int &(vector<int>::*extract)(vector<int>::size_type);是什么意思

[code]
#include <iostream>
#include <vector>
#include <map>
#include <iterator>
#include <algorithm>
#include <hash_set>
#include <functional>
using namespace std;

class Base
{
    public:
        virtual void print()=0;   
};
class B1: public Base
{
    public:
        void print()
        {
            cout<<" this is B1"<<endl;   
        }   
};
class B2: public Base
{
    public:
        void print()
        {
            cout<<" this is B2"<<endl;   
        }   
};
class Op
{
    public:
        virtual double eval(double)=0;
};
class square:public Op
{
    public:
        double eval(double x)
        {
            return x*x;   
        }   
};
class Negate:public Op
{
    public:
        double eval(double x)
        {
            return -x;   
        }   
};
int main()
{
    //mem_fun1_ref_t
    vector<int> v1{1,1,1,1,1};
    vector<int> v2{2,2,2,2,2};
    vector<int> v3{3,3,3,3,3};
   
    vector<vector<int> > v;
    v.push_back(v1);
    v.push_back(v2);
    v.push_back(v3);
   
    vector<int> v4{1,2,3};
    int &(vector<int>::*extract)(vector<int>::size_type);//1
    extract=&vector<int>::operator[];//2
    transform(v.begin(),v.end(),v4.begin(),ostream_iterator<int>(cout," "),mem_fun_ref(extract));//3
    return 0;
}