Can `std::unique_ptr` Be Used with Incomplete Types?

Can std::unique_ptr Operate with Incomplete Types?

In C , certain templates in the standard library require complete types for instantiation. However, std::shared_ptr and std::unique_ptr are partial exceptions, allowing instantiation with incomplete types for specific members.

Reasoning Behind Partial Exception:

To avoid undefined behavior that could arise from deleting incomplete types using raw pointers, std::shared_ptr and std::unique_ptr permit incomplete types in certain instances. This enables idioms like using smart pointers with PIMPL while minimizing risk.

Unique_ptr Requirements for Incomplete Types:

Unlike shared_ptr, unique_ptr requires complete types in a more limited capacity, when ~P(), reset(), and move assignment operator=() are used.

Shared_ptr Requirements for Incomplete Types:

Shared_ptr can be instantiated with incomplete types when used in its default constructor P(), copy constructor P(const P&), and move constructor P(P&&). However, operations like reset() and move assignment =() necessitate complete types.

Conclusion:

While unique_ptr and shared_ptr both offer partial support for incomplete types, their requirements vary. It is essential to understand these requirements to avoid undefined behavior and ensure efficient usage. In case of errors related to forward declarations, it's worth considering the possibility of implementation-specific behaviors.

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