C++ password generator

#include <iostream>
#include <random>


const std::string upperCase = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
const std::string lowerCase = "abcdefghijklmnopqrstuvwxyz";
const std::string digits = "0123456789";
const std::string special = "!\"#$%&'*+,./:;=?@\\^`|~";

std::random_device dev;
std::mt19937 rng(dev());

std::uniform_int_distribution<std::mt19937::result_type> dist4(1, 4);
std::uniform_int_distribution<std::mt19937::result_type> distUC(0, upperCase.length()-1);
std::uniform_int_distribution<std::mt19937::result_type> distLC(0, lowerCase.length()-1);
std::uniform_int_distribution<std::mt19937::result_type> distDig(0, digits.length()-1);
std::uniform_int_distribution<std::mt19937::result_type> distSpec(0, special.length()-1);


std::string generate(int len){
    std::string result{};
    int group{};
    int index{};
    for(size_t i = 0; i < len; ++i){
        group = dist4(rng);

        switch(group){

            case 1:
                index = distUC(rng);
                result += upperCase[index];
                break;

            case 2:
                index = distLC(rng);
                result += lowerCase[index];
                break;

            case 3:
                index = distDig(rng);
                result += digits[index];
                break;

            case 4:
                index = distSpec(rng);
                result += special[index];
                break;
        }
    }

    return result;
}


int main()
{

    int ln = 10;
    for(int i = 0; i < 5; ++i){
        std::cout << generate(ln) << '\n';
    }


    return 0;
}