C++ Program to Implement the Monoalphabetic Cipher

#include <iostream>
#include <istream>
#include <ostream>
#include <fstream>
#include <cstdlib>
#include <string>
 
// the rot13 function
std::string rot13(std::string s)
{
    static std::string const lcalph = "abcdefghijklmnopqrstuvwxyz", ucalph =
            "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
 
    std::string result;
    std::string::size_type pos;
 
    result.reserve(s.length());
 
    for (std::string::iterator it = s.begin(); it != s.end(); ++it)
    {
        if ((pos = lcalph.find(*it)) != std::string::npos)
            result.push_back(lcalph[(pos + 13) % 26]);
        else if ((pos = ucalph.find(*it)) != std::string::npos)
            result.push_back(ucalph[(pos + 13) % 26]);
        else
            result.push_back(*it);
    }
 
    return result;
}
 
// function to output the rot13 of a file on std::cout
// returns false if an error occurred processing the file, true otherwise
// on entry, the argument is must be open for reading
int rot13_stream(std::istream& is)
{
    std::string line;
    while (std::getline(is, line))
    {
        if (!(std::cout << rot13(line) << "\n"))
            return false;
    }
    return is.eof();
}
 
// the main program
int main(int argc, char* argv[])
{
    if (argc == 1) // no arguments given
        return rot13_stream(std::cin) ? EXIT_SUCCESS : EXIT_FAILURE;
 
    std::ifstream file;
    for (int i = 1; i < argc; ++i)
    {
        file.open(argv[i], std::ios::in);
        if (!file)
        {
            std::cerr << argv[0] << ": could not open for reading: " << argv[i]
                    << "\n";
            return EXIT_FAILURE;
        }
        if (!rot13_stream(file))
        {
            if (file.eof())
                // no error occurred for file, so the error must have been in output
                std::cerr << argv[0] << ": error writing to stdout\n";
            else
                std::cerr << argv[0] << ": error reading from " << argv[i]
                        << "\n";
            return EXIT_FAILURE;
        }
        file.clear();
        file.close();
        if (!file)
            std::cerr << argv[0] << ": warning: closing failed for " << argv[i]
                    << "\n";
    }
    return EXIT_SUCCESS;
}

Output:

$ g++ MonoalphabeticCipher.cpp
$ a.out
 
Dharmendra
Qunezraqen
Hingu
Uvath
Sanfoundry
Fnasbhaqel