std::unique_ptr incomplete type error

Question

I have ``` template class queue { private: struct node { T data; std::unique_ptr next; //compile error on incomplete type node(T&& data_): data(std::move(data_)) {} }; std::unique_ptr head; node* tail; public: queue(): tail(nullptr) {} ``` I get compile error on marked line in VS10. Shouldn't I be allowed to use an incomplete type in this case (instantiate the template - the constructor - here for int as an example) ? Is there a work-around ? EDIT ``` singlethreadedqueue.h(62): error C2079: 'queue::node::next' uses undefined class 'std::unique_ptr<_Ty>' 1> with 1> [ 1> T=MyClass 1> ] 1> and 1> [ 1> _Ty=queue::node 1> ] 1> c:\program files\microsoft visual studio 10.0\vc\include\memory(2161) : see reference to class template instantiation 'queue::node' being compiled 1> with 1> [ 1> T=MyClass 1> ] 1> c:\program files\microsoft visual studio 10.0\vc\include\memory(2195) : see reference to class template instantiation 'std::_Unique_ptr_base<_Ty,_Dx,…

Accepted Answer

The requirement of complete type at the template instantiation point is due to `std::default_delete`, so probably you could work-around it by providing a custom deleter.

```
struct node;         // node_delete need to know 'node' is a type.
struct node_deleter 
{                    // std::unique_ptr needs 'node_deleter' to be complete.
    void operator()(node* ptr);  // forward-reference to avoid needing
};                               //  'delete ptr' before the node is complete.

struct node
{
    std::unique_ptr<node, node_deleter> next;
};

void node_deleter::operator()(node* ptr)
{
     delete ptr;
}
```

Mind you, I have not tested it in MSVC, and probably you should try to upgrade to VC 11, or file a bug to Microsoft if even VC 11 cannot compile your original code.

std::unique_ptr<T> incomplete type error

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