Returning const ref

This is a simplified version of the class I once wrote. Please comment
on Container::get() method. Is it good, bad, safe, evil? I admit that I
never saw such use of the static keyword and I wonder if it's OK.

I don't want to use pointers. I don't want to throw an exception,
because it's legal for a key to map into an empty set. And finally, I
don't want to store empty objects when there's no need for doing so.

#include <iostream>
#include <cstdlib>
#include <map>
#include <set>
using namespace std;

class Container
{
     public:
         void add(size_t i, char c);
         const set<char>& get(size_t i) const;
     private:
         map<size_t, set<char> > content;
};

void Container::add(size_t i, char c)
{
     content[i].insert(c);
}

const set<char>& Container::get(size_t i) const
{
     static const set<char> empty;
     map<size_t, set<char> >::const_iterator mit = content.find(i);
     if(mit == content.end())
         return empty;
     return mit->second;
}

int main()
{
     Container c;
     c.add(0, 'a');
     c.add(0, 'b');
     const set<char>& charSet0 = c.get(0);
     const set<char>& charSet1 = c.get(1);
     cout << charSet0.size() << endl;
     cout << charSet1.size() << endl;
}

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