map中的const-interator

#include<string>
//#include<stringstream>
using namespace std;
typedef map<string,vector<string> > Familytree;
int main ()
{
    vector<string>::const_iterator out;////////////////////看这
    Familytree::const_iterator it;////////////////////////看这
   
    Familytree family;
    string fn,cn,line;
    cout<<"enter family name  "<<endl;
    while(1)
    {
        getline(cin,fn);
        if(fn=="@")
            break;
        cout<<"enter children's names ctrl+z to end"<<endl;
        //getline(cin,line);
        //istringsstream stream(line);
        //while(stream>>cn)
        vector<string> nvec;
        while(cin>>cn)
          {      cout<<"here"<<endl; //check
              nvec.push_back(cn);
        }
        cin.clear();
        //cout<<"fail:"<<cin.fail()<<endl;//check istream
        //cout<<"end:"<<cin.eof()<<endl;
        //cout<<"bad:"<<cin.bad()<<endl;
        family.insert(make_pair(fn,nvec));
        cout<<"if you have finished all,enter @;otherwise,enter family name"<<endl;
    }
    cout<<"enter the family name you want to check ctrl+z to end"<<endl;
    while(1)
    {
        getline(cin,fn);//cin>>fn;
        if(fn=="@")
        break;
        it=family.find(fn);
        if(it==family.end())
            cout<<"the family doesn't exist in the list"<<endl;
        else
        {
            for(out=it->second.begin();out!=it->second.end();out++)
                cout<<*out<<'    ';
            cout<<endl;
        }
        cout<<"if you have finished all,enter @;otherwise,enter family name"<<endl;
    }
    return 0;
}是我写的一个存家谱的小程序。
原来在程序中是Familytree::iterator it;vector<string>::iterator out;/(可练习册上就是这样写的)？就会出错，如图  额。。。。改成const_iterator就对了,可练习册上答案也没写const，他错了？一定要用const？