Pass-by-Value vs. Pass-by-Rvalue Reference in Function Parameters

When defining a function that takes an argument of a specific type, you have two primary options: pass-by-value or pass-by-rvalue reference. Pass-by-value creates a copy of the argument, while pass-by-rvalue reference uses the rvalue (temporary) reference to the argument, allowing it to be moved into the function.

Key Differences

Beyond the primary distinction between copying and moving, there are several key differences to consider:

• Control over ownership: Pass-by-value assumes that the function takes ownership of the argument, effectively transferring control from the caller to the function. Pass-by-rvalue reference, on the other hand, leaves the ownership with the caller.
• Explicitness of copying: Pass-by-value hides the cost of copying internally. Pass-by-rvalue reference requires explicit copying by the caller using std::move, forcing developers to be explicit about their intentions.
• Elide potential copy/move: Pass-by-rvalue reference eliminates the need for a single move constructor call in certain cases. However, both pass-by-value and pass-by-rvalue reference allow compilers to elide copies/moves.

Interface Implications

The choice between pass-by-value and pass-by-rvalue reference has implications for the function interface:

• Pass-by-value:

  • Indicates that the function takes ownership of the argument
  • Relieves the caller of managing the argument's lifetime

• Pass-by-rvalue reference:

  • Signals that the caller relinquishes control of the argument
  • Enforces a clear separation of ownership between the caller and function

Efficiency Considerations

The efficiency difference between pass-by-value and pass-by-rvalue reference depends on the semantics of the argument type:

• Large data structures: Pass-by-rvalue reference can significantly improve efficiency if the argument type contains a large data structure that can be cheaply moved. This avoids making an expensive copy.
• Small data structures: For small data structures with minimal contents, there is minimal efficiency difference between pass-by-value and pass-by-rvalue reference.