Are there any reasons not to extend std::set to add a subscript operator?

Question

I'm using std::set to store unique instances of a class. std::set does not have an overloaded subscript operator so you can't do set[0] for example.

I found a way to do it:

auto myClass = *std::next(set.begin(), index);

However, I found it monotonous to copy that code over and over again. So I decided it would be much more convenient to just extend std::set (class sset) and just overload the subscript operator in it.

template <class T>

class sset: public std::set<T>
{
public:
    T operator[](const uint32_t i) const
    {
        if(i <= (this->size()-1))
            return *std::next(this->begin(), i);
        else
            throw std::out_of_range("Index is out of range");
    }
};

int main()
{
    auto myClass = set[0]; //works and no exception thrown

    return 0;
}

I achieved the desired behavior but it occurred to me that there must have been a reason the standard didn't include a subscript operator. Surely not just laziness.

Is there any preceived disadvantage or possible future problems one can see doing this?

Answer 1

Indexing should never be more than logarithmic time, that's what's expected. This indexing is (at least) linear time. That's awfully inefficient. If you run through all items in a set, using that indexing, you get quadratic total time. That's a good reason to not do it.

---

For the code shown, note that

if(i <= (this->size()-1)

doesn't handle size 0 well. In this case you get unsigned wrap-around so that the condition is true. Dereferencing the end iterator is then Undefined Behavior.

Answer 2

The std::set doesn't generally have an efficient way to access the nth element. The method you are using will start at the beginning of the set and advance one element at a time until it gets to the nth. This would be very slow for large sets.

If you need it, then by all means do it, but`be aware of the inefficiency.

Answer 3

Apart from efficiency issues already mentioned, std::set (as most of the other standard containers) is not designed to be inherited from - especially, it does not provide a virtual destructor, so the following is bound to fail:

std::set<MyType>* s = new sset<MyType>();
delete s;

Sure, there should be little reasons for having a set created this way, but the issue still remains...

If you really, really need nth element and don't want to rewrite your sample code all the time, I'd rather have a separate function for than (at least) questionable inheritance:

template <typename T>
T& at(std::set<T>& s, size_t index)
{
    if(i < s.size())
        return *std::next(s.begin(), index);
    throw std::out_of_range("index is out of range");
}
template <typename T>
T const& at(std::set<T> const& s, size_t index)
{
    if(i < s.size())
        return *std::next(s.begin(), index);
    throw std::out_of_range("index is out of range");
}

Don't ever use that in a for loop iterating from 0 to size, use iterators instead or preferably a range based loop (which actually maps to a loop using iterators).