Passing by Value or Reference: When Does \'Want Speed? Pass by Value\' Actually Hold True?

Performance Implications of "Want Speed? Pass by Value"

The "Want speed? Pass by value" principle suggests that passing arguments by value can sometimes enhance performance by allowing the compiler to optimize the copying process. However, this principle has sparked discussions and counter-arguments.

In the case of structures X and Y with string members, the behavior varies:

• X: Constructs a temporary object from the argument using a copy, then moves it into the member variable. If the argument is an rvalue, Return Value Optimization (RVO) may eliminate the copy.
• Y: Binds a constant reference to the argument, but still copies the string into the member variable.

Therefore, passing an rvalue to X can potentially result in only a move, while passing it to Y always requires a copy.

In general, passing by value may perform better than passing by reference for rvalues, but comparable to passing by reference for lvalues. This is because a move is typically comparable in time to passing a pointer (as in the case of references).

However, it's important to note that this principle is not a hard rule and should be considered on a case-by-case basis. Profiling can provide valuable insights into the actual performance impact of different passing mechanisms.

Additionally, passing objects by value has drawbacks, such as increased memory usage and potential for performance degradation when dealing with large objects. Therefore, the trade-offs should be carefully evaluated when making this decision.

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