CODEWITHSUNDEEP
c++c++11unique-ptr
ibp73
ibp73

Reputation: 650

Copy semantics for C++ unique pointer

Is there something wrong if I write something like this:

Try<std::unique_ptr<int> > some_function() {
  std::unique_ptr<int> s(new int(2));
  return s;
}

Is the copy constructor invoked? Should I use std::move?

Upvotes: 2

Views: 410

Answers (2)

David G
David G

Reputation: 96810

In a return statement, overload resolution can be performed as if the id-expression in the return statement designates an rvalue:

When the criteria for elision of a copy/move operation are met, [..], or when the expression in a return statement is a (possibly parenthesized) id-expression that names an object with automatic storage duration declared in the body [..], overload resolution to select the constructor for the copy is first performed as if the object were designated by an rvalue.

So your case does indeed qualify for NRVO since s is declared in the body of the function, thus std::move() is not required as overload-resolution can treat s as an rvalue.

Note that std::move() might still be needed if your compiler doesn't support the first phase of overload resolution treating s as an rvalue when the type of the return expression doesn't have the same cv-unqualified type as the function's return type. This seems to be the case with the trunk version of clang but not gcc. More info in this thread.

Upvotes: 3

Cornstalks
Cornstalks

Reputation: 38218

std::unique_ptr doesn't have a copy constructor. What you're doing there is the same as assigning with a unique_ptr: the pointer is moved. (though in some situations you have to explicitly move() the pointer or else you'll get a compilation error; but if the compiler doesn't complain with an error, then it's quietly moving the pointer)

Upvotes: 3

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