大神不要不鸟我啊，构造函数不懂了

#include "stdafx.h"
#include<iostream>
using namespace std;
class Base
{
    int i;
public:
    Base(int n)
    {
        cout<<"constructing Base class\n";
        i=n;
    }
    ~Base()
    {
        cout<<"destructing Base class\n";
    }
    void showi()
    {
        cout<<i<<endl;
    }
};
class Derived:public Base
{
    int j;
    Base ob;
public:
    Derived(int n):Base(n),ob(n)
    {
        cout<<"constrycting Derived class"<<endl;
        j=2*n;
    }
    ~Derived()
    {
        cout<<"destructing Derived class"<<endl;
    }
    void showj()
    {
        cout<<j<<endl;
    }
};

int _tmain(int argc, _TCHAR* argv[])
{
    Derived ob(10);
    ob.showi();
    ob.showj();
    return 0;
   
}
这个main里ob的i是从哪里来的？派生类构造函数的Base么?话说派生的成员里还有个基类对象ob里也有个i么，在哪里赋值呢，小白求教，大神不要不鸟我，谢谢