Compare vector<T>::iterator with vector<T>::reverse_iterator

Question

I am working on an exercise where I have a vector and I am writing my own reverse algorithm by using a reverse and a normal (forward) iterator to reverse the content of the vector. However, I am not able to compare the iterators.

int vals[] = { 1, 2, 3, 4, 5, 6, 7, 8, 9, 0 };
vector<int> numbers(vals, vals + 10);

vector<int>::iterator       start = numbers.begin();
vector<int>::reverse_iterator end = numbers.rend();

I have a previous algorithm for reversing the vector by using two iterators, however in this task I am not able to compare them using the != operator between them. My guess would be to get the underlying pointers or indexes in the vector with each other but how do I get the pointers/index?

Answer 1

使用base()返回的迭代器进行比较： it == rit.base() - 1 。

Answer 2

You can convert a reverse_iterator to iterator by calling base() .

Be careful however, as there are some caveats. @Matthieu M.'s comment is particularly helpful:

Note: base() actually returns an iterator to the element following the element that the reverse_iterator was pointing to.

Answer 3

Checkouthttp://en.cppreference.com/w/cpp/iterator/reverse_iterator/base

rit.base()

returns a 'normal' iterator.

Answer 4

您可以使用(&*start == &*(end - 1))直接比较迭代器指向的地址。

Answer 5

The two types cannot be compared (which is a very good idea) and calling .base() is not very elegant (or generic) in my opinion. You can convert the types and compare the result. Taking into account the off-by-one rule involving reverse_iterators .

Conversion from iterator to reverse_iterator need to be explicit (fortunately), however, conversion from reverse_iterator to iterator is not possible (unfortunately). So there is only one way to do conversion and then make the comparison.

    std::vector<double> vv = {1.,2.,3.};
    auto it = vv.begin();
    auto rit = vv.rend();
//  assert( it == rit ); // error: does not compile
    assert(std::vector<double>::reverse_iterator{it} == rit);