My version of ceaser cypher in c++ with just alphabets with both upper & lower case

ceasercypher.cpp

#include <iostream>

/*
 (C) SaicharanKandukuri <[email protected]> 2022
  please try not to copy paste this in your class 👌
*/
#define SHIFT 3 // hardoced ( XD )
using namespace std;

string ALPHABET     = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";   // ALPHABETS for encryption
string LALPHABETS   = "abcdefghijklmnopqrstuvwxyz";   // LALPHABETS for encryption
string targetCharset;

int getindexof(char charc)
{
    if (ALPHABET.find(charc) != string::npos)
    {
        ::targetCharset = ALPHABET;
        return ALPHABET.find(charc);
    }
    else if (LALPHABETS.find(charc) != string::npos)
    {
        ::targetCharset = LALPHABETS;
        return LALPHABETS.find(charc);
    }
    else
    {
        return -1;
    }
}

string encrypt(string plaintext)
{
    string ciphertext = "";
    for (int i = 0; i < plaintext.length(); i++)
    {
        int index = (getindexof(plaintext[i]) + SHIFT) % 26;
        ciphertext += ::targetCharset[index];
    }
    return ciphertext;
}

string decrypt(string ciphertext)
{
    string plaintext = "";
    for (int i = 0; i < ciphertext.length(); i++)
    {
        int index = (getindexof(ciphertext[i]) - SHIFT) % 26;
        plaintext += ::targetCharset[index];
    }
    return plaintext;
}

// command line interface
int main()
{
    /* 
        just to display usability of functions
        try to use cmd line way instead for better experience
    */
    string plaintext, ciphertext;
    cout << "Enter plaintext: ";
    cin >> plaintext;
    ciphertext = encrypt(plaintext);
    cout << "Encrypted text: " << ciphertext << endl;
    cout << "Decrypted text: " << decrypt(ciphertext) << endl;
    return 0;
}