HowTo sort std::map?

Here is my map:

typedef std::map<int/*security id*/, PositionMonth> PortfolioMonth;

where PositionMonth is a structure, ex.:

struct PositionMonth
        {
            Nav::Shares shares_;
            Nav::Amount market_value_;

            PositionMonth(void)
                {}
            PositionMonth(const Nav::Amount& market_value
                    , const Nav::Shares& shares)
                : market_value_(market_value)
                , shares_(shares)
                {}
        };

Question: how to sort std::map by second value key param (by market_value_, let it be int)? Examples or links?

PS. Boost methods not interested!

PPS. I'm unable to initialise my std::map with compare functor!

Thanks for help!

My solution (or how I did it myself):

template<class T>
    struct LessSecondCcy
        : std::binary_function<T,T,bool>
    {
        inline bool operator ()(const T& _left, const T& _right)
        {
            return _left.second.market_value_.currency() < _right.second.market_value_.currency();
        }
    };

and in function:

typedef std::pair<int/*security id*/, _Entry> data_t;

where _Entry is PositionMonth

std::vector<data_t> vec(item.funds_end_.begin(), item.funds_end_.end());
std::sort(vec.begin(), vec.end(), Nav::LessSecondCcy<data_t>());

Done!