How to move ownership through unique_ptrs
I have two smart pointers:
Foo *p1 = new Foo;
Foo *p2 = new Foo;
std::unique_ptr<Foo> u1(p1);
std::unique_ptr<Foo> u2(p2);
Now I want to u1
have p2
. And I want u2
to have nothing (or nullptr
). Of course, at the same time p1
must be removed gracefully.
What C ++ code should I write to execute it?
Use std::move
to apply u2
to rvalues, then use move assignment to move u2
to u1
:
u1 = std::move(u2);
After moving there u2
will be nullptr
, although this is a special case for unique_ptr
, don't expect moving from objects to be in any particular state in general. And, of course, it p1
will be gracefully removed.
I recommend that you do not create p1
or p2
and never have any pointers of your own:
std::unique_ptr<Foo> u1(new Foo);
Or better yet, in C ++ 14:
auto u1 = std::make_unique<Foo>();
What you want to do is basically describe the move assignment for unique_ptr
, modulo part of the deleter (not relevant here):
u1 = std::move(u2);
The two versions below will do the same:
u1 = std::move(u2);
and
u1.swap(u2);
u2.reset(nullptr);
although the former is more efficient. u2
will contain nullptr
in both cases.
I am also joining the chorus for use std::make_unique
in C ++ 14 (more efficient and safer in many cases).