迭代器无法访问完整向量

Question

int main()
{
    vector<int> vi;

    vi.reserve(10);
   // back_insert_iterator<vector<int> > iter(vi);

    vector<int>::iterator iter = vi.begin();

    *iter = 1;
    ++iter;
    *iter = 2;
    ++iter;
    *iter = 3;

    back_insert_iterator<std::__1::vector<int> > iterb(vi);
    back_inserter(vi) = 4;
    back_inserter(vi) = 5;

    vi.reserve(vi.size() * 2);
    copy(vi.begin(), vi.end(), vi.end());

    ostream_iterator<int> os(cout, " ");
    copy(vi.begin(), vi.end(), os);
}

Question 1, why does cout print 4 5 , when I expected it will print 1 2 3 4 5 1 2 3 4 5 ? Question 2, when I replace it with copy(vi.begin(), vi.end(), back_insert(vi)) , it will print 4 5 4 5 , why?

Answer 1

Your code has undefined behavior, because you assign to iterators which are at or beyond the end. The reserve() function does not update the size, only the capacity. You might try resize() instead.

Answer 2

copy(vi.begin(),vi.end(),vi.end()) doesn't work because std::copy writes to the destination iterator, and you can't write to the end iterator.

copy(vi.begin(),vi.end(),back_insert(vi)) doesn't work because the back_insert_iterator is changing the vector as it inserts, which invalidates the source iterators.

You might try this instead:

 size_t vi_size = vi.size();
 vi.resize(vi_size*2);
 copy(vi.begin(),vi.begin()+vi_size,vi.begin()+vi_size);