When Does `std::unique_ptr` Require a Complete Type Definition?

std::unique_ptr: Dependency on Complete Type Definition

In C , it's generally required that templates in the standard library be instantiated with complete types. However, std::unique_ptr and std::shared_ptr are notable exceptions. They allow for partial instantiation with incomplete types, but certain operations necessitate complete types.

This distinction arises from potential undefined behavior when deleting an object of an incomplete type. Smart pointers like std::unique_ptr and std::shared_ptr aim to prevent this by requiring complete types where necessary, ensuring that destructors are invoked correctly.

Type Completeness Requirements for std::unique_ptr

std::unique_ptr requires complete types in specific instances, as summarized below:

**Operation

Complete Type Requirement**

Default constructor

Incomplete

Copy constructor

N/A (unique_ptr does not have a copy constructor)

Move constructor

Incomplete

Destructor

**Operation	Complete Type Requirement**
Default constructor	Incomplete
Copy constructor	N/A (unique_ptr does not have a copy constructor)
Move constructor	Incomplete
Destructor	Complete
Constructor from pointer	Incomplete
Copy assignment	N/A (unique_ptr does not have a copy assignment operator)
Move assignment	Complete
reset()	Complete
reset(A*)	Complete

Complete

Constructor from pointer

Incomplete

Copy assignment

N/A (unique_ptr does not have a copy assignment operator)

Move assignment

Complete

reset()

Complete

reset(A*)

Complete

Differences from std::shared_ptr

std::shared_ptr and std::unique_ptr differ in their completeness requirements. std::shared_ptr allows incomplete types in additional operations, including the copy constructor, copy assignment, and destructor. This is due to its use of a dynamic deleter, while std::unique_ptr employs a static deleter.ConclusionDespite the partial exceptions for std::unique_ptr and std::shared_ptr, it's crucial to ensure that complete types are used when necessary. The compiler will generate compile-time errors if incomplete types are employed in such cases. This helps prevent undefined behavior and ensures the correct execution of destructors.

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