Vector read functions don't use command queue

[Slonegg]

I've noticed inconsistent behavior when reading values from vector using .back() routine. Digging it I've realized that essentially most of the vector read routines rely on the buffer_value class to read values from buffer and it uses some dummy command queue.

    operator value_type() const
    {
        if(m_buffer.get()){
            const context &context = m_buffer.get_context();
            const device &device = context.get_device();
            command_queue queue(context, device);

            return detail::read_single_value<T>(m_buffer, m_index / sizeof(T), queue);
        }
        else {
            return m_value;
        }
}

I suggest to add command queue argument to read routines, this way you can ensure that operations on the buffer have been performed before actually reading values from it.

[jszuppe]

I guess there's nothing wrong in adding front(), back() and at() functions that take command queue argument.

[jszuppe]

After looking at the code, I think we can't do it without adding command_queue to buffer_value class.

I've noticed inconsistent behavior when reading values from vector using .back() routine.

I think commit https://github.com/boostorg/compute/pull/710/commits/21ff71e2814c3c2e61e4bdc858ac6ecad030ab1d should fix your problem.