Passing a temporary unique_ptr to a constructor in VS2010

Question

I have a class that I want to be able to construct with a temporary unique_ptr, like this:

MyCollection foo(std::unique_ptr<MyObj>(nullptr));

The object should take ownership of the pointer. My question is, what is the correct constructor signature for this?

1. MyCollection(std::unique_ptr<MyObj> foo);
2. MyCollection(std::unique_ptr<MyObj>&& foo);

The first option does not link. The second does, however what happens if I go with this, and then try to construct MyCollection with a non R-value? ie

std::unique_ptr<MyObj> pointer(nullptr);
MyCollection(pointer);

The answer here: How do I pass a unique_ptr argument to a constructor or a function? suggests that I should take the unique_ptr by value, but as I said above, it does not link in VS2010 (the error looks like this...

Error   5   error LNK2028: unresolved token (0A00075A) "private: __thiscall std::unique_ptr<class IVDSDocCore,struct std::default_delete<class IVDSDocCore> >::unique_ptr<class IVDSDocCore,struct std::default_delete<class IVDSDocCore> >(class std::unique_ptr<class IVDSDocCore,struct std::default_delete<class IVDSDocCore> > const &)" (??0?$unique_ptr@VIVDSDocCore@@U?$default_delete@VIVDSDocCore@@@std@@@std@@$$FAAE@ABV01@@Z) referenced in function "public: static void __clrcall std::unique_ptr<class IVDSDocCore,struct std::default_delete<class IVDSDocCore> >::<MarshalCopy>(class std::unique_ptr<class IVDSDocCore,struct std::default_delete<class IVDSDocCore> > *,class std::unique_ptr<class IVDSDocCore,struct std::default_delete<class IVDSDocCore> > *)" (?<MarshalCopy>@?$unique_ptr@VIVDSDocCore@@U?$default_delete@VIVDSDocCore@@@std@@@std@@$$FSMXPAV12@0@Z)  C:\sviluppo\FerrariGes\GesDB\VDS.NET\VDS\vdsdoc.obj

Some replies suggest I need to use the move function. If I use constructor 1, and try to create the object like this:

If I use constructor 1, and try to create the object like this:

MyCollection foo(move(std::unique_ptr<MyObj>(nullptr)));

I get the same link error.

the compiler rightfully complains: it doesn't move from under your feet. You have to be explicit and use `std::move(pointer)`. — juanchopanza, Feb 28 '13 at 15:24
juan - how do I use the move? see my edited question. I thought you didn't have to move temporary values? — Tom Davies, Feb 28 '13 at 15:26
Has it occurred to you that you're not getting that error because of the constructor prototype in question? Or that you're getting it because of what is *in* the constructor? — Nicol Bolas, Feb 28 '13 at 15:31
My only explanation at the moment is that I'm using VS2010, which is not fully compliant with C++11 yet. — Tom Davies, Feb 28 '13 at 15:45

score 0 · Answer 1 · edited May 23 '17 at 12:21

Youa re right that you should accept the unique_ptr by value, but since it cannot be copied, you have to move it into the constructor:

MyCollection(std::unique_ptr<MyObj> foo);

//..
std::unique_ptr<MyObj> pointer(/*something*/);
MyCollection(std::move(pointer));

Edit my bad, I just noticed I overread the first use of the constructor.

MyCollection foo(std::unique_ptr<MyObj>(nullptr));

should work, of course, with the constructor taking the unique_ptr by value.

However, the function signature mentioned in the linker error looks a bit weird to me, it basically is

void __clrcall unique_ptr<X>::<MarshalCopy>(unique_ptr<X>*, unique_ptr<X>*)

So maybe it's a completely different issue. I'm not very firm with CLR, so forgive me if I am wrong: Are you using unique_ptr in C++/CLI? Is that supported? What does <MarshalCopy> do internally - looks like something generated, maybe it does not work for move-only types?

This could be related:Linker error when using unique_ptr in C++/CLI

Yes, this is my understanding. So, I'm confused about the link error! — Tom Davies, Feb 28 '13 at 15:35
I think it must be related to this. The app is a mix of managed and unmanaged C++. As in the link you posted, using the && constructor works ok. No idea why, and no time to figure it out now. Thanks. — Tom Davies, Feb 28 '13 at 16:14

Passing a temporary unique_ptr to a constructor in VS2010

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