Is unique_ptr guaranteed to store nullptr after move?

c++ c++11 move-semantics unique-ptr
lizarisk picture lizarisk · Jun 5, 2014 · Viewed 8.3k times · Source

Is unique_ptr guaranteed to store nullptr after move?

std::unique_ptr<int> p1{new int{23}};
std::unique_ptr<int> p2{std::move(p1)};
assert(!p1); // is this always true?

Answer

Praetorian picture Praetorian · Jun 5, 2014

Yes, you can compare it to nullptr after the move and it is guaranteed to compare equal.

From §20.8.1/4 [unique.ptr]

Additionally, u can, upon request, transfer ownership to another unique pointer u2. Upon completion of such a transfer, the following postconditions hold:
— u2.p is equal to the pre-transfer u.p,
— u.p is equal to nullptr, and
...

(the member p is described earlier as — a unique pointer is an object u that stores a pointer to a second object p)

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