I try to use equal_range() traverse my all elements of one key, why there are compile errors?

@Zerp, I try to use the http://ift.tt/1BQQqUj descrbing way to traverse all elements of the same map, but I met some compile error of '=',

error   1   error C2679: binary “=”: not find “std::_List_iterator<std::_List_val<std::_List_simple_types<std::pair<const _Kty,_Ty>>>>”

Code:

#include <iostream>
#include <string>
#include <memory>
#include <map>
#include <unordered_map>
using namespace std;

class Instrument{

public:
    virtual void display() = 0;
    virtual ~Instrument(){};
};
class Stock :
    public Instrument{
public:
    Stock(){
        cout << "This is stock, please input its information: ";
        cin >> name >> bidPrice >> askPrice >> lastPrice >> issueExchange;
    }
    virtual void display(){
        cout << "This is to display stock: " << name << " "
            << bidPrice << " "
            << askPrice << " "
            << lastPrice << " "
            << issueExchange << " "
            << endl;
    }
    virtual ~Stock(){}
    inline string getname(){
        return name;
    }
private:
    string name;
    double bidPrice;
    double askPrice;
    double lastPrice;
    int issueExchange;

};
class Option :
    public Instrument{
public:
    Option(){
        cout << "This is option, please input its information: ";
        cin >> name >> uname >> bidPrice >> askPrice >> lastPrice >> contractSize >> exp;
    }
    virtual void display(){
        cout << "This is to display option: "
            << name << " "
            << uname << " "
            << bidPrice << " "
            << askPrice << " "
            << lastPrice << " "
            << contractSize << " "
            << exp << " "
            << endl;
    }
    virtual ~Option(){}
    inline string getuname(){
        return uname;
    }
private:
    string name;
    string uname;
    double bidPrice;
    double askPrice;
    double lastPrice;
    int contractSize;
    double exp;
};

int main(){

    int N = 2; int i = 5;
    // Construct the Stock, Option maps
    map<int, Stock> myStockMap;
    for (i = 0; i < N; i++) myStockMap.insert(pair<const int, Stock>(i, Stock()));
    for (map<int, Stock>::iterator it = myStockMap.begin(); it != myStockMap.end(); it++){
        it->second.display();
    }


    unordered_multimap<string, Option> myOptionMultiMap;
    for (i = 0; i < N; i++){
        Option a = Option();
        myOptionMultiMap.insert(pair<string, Option>(a.getuname(), a));
    }
    for (auto it = myOptionMultiMap.begin(); it != myOptionMultiMap.end(); ++it)
        it->second.display();


    pair < unordered_map<string, Option>::iterator,
        unordered_map<string, Option>::iterator > ret;

    for (auto it = myStockMap.begin(); it != myStockMap.end(); it++){
        it->second.display();
        ret = myOptionMultiMap.equal_range(it->second.getname());
        while (ret.first != ret.second){// there is some problem 
            cout << "\t";
            (ret.first++)->second.display();
        }
    }


    system("pause");
    return 0;

}