C ++的向量元素的顺序

Question

The following piece of c++ code gives

int main()
{
    vector <int> myvect(3,0);
    vector <int> :: iterator it;
    it = myvect.begin();
    myvect.insert(it,200);
    myvect.insert(it+5,400);         //Not sure what 5 makes the difference here
    cout << myvect[0] << endl << myvect[1];
}

Output :

200
400

And the same code with minor changes gives

int main()
{
    vector <int> myvect(3,0);
    vector <int> :: iterator it;
    it = myvect.begin();
    myvect.insert(it,200);
    myvect.insert(it+4,400);         //Not sure what 4 makes the difference here
    cout << myvect[0] << endl << myvect[1];
}

Output:

400
200

Can someone tell me why adding 4 or 5 to the iterator changes the order of elements?

Thanks

Answer 1

Your program has Undefined Behavior .

You are creating a vector of 3 elements (all initialized to 0), and you are inserting elements at position v.begin() + 5 , which is beyond the end of the vector.

Moreover, you are using an iterator ( it) after inserting an element before the position it points to. According to Paragraph 23.3.6.5/1 of the C++11 Standard:

[...] If no reallocation happens, all the iterators and references before the insertion point remain valid. [...]

Therefore, iterator it itself is not guaranteed to be valid after the statement myvect.insert(it, 200) , and using it in the next instruction ( myvect.insert(it + 4, 400) ) is again Undefined Behavior.

You cannot expect anything of a program with Undefined Behavior. It may crash, give you bizarre results, or (in the worst case) behave just as you would expect.

Answer 2

The member function vector::insert(const_iterator, const value_type&) requires a valid iterator that refers to the vector but it+4 and it+5 are not valid iterators.

Before the first insertion, it+3 is a valid (non-dereferencable) iterator, pointing just past-the-end of the vector sequence, but it+4 is invalid. After the insertion it might get invalidated, in which case no expression using it is valid, certainly not it+5 because the sequence only has four elements at that point.

The code would be valid if changed like so:

it = myvect.begin();
myvect.insert(it,200);
it = myvect.begin();     // make it valid again
myvect.insert(it+4,400);