Not used set_intersection before, but I believe it will work with maps. I wrote the following example code but it doesn't give me what I'd expect:

#include <map>
#include <string>
#include <iostream>
#include <algorithm>

using namespace std;

struct Money
{
    double amount;
    string currency;

    bool operator< ( const Money& rhs ) const
    {
        if ( amount != rhs.amount )
            return ( amount < rhs.amount );
        return ( currency < rhs.currency );
    }
};

int main( int argc, char* argv[] )
{
    Money mn[] =
    {
        { 2.32,  "USD" },
        { 2.76,  "USD" },
        { 4.30,  "GBP" },
        { 1.21,  "GBP" },

        { 1.37,  "GBP" },
        { 6.74,  "GBP" },
        { 2.55,  "EUR" }
    };

    typedef pair< int, Money > MoneyPair;
    typedef map< int, Money > MoneyMap;

    MoneyMap map1;
    map1.insert( MoneyPair( 1, mn[1] ) );
    map1.insert( MoneyPair( 2, mn[2] ) );
    map1.insert( MoneyPair( 3, mn[3] ) );  // (3)
    map1.insert( MoneyPair( 4, mn[4] ) );  // (4)

    MoneyMap map2;
    map1.insert( MoneyPair( 3, mn[3] ) );  // (3)
    map1.insert( MoneyPair( 4, mn[4] ) );  // (4)
    map1.insert( MoneyPair( 5, mn[5] ) );
    map1.insert( MoneyPair( 6, mn[6] ) );
    map1.insert( MoneyPair( 7, mn[7] ) );

    MoneyMap out;
    MoneyMap::iterator out_itr( out.begin() );
    set_intersection( map1.begin(), map1.end(), map2.begin(), map2.end(), inserter( out, out_itr ) );

    cout << "intersection has " << out.size() << " elements." << endl;
    return 0;
}

Since the pair labelled (3) and (4) appear in both maps, I was expecting that I'd get 2 elements in the intersection, but no, I get:

intersection has 0 elements.

I'm sure this is something to do with the comparitor on the map / pair but can't figure it out.