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

std::unique_ptr with Incomplete Types: Exploring the Compiler Error

The std::unique_ptr is a powerful tool for managing dynamic memory, and it is often used with the private implementation, or "pimpl" idiom, to improve code organization and encapsulation. However, when attempting to use std::unique_ptr with an incomplete type, such as the example provided, you may encounter a compiler error.

The issue stems from the fact that the compiler requires a complete definition of the type being stored in the unique_ptr, in this case, the window_impl class. However, it is essential to note that std::unique_ptr itself does not have any requirements for the type it stores, including completeness.

To address this error, we need to understand the role of the destructor in this context. When a pointer stored in std::unique_ptr is destroyed, the destructor of the stored type is called to free its resources. In the provided code, the destructor for the window_impl class is defined elsewhere, preventing the compiler from generating a default destructor for this class.

To resolve the issue, you should explicitly define a destructor for the window class and ensure the definition is visible to the compiler before the use of std::unique_ptr. This destructor can be either empty or can call the destructor of the window_impl class, if needed.

By implementing a destructor for the window class, the compiler has the complete information it requires to handle the destruction of the stored window_impl object, allowing you to successfully use std::unique_ptr with an incomplete type as desired.

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