How Should I Pass Unique Pointers as Function or Constructor Arguments in C ?

Managing Unique Pointers as Parameters in Constructors and Functions

Unique pointers (unique_ptr) uphold the principle of unique ownership in C 11. When dealing with unique pointers as function or constructor arguments, several options arise with distinct implications.

Passing by Value:

Base(std::unique_ptr<Base> n)
  : next(std::move(n)) {}

This method transfers ownership of the unique pointer to the function/object. The pointer's contents are moved into the function, leaving the original pointer empty after the operation.

Passing by Non-const L-Value Reference:

Base(std::unique_ptr<Base> &n)
  : next(std::move(n)) {}

Allows the function to both access and potentially claim ownership of the unique pointer. However, this behavior is not guaranteed and requires inspection of the function's implementation to determine its handling of the pointer.

Passing by Const L-Value Reference:

Base(std::unique_ptr<Base> const &n);

Prevents the function from claiming ownership of the unique pointer. The pointer can be accessed but not stored or modified.

Passing by R-Value Reference:

Base(std::unique_ptr<Base> &&n)
  : next(std::move(n)) {}

Comparable to passing by non-const L-Value reference, but requires the use of std::move when passing non-temporary arguments. ownership may or may not be claimed by the function, making it less predictable.

Recommendations:

• Pass by Value: For functions that expect to claim ownership of the unique pointer.
• Pass by Const L-Value Reference: When the function needs temporary access to the pointer.
• Consider Alternative Approaches: Avoid passing by R-Value reference, as it introduces uncertainty about ownership.

Manipulating Unique Pointers:

To move a unique pointer, use std::move. Copying a unique pointer is not allowed:

std::unique_ptr<Base> newPtr(std::move(oldPtr));

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