How to fix a bug in C++ code?

Question

C++ / C#

Please help me fix the bug in the code.
Error code in VS:
Error C3861 gets: identifier not found

#include<iostream>
#include<fstream>
#include<stdio.h>
#include<string>
using namespace std;
int main()
{
    char fileName[30], ch;
    fstream fps, fpt;
    cout << "Enter the Name of File: ";
    gets(fileName);
    fps.open(fileName, fstream::in);
    if (!fps)
    {
        cout << "\nError Occurred, Opening the Source File (to Read)!";
        return 0;
    }
    fpt.open("tmp.txt", fstream::out);
    if (!fpt)
    {
        cout << "\nError Occurred, Opening/Creating the tmp File!";
        return 0;
    }
    while (fps >> noskipws >> ch)
    {
        ch = ch + 100;
        fpt << ch;
    }
    fps.close();
    fpt.close();
    fps.open(fileName, fstream::out);
    if (!fps)
    {
        cout << "\nError Occurred, Opening the Source File (to write)!";
        return 0;
    }
    fpt.open("tmp.txt", fstream::in);
    if (!fpt)
    {
        cout << "\nError Occurred, Opening the tmp File!";
        return 0;
    }
    while (fpt >> noskipws >> ch)
        fps << ch;
    fps.close();
    fpt.close();
    cout << "\nFile '" << fileName << "' Encrypted Successfully!";
    cout << endl;
    return 0;
}

Answer 1

https://docs.microsoft.com/ru-ru/cpp/c-runtime-lib...

Answer 2

There is no function in the standard library gets.
You can do the same with this code

#include <iostream>
#include <fstream>
#include <string>

int main()
{
  std::string file_name;
  std::cout << "Enter file name: " << std::endl;
  std::getline(std::cin, file_name);

  std::ifstream ifstream(file_name);

  std::string data_to_encrypt;
  std::getline(ifstream, data_to_encrypt);

  ifstream.close();

  for (auto& ch : data_to_encrypt)
  {
    ch += 100;
  }

  std::ofstream ofstream(file_name, std::ios::trunc | std::ios::out);
  
  ofstream << data_to_encrypt;
  ofstream.close();

  return 0;
}