C++ code for serial number

#include<iostream>
// it is code for serial number

using namespace std;

int main(){



    string serial=""; //serial
    int length=20;    //serial length

    string alp ="0123456789";
    for(int i=0;i<26;i++)alp+='a'+i;//a-z
    for(int i=0;i<26;i++)alp+='A'+i;//A-Z

    int index;//random index
    int size=alp.size();//chars size
    srand(time(0));

    for(int i=0;i<length;i++){
        index=rand()%size;
        serial+=alp[index];
    }

    cout<<serial<<endl;


    return 0;

}