c++ stl map根据value 排序

std::map<int, int, less<int>  > sortArmorAttr(std::map<int, int> & attr )
{
    std::map<int, int, less<int>  > ss;
    //ll.swap()
    for (std::map<int, int>::iterator it=attr.begin(); it!=attr.end(); ++it)
    {
        int type=it->first;
        int value=it->second;

        ss.insert( std::map<int, int, less<int> >::value_type(value, type)  );
    }

    return ss;
}