Why Does `std::unique_ptr` Fail with Incomplete Types, and How Can I Fix It?

Incomplete Type Obstacle for std::unique_ptr

Utilizing std::unique_ptr with an incomplete type raises compilation concerns. When employing the pimpl idiom with std::unique_ptr, code resembling the following may trigger such issues:

class window {
  window(const rectangle& rect);

private:
  class window_impl; // defined elsewhere
  std::unique_ptr<window_impl> impl_; // compilation failure
};

A compilation error regarding an incomplete type is encountered, as std::unique_ptr expects a complete type definition.

Unveiling the Root Cause: Destruction Deficiencies

Despite expected compatibility between std::unique_ptr and incomplete types, the problem stems from the lack of a destructor declaration. Default constructors generated by the compiler require a complete declaration of the implemented class.

To rectify this issue, explicitly define a destructor in the outer class, ensuring its availability when the implemented class is complete:

class foo
{ 
    class impl;
    std::unique_ptr<impl> impl_;

public:
    foo(); // May require a default constructor defined elsewhere

    ~foo(); // Implement with an empty body or use "= default;"
};

Caveats with Template Constructors

Defining a template constructor complicates matters, even if the implemented member is left uninitialized:

template <typename T>
foo::foo(T bar) 
{
    // Compiler requires knowledge of impl_ destruction in case of exceptions
}

Namespace-Level Restrictions

Using std::unique_ptr with incomplete types at namespace scope also poses challenges:

class impl;
std::unique_ptr<impl> impl_;

The compiler needs to know how to destroy the static duration object. A remedy lies in wrapping the std::unique_ptr within a custom struct with a defined destructor:

class impl;
struct ptr_impl : std::unique_ptr<impl>
{
    ~ptr_impl(); // Implement with an empty body
} impl_;

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