What is the official behavior for std::vector range constructor when the first itr comes after the last?

c++, stdvector, vector

Solution

It will be undefined behavior.

Looking at the Standard:

N3690 23.3.7.2`[vector.cons]`

template <class InputIterator>
    vector(InputIterator first, InputIterator last,
    const Allocator& = Allocator());

`9.` Effects: Constructs a vector equal to the range [first,last), using the specified allocator.

It says that the range has to be [first, last), but the standard doesn't mention what happens if that isn't the case. It therefore is undefined behavior.

Problem

Assuming you have a valid starting point: ``` std::vector<UINT32> host = {1,2,3,4,5}; ``` When you try to construct another vector using iterators: ``` std::vector<UINT32> client(host.begin(),host.end()); // client.size() is 5. Elements begin -> end look just like host. ``` But what happens if the iterators are backwards? What if the start is after the end? ``` std::vector<UINT32> backwardsClient(host.end(), host.begin()); // What happens? ```

Original source