CODEWITHSUNDEEP
c++iterator
YiFei
YiFei

Reputation: 1802

How to get a guaranteed invalid iterator (for vector)?

I have a container class which is essentially (showing relevant parts only)

class Foo {
    typedef std::map<int, std::vector<int>> data_t;
    data_t data;
    struct iterator {
        data_t::iterator major_it;
        data_t::mapped_type::iterator minor_it;
        // ...
    };
    iterator begin();
    iterator end() {
        return { data.end(), /* XXX */ };
    }
};

As you see, I want to have iterator implemented for this container. The iterator shall go through each node in map and iterator through each element in the vector the node refers to. I have no problem implementing the iterator, but I had some trouble implementing end() for the container.

The iterator consists of two levels of iterators, and for the past-end iterator, major_it will have to be data_t.end(), but I won't have anything to initialize minor_it.

In the same vein, begin() will also be broken when the map is empty.

Any idea?

Upvotes: 1

Views: 92

Answers (1)

Potatoswatter
Potatoswatter

Reputation: 137780

std::vector::iterator is always invalid when value-initialized:

std::vector<…>::iterator invalid_iter_value = {};

Incidentally, it's unusable when default-initialized (i.e. uninitialized), which might be good enough for you. If major_it is already at the end, then simply don't access minor_it.

std::vector<…>::iterator unusable_iter_value;

However, note that it's also illegal to copy the default-initialized object, so value-initialization is a good idea unless you're customizing the copy constructor and operator.

Upvotes: 3

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