C++ vector resize elements re-ordered?

Question

If my vector starts out with some information in it such as:

vector<int> ordered_set;
ordered_set.resize(5);
for (int counter=0; counter<5; counter++){
ordered_set[counter]=counter+1;
}

Then I later resize it as:

ordered_set.resize(10,0);

will the 1 through 5 still be guaranteed as the first five elements through the pointer arithmetic ordered_set[0,1,2..4]? or does the standard enable the contents to permute if contiguous memory is not found for the resize and a reallocation is required? In other words will ordered_set[0,1,2..4] potentially encounter a 0?

Answer 1

A std::vector is defined as a contiguous container of elements. If the vector's allocator cannot provide enough memory when resizing, the vector won't "fracture". It cannot maintain its invariant so the operation simply won't finish, on account of an exception being thrown.

As for pointer arithmetic, you can be sure that traversing the vector after resizing will produce the same integer values you previously placed there. But be sure to not use any pointers or iterators you obtained prior to resizing, as those would have become invalid.