How Should I Pass `unique_ptr` Arguments in C Constructors and Functions?

Understanding Unique_ptr Arguments in Constructors and Functions

In C 11, move semantics enable efficient resource management through the unique_ptr class. To effectively handle unique_ptr parameters in constructors or functions, it's crucial to understand different parameter passing mechanisms and their implications.

A. By Value:

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

• The function takes ownership of the unique_ptr, meaning it becomes responsible for managing its lifetime.
• To call this function, one must transfer ownership explicitly using std::move:

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

B. By Non-Const L-Value Reference:

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

• This does not explicitly transfer ownership. The function can access the unique_ptr but may or may not claim it.
• Using a non-const reference allows the function to potentially modify the referenced object.

C. By Const L-Value Reference:

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

• Ownership cannot be transferred. The function can only access the referenced object without modifying or claiming it.

D. By R-Value Reference:

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

• Similar to a non-const l-value reference, but requires std::move when passing an l-value.
• Ownership transfer may or may not occur based on function implementation.

Recommendations:

• To transfer ownership, pass unique_ptr by value.
• To access a unique_ptr without transferring ownership, pass by const l-value reference or pass a reference to the underlying object directly.
• Avoid passing unique_ptr by r-value reference due to potential ambiguity in ownership transfer.

Manipulation of Unique_ptr:

• Copy is prohibited; only movement is allowed using std::move.
• Movement occurs implicitly in constructors through move semantics:

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

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