When to Use Pass by Value vs Pass by Rvalue Reference?

Understanding Pass by Value vs Pass by Rvalue Reference

When defining a function's parameter, the choice between pass by value and pass by rvalue reference can significantly impact the interface and efficiency of the function.

Pass by Value vs Pass by Rvalue Reference

In pass by value, a copy of the argument is created within the function. In pass by rvalue reference, a reference to the original argument is created, allowing direct manipulation of the argument.

Distinction in Interface

The use of an rvalue reference parameter conveys the following message to the caller:

• The function requires ownership of the argument.
• The function may modify the argument, and the caller should not rely on the original state.

On the other hand, pass by value indicates that:

• The function creates its own local copy of the argument.
• The caller can continue to use the original argument independently of the function's actions.

Efficiency Considerations

• No Elision: Using pass by rvalue reference eliminates the need for a move constructor call, which can improve performance.
• Compiler Optimization: Pass by value may provide better opportunities for compiler optimizations, as it can safely elide copies or moves. However, this claim requires concrete evidence through code optimization analysis.

Additional Considerations

• Pass by rvalue reference allows the argument to be moved, but does not mandate it.
• Pass by value ensures that a move has occurred, assuming no elisions.
• Pass by value can be less efficient when handling large data structures with pointers to external data. Rvalue references offer performance benefits in such scenarios by allowing direct access to the pointed-to contents.

Application Guidelines

• Use pass by value when you don't require ownership of the argument and want to preserve its original state.
• Use pass by rvalue reference when you intend to take ownership of the argument and may modify it.
• Consider performance implications when dealing with large data structures with external data pointers, as pass by rvalue reference offers potential performance advantages.

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