如何检查简单的C ++标准库迭代器是否可以高级？

Question

Suppose I have a class encapsulating std container:

class Stash
{
    list<int> Data;

public:
    list<int>::const_iterator GetAccess() const { return Data.begin(); }
};

It's very convenient way of forcing the user to read the data in form of the iterator. However, I can't find the way other that comparing the iterator to container.end() . So, I would like to know if there's an option to do it solely by stdlib or I have to write iterator class myself (with can_advance method, for example).

Relevant question might be this one , but it asks whether the iterator is valid , not whether it can advance . I weren't able to find any information about the latter.

Answer 1

You can't do this, a single iterator does not contain the information when it is at the end of the sequence its pointing into.

Normally, this is solved by either providing a range (think std::make_pair(cont.begin(), cont.end()) ), or providing begin() and end() methods to your class, effectively making it a range.

Answer 2

Iterators work in pairs: an iterator that points to the beginning of a sequence and an iterator that points past the end of the sequence. That's why all the containers have begin() and end() member functions: so you can look at the sequence of values that the container manages.

It would be far more idiomatic to change the name of GetAccess to begin and to add end . Having end() would also make it possible to apply standard algorithms to the data.

Answer 3

What you seem to be asking for is a "lookahead" iterator. You could write a class to "adapt" an iterator to have lookahead, where the adaptor just stays one step ahead of your code:

template<class FwdIter>
class lookahead_iterator
{
public:
    lookahead_iterator(const FwdIter& begin): cur_iter(begin), next_iter(++begin) {}
    operator FwdIter() const { return cur_iter; }
    lookahead_iterator<FwdIter>& operator ++() { cur_iter = next_iter++; return *this; }
    // Other methods as needed.
private:
    FwdIter cur_iter;
    FwdIter next_iter;
};

Needless to say, this gets a lot more complicated if you need more than a forward iterator.