Function checking if string is a palindrome

palindrome.cpp

#include <iostream>
#include <string>

bool isPalindrome(std::string s) {
    std::string p = "", f = "";
    for(auto c:s) {
        c = std::tolower(c);
        int i = (int)c;
        if(i > 96 && i < 123 || i > 47 && i < 58) {
            f += c;
            p = c + p;
        }
    }
    return f == p;
}