迭代器使用end（）持续std :: vector的元素 -

Question

I have a std::vector and I want the iterator to the last element in the vector; I will be storing this iterator for later use.

NOTE: I want an iterator reference to it, not std::vector::back . Because I want to be able to compute the index of this object from the std::vector::begin later on.

The following is my logic to get the iterator to the last element:

std::vector<int> container;
std::vector<int>::iterator it = container.end()--;

Since std::vector::end has O(1) time complexity, is there a better way to do this?

Answer 1

I think you mean either:

std::vector<int>::iterator it = --container.end();
std::vector<int>::iterator it = container.end() - 1;
std::vector<int>::iterator it = std::prev(container.end());

You're unintentionally just returning end() . But the problem with all of these is what happens when the vector is empty, otherwise they're all do the right thing in constant time. Though if the vector is empty, there's no last element anyway.

Also be careful when storing iterators - they can get invalidated.

Note that if vector<T>::iterator is just T* (which would be valid), the first form above is ill-formed. The second two work regardless, so are preferable.

Answer 2

The way you are doing it will give you the wrong iterator because post increment will not change the value until after the assignment.

There is always this:

auto it = std::prev(container.end());

Remember to check first that the container is not empty so your iterator exists in a valid range.

Answer 3

You have rbegin that does what you need

cplusplus reference

auto last = container.rbegin();