怎么改进这个程序

#include <iostream>
#include <string>
using namespace std;
class user
{
   
    double money;
    string name;
   
public:
    void add(double& mey)
    {
        money=money+mey;
        cout<<money;
    }

    user(string Name,double Money);
};
user::user(string Name,double Money)
{
    name=Name;
    money=Money;
    cout<<"please input the user's name and money:"<<name<<"  "<<money<<endl;
   
}

void main()
{
    user a("li",100);
    double mey;
    cout<<"input the number you want deposit"<<endl;
    cin>>mey;
      a.add(double& mey);//加上这句话就有错误，怎么改进实现加，且使他不出错
}