Why am I getting "vector iterators incompatible"?

Question

Why does this code

#include <algorithm>
#include <iterator>
#include <vector>

int main()
{
    std::vector<int> v;
    v.push_back(1);
    v.push_back(2);
    v.push_back(3);
    v.reserve(v.size() * 2);  // Reserve enough space to keep iterators valid
    std::copy(v.begin(), v.end(), std::back_inserter(v));
    return 0;
}

give me the debug assertion failure, Expression: vector iterators incompatible (Visual C++ 2008)?

Answer 1

Iterators corresponding to elements are only invalidated when the vector has to be reallocated, which reserve avoids.

However, v.end() won't stay valid.

The Standard's description of push_back and insert guarantees that

Causes reallocation if the new size is greater than the old capacity. If no reallocation happens, all the iterators and references before the insertion point remain valid.

v.end() is not "before the insertion point".