When Do `std::unique_ptr` and `std::shared_ptr` Require a Complete Definition of T?

Is std::unique_ptr Required to Have a Complete Definition of T?

In C programming, the behavior of templates within the standard library often depends on the completeness of the types they instantiate. While most templates require complete types, std::unique_ptr and std::shared_ptr are exceptions.

std::unique_ptr and Incomplete Types

std::unique_ptr allows incomplete types in certain contexts, but not all. Specifically, it requires a complete type when:

• Calling its destructor (~std::unique_ptr)
• Resetting it with a raw pointer (reset(T*))

Otherwise, it can work with incomplete types, such as for:

• Default construction
• Copy and move construction from another std::unique_ptr
• Transfer of ownership via move assignment

std::shared_ptr and Incomplete Types

std::shared_ptr follows similar rules as std::unique_ptr, but with two key differences:

• It requires a complete type when:

  • Constructing it from a raw pointer
  • Calling its destructor (~std::shared_ptr)
  • Resetting it with a raw pointer when the shared_ptr has multiple owners
• It doesn't provide a move assignment operator when working with incomplete types.

Implications for Implementation

These completeness requirements mean that different implementations of std::unique_ptr and std::shared_ptr may handle incomplete types differently. For example, the Visual Studio 2010-SP1 implementation may require a complete definition of the type T to instantiate std::unique_ptr, while other implementations may allow incomplete types.

Standard Requirements

The standard for std::unique_ptr and std::shared_ptr does not explicitly state that they cannot work with incomplete types. However, the requirements on their behavior under certain circumstances imply that a complete type is necessary. This is explained in the C Standard, which states that the validity of certain operations depends on the completeness of the type instantiated with the template.

In conclusion, while std::unique_ptr and std::shared_ptr allow incomplete types in specific contexts, they require complete types for certain operations. This requirement stems from the standard's specifications and may impact the behavior of the templates in different implementations.

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