Not working: override the default less-than operator of shared_ptr of a class

all. I'm trying to use a std::multiset to have automatic ordering upon insertion. Its elements are std::shared_ptrs to DataCell objects. Here is the code in the header file:

typedef std::shared_ptr<DataCell> DataCellPtr;

inline bool operator<(const DataCellPtr &d1, const DataCellPtr &d2){
    std::cout << "lalala" << std::endl;
    return d1->_cartesianDistance < d2->_cartesianDistance;
}

typedef std::multiset<DataCellPtr, std::less<DataCellPtr>> DataCellPtrMultiset;

All works fine, except that my custom ordering does not take place, as I was supposed to get "lalala"s printed to my std::out, which is not happening. I imagine the multiset is using the shared_ptr's default ordering. The _cartesianDistance member is a double. Question: how do I override the default shared_ptr ordering?