How Should I Pass Unique Pointers as Function Parameters in C ?

Using Unique Pointers as Function Parameters

Unique pointers provide memory management capabilities in C 11. When passing a unique_ptr as a function argument, there are several approaches to consider, each with distinct implications.

Passing by Value (Ownership Transfer)

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

Calling this constructor requires passing the pointer by value, effectively transferring its ownership to the function:

Base newBase(std::move(nextBase));

After this operation, nextBase will be empty as the unique_ptr's ownership has been transferred.

Passing by Non-const L-Value Reference (Ownership Ambiguity)

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

This approach requires an actual L-value (named variable) as an argument:

Base newBase(nextBase);

In this case, it is uncertain whether the function will claim ownership of the pointer or not. The implementation of Base::Base(std::unique_ptr &n) determines this behavior, making it less transparent.

Passing by Const L-Value Reference (No Ownership Transfer)

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

Passing by const L-value reference prevents the function from claiming ownership of the pointer. The function can access the pointed object but cannot modify its value.

Passing by R-Value Reference (Potential Ownership Transfer)

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

This approach is similar to passing by non-const L-value reference. It allows passing temporary objects:

Base newBase(std::unique_ptr<Base>(new Base)); // Legal

However, it requires the use of std::move when passing non-temporary arguments. The function may or may not claim ownership of the pointer, again making its behavior less evident without examining the implementation.

Recommendations

• Passing by Value: Use this approach when the function should take ownership of the pointer.
• Passing by Const L-Value Reference: Use this approach when the function needs to access the pointer without ownership.
• Avoid Passing by R-Value Reference: This approach can lead to ambiguity in ownership transfer.

Manipulating Unique Pointers

To manipulate unique pointers:

• Use std::move to transfer ownership.
• Do not copy unique pointers.
• Store unique pointers using std::move to prevent ownership issues.

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