encoding game help!

[beginnervn]

#include <iostream>
#include <cstdlib>
#include <string>
using namespace std;

class Encoding
{
private:
string coding_words;
int shift;
int wlen;
public:
void getshift()
{
do
{
cout <<"What interger cipher shift key will you use? (0-20) ";
cin>> shift;
}
while (shift < 0 || shift > 20);
}

void get_encoding(string ent_words)
{
coding_words = ent_words;
int wlen = coding_words.length();
for (int j=0; j < wlen; j++)
coding_words.at(j) = coding_words.at(j) + shift;
}


void get_decoding(string ent_words)
{
coding_words = ent_words;
int wlen = coding_words.length();
for (int i=0; i < wlen; i++)
coding_words.at(i) = coding_words.at(i) -shift;
}

void print_result (string key)
{
cout << "Your " <<key<<"d" <<" string is:\n";
cout << coding_words;
}
};

int main()
{
Encoding encode, decode, cipher_shift ;
string keycode, entered_words, again;
do
{
cout <<"Welcome to the Caesar Encoder/Decoder!" << endl;
cout <<"Would you like to Encode or Decode or Quit? ";
getline(cin, keycode);

cout <<"Please enter a string that you would like to "<<keycode <<" :\n";
cout <<"(Please end your string with a ~ symbol and then press Enter key)\n";
getline(cin, entered_words);

cipher_shift.getshift();

if (keycode=="Encode")
{
encode.get_encoding(entered_words);
encode.print_result(keycode);
}
else if (keycode =="Decode")
{
decode.get_decoding(entered_words);
decode.print_result(keycode);
}
else if (keycode=="Quit")
exit;

cout <<"Would you like to encode/decode another string? (Yes/No): ";
getline(cin, again);
system("cls");

}
while ( again == "Yes");



system ("pause");
return 0;
}

please help!
this is an encoding game
i don't know how to convert string to integer before i do the operation:
for (int i=0; i < wlen; i++)
coding_words.at(i) = coding_words.at(i) -shift;