C++ Error: 'no match for operator<...'

I have been attempting to create a password generator. The program is supposed to take input, and put out every possible combo of characters (brute force without the force). I am encountering this error: error: no match for 'operator<=' in 'i <= pear'

I have no idea what to do. However, here is the code. Please let me know if I also messed up on anything else, but the error described is the main problem right now:

#include <iostream>
#include <string>

using namespace std;

void generate() {
    int i=0;
    string pass;
    string r;
    string pear;
    for(i=0; i <= pear; i++) {
        pear = pass;
            r = pass[i];
}


    return r;
}

int main() {
    int i;
    string apple;
    cin >> apple;
    generate(apple,i);
    cout << apple;
}