Assign part of a vector to itself using std::vector::assign()

Question

Let's say I have a vector(v1) of size 10. Now I want to only keep parts of the elements, using:
v1.assign(v1.begin() + 2, v1.begin() + 6);

What I'm not sure is, whether the original elements will be destroyed before assignment. If they are destoyed first, then I cannot rely on the original data.

This page seems to indicate that elements are erased first. However, a quick experiment tells me that the data are correctly assigned to itself.

So what is happening when assigning part of a vector to itself ?

Answer 1

Per the C++14 standard Table 100 — Sequence container requirements (in addition to container)

The expression a.assign(i,j) has a pre-condition that i and j are not iterators into a. Since

v1.assign(v1.begin() + 2, v1.begin() + 6);

Uses iterators that are iterators into v1 so you have violated that pre-condition

If you want to reset the vector to contain a subrange then you can copy those elements into a temporary vector and then assign that temporary vector back to main vector. This should be a move operation(C++11 and above) so no additional copies are made.

v = std::vector<decltype(v)::value_type>(v.begin() + 2, v.begin() + 6);

As pointed out in the comments from Benjamin Lindley and Barry we can use std::copy and move iterators like

std::copy(std::make_move_iterator(v.begin() + 2), std::make_move_iterator(v.begin() + 6), 
          std::make_move_iterator(v.begin()));

Answer 2

From the C++11 Standard:

23.3.6.2 vector constructors, copy, and assignment

template <class InputIterator>
void assign(InputIterator first, InputIterator last);

11 Effects:

erase(begin(), end());
insert(begin(), first, last);

In other words, don't use:

v1.assign(v1.begin() + 2, v1.begin() + 6);

By the time insert is called, first will be an invalid iterator.