How to save variable in for loop?

I have made a random password generator.In function void passwordGenerator(int sizeOfPassword) The problem here is this I am trying to save the password generated by the program in sum but I don't know how to do it properly.

How do I save random digits password in sum.

#include <iostream>
#include <ctime>
#include <cstdlib>

using namespace std;

void passwordGenerator(int sizeOfPassword)
{
    srand(time(NULL));

    char allChars[] = {"0123456789!@#$%^&*abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ"};
    char sum;
    for(int i = 0; i < sizeOfPassword; i++){
        sum = sum + allChars[rand()%sizeOfPassword];
    }
    std::cout<<sum<<std::endl;
}

int main()
{
    int sizeOutput;
    char wannaPlay = 'y';
    while(wannaPlay == 'y'){
        std::cout<<"Enter the size of password: ";
        std::cin>>sizeOutput;
        passwordGenerator(sizeOutput);
        std::cout<<"\nRun Again[y/n]? : ";
        std::cin>>wannaPlay;
    }
    return 0;
}

Thank you