c++ set and shared_ptr

Question

I have class X like:

class X {
    public:
        bool operator<(const SCN& other) const;
};

Then I have the following code:

std::multiset<std::shared_ptr<X>> m;

My questions are:

1. how the data in m is ordered? the address of X(shared_ptr) or the X.operator<? If it is ordered by address of X, how can I make it order by X.operator<?

2. for this m, if I want to access its elements from smallest to biggest, can the following code guarantee that? If not, How?

   for (auto& i : m) {
       f(i);
   }
   

Answer 1

Your set is ordered based on your key_type which is std::shared_ptr<X>. As your std::shared_ptr<X> is comparable, the ordering of the std::shared_ptr prevails.

For the sake of reference, multiset is defined as

template<
    class Key,
    class Compare = std::less<Key>,
    class Allocator = std::allocator<Key>
> class multiset;

As can be seen, typename Compare is std::less<Key> and std::less should overload the function overload which would possibly be implemented as

constexpr bool operator()(const T &lhs, const T &rhs) const 
{
    return lhs < rhs;
}

both lhs and rhs is of type T which in this case is Key which is the type that we have instantiated multiset with which is std::shared_ptr<X>.