# unique_ptr private member

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Hello,

Can someone tell me why line (1) does not generate a compiler error while line (2) does?

(1)mAtlasList.begin()->operator*().GetName();
(2)auto IsLevel1Atlas = [](std::unique_ptr<Atlas> atlas) -> bool { return atlas->GetName() == L"Level1Atlas"; };
auto tempAtlasPtr = std::find_if(mAtlasList.begin(), mAtlasList.end(), IsLevel1Atlas);
const Sprite * tempSpritePtr = &(tempAtlasPtr->operator*().GetSpriteList().find(L"Background")->second);

The error generated is: error C2248: 'std::unique_ptr<_Ty>::unique_ptr' : cannot access private member declared in class 'std::unique_ptr<_Ty>'

Thanks.

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IsLevel1Atlas's parameter is a value of unique_ptr, you can't copy a unique_ptr.
Change IsLevel1Atlas to
auto IsLevel1Atlas = [](const std::unique_ptr<Atlas> & atlas) -> bool { return atlas->GetName() == L"Level1Atlas"; };

The first one works I guess your list::push_back must implement rvalue reference (&&) version? Edited by wqking

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wqking,

WOW! Thank you. You really solved my problem this time, and that makes sense.

Best,

-Dave ottley

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Hello again,

Is something like this possible? I am getting the same error here, which leads me to believe that a vector<unique_ptr<somethin> > is not possible:

const Rect GetSourceRect(const std::unique_ptr<Animation>& currentAnimation, uint32_t frameNumInAnimation) const
{
// Calculate position of current animation frame
auto i = std::find(mAnimations.begin(), mAnimations.end(), currentAnimation); // error C2248: 'std::unique_ptr<_Ty>::unique_ptr' : cannot access private member declared in class 'std::unique_ptr<_Ty>'

As you can see I passed the unique_ptr as a const &, but when I use algorithms, there appear to be copies being made here and there. Is there a solution to this?

Thanks,

-Dave Ottley Edited by KingofNoobs

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When running into a problem it is important to post the exact error.

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BitMaster,

Sorry about that. I have edited the post above. Also, the error is:

error C2248: 'std::unique_ptr<_Ty>::unique_ptr' : cannot access private member declared in class 'std::unique_ptr<_Ty>'

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Without knowing the details it is not possible to be sure but does a comparison like
(instance of std::unique_ptr<somethin>) == (instance of std::unique_ptr<Animation>)
make sense at all? You might have to define a custom predicate for whatever it is you want and use std::find_if.

On a sidenote, the error is still not complete the output will contain a few more lines after the actual error codes to define exactly what kind of types T is in each case.

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The original problem was that I was having to default construct animations. For my game, animations come out of a file called an "atlas" file. There should be no way to default construct either an atlas or a sprite, or an animation (the ordering is atlas->sprite->animation, each one owning one or more of the item following it on the list). Thus, when I want to create a SpriteInstance, I have to pass a "starting animation" but rather than default construct one and fill in the fields by hand, I want to force myself to use the "unique" and only copy instantiated by the file. So I thought that changing the atlas and sprite classes to hold pointers to animations rather than animations themselves, that I could solve this problem. Wanting to avoid memory leaks, I thought I would go with the unique_ptr, however, I'm not sure if it is possible given the current errors. I think it should be but don't want to waste too much time on the issue, when I could just use raw pointers and delete.

The full error is below:

error C2248: 'std::unique_ptr<_Ty>::unique_ptr' : cannot access private member declared in class 'std::unique_ptr<_Ty>'
1> with
1> [
1> _Ty=Animation
1> ]
1> c:\program files (x86)\microsoft visual studio 11.0\vc\include\memory(1447) : see declaration of 'std::unique_ptr<_Ty>::unique_ptr'
1> with
1> [
1> _Ty=Animation
1> ]
1> c:\program files (x86)\microsoft visual studio 11.0\vc\include\xmemory0(605) : while compiling class template member function 'void std::allocator<_Ty>::construct(_Ty *,const _Ty &)'
1> with
1> [
1> _Ty=std::unique_ptr<Animation>
1> ]
1> c:\program files (x86)\microsoft visual studio 11.0\vc\include\xmemory0(751) : see reference to function template instantiation 'void std::allocator<_Ty>::construct(_Ty *,const _Ty &)' being compiled
1> with
1> [
1> _Ty=std::unique_ptr<Animation>
1> ]
1> c:\program files (x86)\microsoft visual studio 11.0\vc\include ype_traits(743) : see reference to class template instantiation 'std::allocator<_Ty>' being compiled
1> with
1> [
1> _Ty=std::unique_ptr<Animation>
1> ]
1> c:\program files (x86)\microsoft visual studio 11.0\vc\include\vector(655) : see reference to class template instantiation 'std::is_empty<_Ty>' being compiled
1> with
1> [
1> _Ty=std::allocator<std::unique_ptr<Animation>>
1> ]
1> c:\users\mrmerchantman\dropbox\zzz_c++\projects\blockbuster\source\graphics\sprite.h(37) : see reference to class template instantiation 'std::vector<_Ty>' being compiled
1> with
1> [
1> _Ty=std::unique_ptr<Animation>

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Hello,

I just realized that I am trying to share the pointers, so shared_ptr will be better. Thanks for your help.

-Dave Ottley

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I just want to point out that this

should be

mAtlasList.push_back( std::move( std::unique_ptr<Atlas>(new Atlas(MASTER_ATLAS_MAINMENU)) ) );

But if you really are trying to pass around multiple smart pointers to the same object you should use shared_ptr

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There's no point in adding the std::move() call. The unique_ptr is already an rvalue because it's a temporary.

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There's no point in adding the std::move() call. The unique_ptr is already an rvalue because it's a temporary.

I'm myself still a bit unsure about all the details of rvalues, but wouldn't it be more correct to say "unnamed temporary" because not every temporary automatically is an rvalue?

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No, every temporary is automatically an rvalue; it's part of the definition of rvalue. It's possible to bind a temporary to a named non-rvalue reference, but that named reference is itself not a temporary.

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For the record, I think I found where I got myself confused from back where I read this. An rvalue reference can (easily) become an unexpected lvalue (because it has a name). In the following scenario the std::move is needed to use the move assignment operator:
 void doSomething(X&& x) { X anotherX = std::move(x); // whatever... } 

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Of course in this case neither a temporary unique_ptr nor a rvalue reference to one needs to be used. std::vector::emplace_back() can be used to construct the object in place directly without bothering with a move.
 mAtlasList.emplace_back(new Atlas(MASTER_ATLAS_MAINMENU));