Does `std::unique_ptr` Need a Complete Definition of T?

Is std::unique_ptr Required to Know the Full Definition of T?

Is std::unique_ptr required to have a complete definition of the type T it manages? This question arises from observations that std::unique_ptr fails to compile in Visual Studio 2010 SP1 when Thing is forward-declared.

Implementation-Dependent Behavior in Visual Studio 2010 SP1

The behavior in Visual Studio 2010 SP1 suggests that its implementation of std::unique_ptr requires a complete definition of Thing. This is not a standard requirement, but rather an implementation-specific choice.

Partial Exceptions in the Standard Library

While most C standard library templates require complete types, std::shared_ptr and std::unique_ptr are partial exceptions. Some of their functions can be instantiated with incomplete types, due to their role in supporting idioms like the pointer-to-implementation (pimpl) pattern. However, using incomplete types in certain operations can lead to undefined behavior, such as deleting an incomplete object.

Type Completeness Requirements for Different Operations

std::unique_ptr and std::shared_ptr require complete types in different situations. For example, ~std::unique_ptr requires a complete type for its destructor, while std::unique_ptr(A*) allows an incomplete type if the pointer is being taken over from. A table summarizes these requirements:

Operation	std::unique_ptr	std::shared_ptr
Default constructor	incomplete	incomplete
Copy constructor	-	incomplete
Move constructor	incomplete	incomplete
Destructor	complete	incomplete
Constructor from pointer	incomplete	complete
Copy assignment	-	incomplete
Move assignment	complete	incomplete
Reset without argument	complete	incomplete
Reset with argument	complete	complete

In conclusion, while std::unique_ptr does not inherently require a complete type definition in the C standard, some implementations (like Visual Studio 2010 SP1) may impose such a requirement. It is important to be aware of the specific requirements of the implementation being used to avoid undefined behavior.

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