When Should You Use Pass By Value vs Pass By Rvalue Reference?

Pass By Value vs Pass By Rvalue Reference

When to Declare:

In situations where the function requires ownership of the passed parameter, declaring the function as:

• void foo(Widget w); passes by value, requiring implicit copying of the passed argument.
• void foo(Widget&& w); passes by rvalue reference, allowing for explicit moves of the passed argument.

Key Differences:

1. Copy Control:

• Pass by value: Implicit copying occurs, potentially introducing additional costs.
• Pass by rvalue reference: Explicit moves are required for copy construction, forcing the caller to manage copies.

2. Interface Semantics:

• Value parameter: The function signifies that it wants a copy of the parameter to operate on.
• Rvalue reference parameter: The function intends to take ownership of the argument and may make modifications.

3. Efficiency:

• Pass by value: Can eliminate a single move constructor call.
• Pass by rvalue reference: Compiler optimizations can eliminate copies or moves on the caller side.
• However, efficiency gains may be negligible when the passed parameter doesn't have large or expensive members.

Interface Implications:

Rvalue reference parameters convey the following intentions:

• The function claims ownership of the passed value.
• The function may modify the passed value without affecting the caller's reference.
• The caller relinquishes all ownership and responsibility for the passed value.

In contrast, value parameters indicate that:

• The function operates on a copy of the passed value.
• The caller retains ownership of the passed value and any changes made by the function do not affect it.

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