ايش مشكله هذا الكود

// test.cc

#include <iostream>

#include <string>

using namespace std;

char caesar (char);

char caesar(char c)

{

if(isalpha(c))

{

c=toupper(c);

c=(((c-65)+3)%26)+65;

}

return c;

}

int main ()

{

string input;

do {

cout<<"enter cipertext and prees enter continue."<< endl;

cout << "enter blanc lin quit."<<endl;

getline (cin,input);

string output="";

for (int x=0; x<input.length();x++)

{

output+=caesar(input[x]);

}

cout<<output<<endl;

}while(!input.length()==0);

return 0;

}