unique_ptr constructor with rvalue deleter returns null?

Question

Here's a code snippet I'm using to understand the behavior of std::unique_ptr:

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';

Can somebody explain why null is printed for u5 and u6, but not for u7? Thanks in advance!

std::unique_ptr always has a single owner. When you move the ownership, the original pointer become null. So if you insert printing statements between some declaration, it would become obvious when value become null. Using a debugger is another way to help understand code as it would be reading the documentation. — Phil1970
– Phil1970, Commented Jan 12, 2018 at 2:53
thanks a lot for the explanation, I was scratching my head wondering why, now I see it. — Andy
– Andy, Commented Jan 13, 2018 at 3:18

TobiMcNamobi · Accepted Answer · 2018-01-12 07:53:02Z

As Phil1970 pointed out in his comment a std::unique_pointer instance becomes (equivalent to) nullptr after it is assigned to another instance (using std::move as seen in your code example). Actually this is what std::unique_ptr is made for. It is, well, unique.

Now consider this piece of code:

#include <memory> #include <iostream> int main() { std::default_delete<int> d; std::unique_ptr<int> u1; std::cout << "u1: " << (u1 ? "not null" : "null") << '\n'; std::unique_ptr<int> u2(nullptr); std::cout << "u2: " << (u2 ? "not null" : "null") << '\n'; std::unique_ptr<int> u3(new int); std::cout << "u3: " << (u3 ? "not null" : "null") << '\n'; std::unique_ptr<int> u4(new int, d); std::cout << "u4: " << (u4 ? "not null" : "null") << '\n'; std::unique_ptr<int> u5(new int, std::default_delete<int>()); std::cout << "u5: " << (u5 ? "not null" : "null") << '\n'; std::unique_ptr<int> u6(std::move(u5)); std::cout << "u6: " << (u6 ? "not null" : "null") << '\n'; std::unique_ptr<int> u7(std::move(u6)); std::cout << "u7: " << (u7 ? "not null" : "null") << '\n'; }

When executed you see that u5 and u6 are "not null" immediately after instantiation.

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