std::unique_ptr constructor behaviour

Question

First of all, I know that we should use std::make_unique() instead of calling the std::unique_ptr constructor and I know why.

But I was looking at the documentation of std::unique_ptr to pass the time and sharpen my knowledge about it and I found the following examples about the constructor usage:

// unique_ptr constructor example
#include <iostream>
#include <memory>

int main () {
  std::default_delete<int> d;
  std::unique_ptr<int> u1;
  std::unique_ptr<int> u2 (nullptr);
  std::unique_ptr<int> u3 (new int);
  std::unique_ptr<int> u4 (new int, d);
  std::unique_ptr<int> u5 (new int, std::default_delete<int>());
  std::unique_ptr<int> u6 (std::move(u5));
  std::unique_ptr<int> u7 (std::move(u6));
  std::unique_ptr<int> u8 (std::auto_ptr<int>(new int));

  std::cout << "u1: " << (u1?"not null":"null") << '\n';
  std::cout << "u2: " << (u2?"not null":"null") << '\n';
  std::cout << "u3: " << (u3?"not null":"null") << '\n';
  std::cout << "u4: " << (u4?"not null":"null") << '\n';
  std::cout << "u5: " << (u5?"not null":"null") << '\n';
  std::cout << "u6: " << (u6?"not null":"null") << '\n';
  std::cout << "u7: " << (u7?"not null":"null") << '\n';
  std::cout << "u8: " << (u8?"not null":"null") << '\n';

  return 0;
}

It generates (and I verified it by executing the code) the following results:

u1: null
u2: null
u3: not null
u4: not null
u5: null
u6: null
u7: not null
u8: not null

What I'm struggling to understand is:

• Why is u4 valid while u5 is not (nullptr) ?
• Why is u7 valid but not u6 ?

Perhaps these questions are very basic but I'm completely missing the point.

If anyone could enlighten me about these questions, I would be grateful.

Answer 1

std::move is named move for a reason. When you move from one std::unique_ptr to another, the one you move from becomes nullptr. It couldn't be any other way, it's a unique ptr after all, and two unique_ptr instances sharing the same data would violate that. (Also both going out of scope would call the deleter twice, and then all hell is loose.)