Why Does Using `std::unique_ptr` with an Incomplete Type Cause a Compile Error?

std::unique_ptr with an Incomplete Type Compile Error

The std::unique_ptr class is designed to manage the lifetime of objects, and it can be used with both complete and incomplete types. However, when attempting to declare a std::unique_ptr with an incomplete type, as seen in the code snippet below:

class window {
  window(const rectangle& rect);

private:
  class window_impl; // defined elsewhere
  std::unique_ptr<window_impl> impl_; // won't compile
};</window_impl>

a compile error regarding the use of an incomplete type may occur. This error stems from the fact that when using pimpl with std::unique_ptr, a destructor must be declared:

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

public:
  foo(); // You may need a def. constructor to be defined elsewhere

  ~foo(); // Implement (with {}, or with = default;) where impl is complete
};</impl>

If a destructor is not explicitly declared, the compiler generates a default one, which requires a complete declaration of the foo::impl class. This can lead to compile errors when working with incomplete types.

To resolve this issue, implement a destructor for the class, either with an empty body or by setting it to default, as seen in the code snippet above. This will ensure that the compiler can properly handle the destruction of the object managed by the std::unique_ptr.

Additionally, using std::unique_ptr with incomplete types is not supported at namespace scope. Instead, a workaround is available by wrapping the std::unique_ptr within a custom struct that provides a destructor, as demonstrated in the code snippet below:

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

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