Inout parameters are a type of function parameter that may have its value updated by the function, rather than returning a new value as a part of the return type. This is similar to, and may be optimized to the similar call by reference concept, but may use value copying rather than direct referencing of the value's location.
In-out parameters are passed as follows:
- When the function is called, the value of the argument is copied.
- In the body of the function, the copy is modified.
- When the function returns, the copy’s value is assigned to the original argument.
This behavior is known as copy-in copy-out or call by value result.
As an optimization, when the argument is a value stored at a physical address in memory, the same memory location may be used both inside and outside the function body. The optimized behavior is known as call by reference; it satisfies all of the requirements of the copy-in copy-out model while removing the overhead of copying. Write your code using the model given by copy-in copy-out, without depending on the call-by-reference optimization, so that it behaves correctly with or without the optimization.1
[1] In-Out Parameters, Swift.org. (2020). https://docs.swift.org/swift-book/ReferenceManual/Declarations.html#ID545 (accessed September 3, 2020).