为什么我不能在 c++ 的 `map` 的迭代器中使用 `+1`？

Question

I have a problem about the iterator using in map . I have a map with type of map<int, vector<int> > vpmap; I want to loop through the whole map. Then I use

 for (size_t i = 0; i < vpmap.size(); i++) {
    {
        auto it = vpmap.begin();
        it++;
        /*code*/
    }

This is well. But when I replace the it++ to it=it+1 . The complier gives an error. I haven't get this error before when using vector . I would like to know the reason of it.

Answer 1

std::vector::iterator is a LegacyRandomAccessIterator . It is required to support the operation it + 1 .

On the other hand, std::map::iterator is a LegacyBidirectionalIterator . It is not required to support it + 1 but it is required to support it++ as well as it-- .

Relevant detail from an answer to a different post :

The reason behind this is that adding N to a random access iterator is constant time (eg add N*sizeof(T) to a T* ), whereas doing the same thing for a bidirectional iterator would require applying ++ N times.

Answer 2

it is the iterator of std::map , it's an Bidirectional Iterator and doesn't support operator+ .

RandomAccess Iterator supports operator+ , eg the iterator of std::vector , that's why you did not get this error with std::vector .