#include<iostream>
#include<algorithm>
#include<functional>
#include<iterator>
#include<vector>
using namespace std;

class even{
    int x;
public:
    even():x(0){}
    int operator()(){return x += 2;    }
};
int main(){
    vector<int>ivector3(2);
    fill(ivector3.begin(), ivector3.end(), -1);
    ostream_iterator<int>output(cout," ");
    copy(ivector3.begin(), ivector3.end(), output);
    cout << endl;

    generate(ivector3.begin(), ivector3.end(), even());             //

    copy(ivector3.begin(), ivector3.end(), output);
    cout << endl;
    return 0;
}

因为 函数对象是有状态的
你的代码移除不相干代码部分后，就是

#include <iostream>
#include <algorithm>
using namespace std;

class even {
    int x;
public:
    even():x(0){}
    int operator()(){return x += 2;    }
};

int main( void )
{
   even tmp;
   cout << tmp() << ' ';
   cout << tmp() << endl;
}