c++ caesar cipher decoding trouble

I'm trying to implement a program that reads a text file that has a caesar cipher encryption. My program's purpose is to decode it. With my code it only reads the file and doesnt decode anything. Where am I going wrong? It echoprints the file but it doesnt decode it. I think my logic is flawed at the end of the CaesarCipher function

#include <iostream>         //for standard I/O
#include <fstream>          //necessary for file input
#include <string>           //necessary for fileName input

using namespace std;

void PrintHeading ();           //prototype for printing the initial heading
char PrintMenu ();              //prototype for printing menu and gaining user's choice
void OpenFile (ifstream &);     //prototype for opening file
void CaesarCipher (ifstream &); //prototype for performing a caesar decryption
//void SubCipher (ifstream &);  //prototype for performing a substitution decryption

const int ALPHA_SIZE = 26;
typedef char AlphaArray[ALPHA_SIZE];        //alphabet array with 26 elements
AlphaArray realAlphas;
AlphaArray encryptionArray;
ifstream inFile;



int main()
{
char choice;            //menu choice from user
ifstream inFile;        //opens file
string fileName;        //user inputs name of file
bool condition = true;

PrintHeading();         // prints heading

while(condition)
{
    choice = PrintMenu();       //passes choice to printMenu function
    switch(choice)
    {
        case 'C':                           //choice for caesar substitution
        case 'c': cout << choice << endl;
            OpenFile(inFile);
            CaesarCipher (inFile);
            break;
        case 'S':                           //choice for substitution cipher
        case 's': cout << choice << endl;
            OpenFile(inFile);
            //SubCipher(inFile);
            break;
        case 'Q':                           //choice for quitting the program
        case 'q': condition = false;        //prints closing message when false
            cout << "***********************************************" << endl
                 << "\tThank you for using the DECRYPTER!" << endl
                 << "***********************************************" << endl << endl;
            break;
        default:
            //default statement for incorrect input
            cout << "Not a valid choice. Try again." << endl;
            break;
    }

}

return 0;
}

void PrintHeading ()    //prints heading for decryption program
{
cout << "***********************************************" << endl
     << "\t Welcome to The DECRYPTER!\t\t" << endl
     << endl
     << " You have the option of performing either a" << endl
     << "\t  Caesar decryption or " << endl
     << "\t  a Substitution decryption" << endl
     << endl
     << " You will be asked for a file to be decoded." << endl
     << endl
     << " The DECRYPTER will then echoprint one line " << endl
     << " of your encoded text from your specified file" <<endl
     << " followed by the same text decoded." << endl << endl
     << "***********************************************" << endl
     << endl
     << endl;
}

char PrintMenu ()       //function for displaying menu and gathering user's input
{
char userChoice;    //menu choice from user

//menu for user input
cout << "What would you like to do?" << endl
     << "To decode a text file using the Caesar cipher, press c" << endl
     << "To decode a text file using the substitution cipher, press s" << endl
     << "To quit decoding, press q" << endl
     << "What is your selection ? ";
cin >> userChoice;

while(userChoice != 'C' && userChoice != 'c' && userChoice != 'S' && userChoice != 's' && userChoice != 'Q' && userChoice != 'q')
{
    cout << "Please re-enter a valid menu choice: ";    //prompts user to input choice until it is valid
    cin >> userChoice;
}

return userChoice;              //return user's menu choice
}

void OpenFile (ifstream & infile)
{
string fileName;                //file input from user
cout << endl;
cout << "Please enter the file name to decode -> " << endl;      //asks for file name from user
cin >> fileName;                                                 //user file name
inFile.clear();
inFile.open(fileName.c_str());


while (!inFile)     //loop for if file doesnt exist
{
    cout << "File doesn't exist. Try again: " << endl;
    cin >> fileName;
 
void CaesarCipher (ifstream & infile)
{
char ch, temp;
for (int i = 0; i < 26; i++)
{
    realAlphas[i] = 65 + i;
}

int caesarConstant = 4;
for (int i = 0; i < 26; i++)
{
    encryptionArray[i] = realAlphas[(i + caesarConstant) % ALPHA_SIZE];
}

cout << "Original alphabet: " << realAlphas << endl;
cout << "Encrypted alphabet: " << encryptionArray << endl;
cout << endl;


int i;
while (inFile)
{
    if (!inFile.eof())
    {
        ch = inFile.get();
        cout << ch;
        for (i = 0; i < 26; i++)
        {
            encryptionArray[i] = realAlphas[i];
        }
        if (islower(encryptionArray[i]))
        {
            encryptionArray[i] = (encryptionArray[i] - 'a' + caesarConstant) % 26 + 'a';
        } else if (isupper(encryptionArray[i])) {
            encryptionArray[i] = (encryptionArray[i] - 'A' + caesarConstant) % 26 + 'A';
        }

    }
}


}