
// Driver code
int main()
{
    int n = 3;
    delete_line("a.txt", n);
    return 0;
}

#include <fstream>
#include <string>
 
///----------------------------------------------------------------------------|
/// Удалить n-ую строку txt
///----------------------------------------------------------------------------:
void erase_str_file(const char* filename, unsigned number)
{
    std::string s; std::getline(std::ifstream(filename), s, '\f');
 
    size_t i = number - 1,
           a = 0;
 
    for (;   0 != i&&(a = s.find('\n', a)) != s.npos; a++, i--);
    if  (    0 == i)
    {   auto b  = s.find('\n', a--);
        s.erase  (a,     b  -  a  );
        std::ofstream(filename).write(&s[0], s.size()*sizeof s[0]);
    }
}
 
///----------------------------------------------------------------------------|
/// ...
///----------------------------------------------------------------------------:
int main()
{   erase_str_file("test.txt", 5);
}