One thing that i didn't see in other answers is To clarify another answers that there is a difference between returning std::unique_ptr that has been created within a function, and one that has been given to that function.
The example could be like this:
class Test
{int i;};
std::unique_ptr<Test> foo1()
{
std::unique_ptr<Test> res(new Test);
return res;
}
std::unique_ptr<Test> foo2(std::unique_ptr<Test>&& t)
{
// return t; // this will produce an error!
return std::move(t);
}
//...
auto test1=foo1();
auto test2=foo2(std::unique_ptr<Test>(new Test));