I'm not familiar with the STL implementation, but it seems the pop would need to be coherent with the push in an empty stack. Like if a thread was trying to pop

I'm not familiar with the STL implementation, but it seems the pop would need to be coherent with the push in an empty stack. Like if a thread was trying to pop the only entry while another was trying to push, 'intuitively' there'd be a race condition there if STL isn't thread safe (which is the whole point of the article I think).