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[SR-12942] In-Out documentation is misleading or wrong.
| Previous ID | SR-12942 |
| Radar | None |
| Original Reporter | erickpc (JIRA User) |
| Type | Bug |
Attachment: Download
Additional Detail from JIRA
| Votes | 0 |
| Component/s | |
| Labels | Bug, Documentation |
| Assignee | None |
| Priority | Medium |
md5: 40885081a51d49565b9dd89413fc2e4b
Issue Description:
The documentation here https://docs.swift.org/swift-book/ReferenceManual/Declarations.html#ID545 states:
In-out parameters are passed as follows:
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When the function is called, the value of the argument is copied.
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In the body of the function, the copy is modified.
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When the function returns, the copy’s value is assigned to the original argument.
By reading that, if any changes are made to the argument outside of the function between. step 1 and 3, those changes would be lost when the function returns. The scenario I'm describing would be:
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Pass array a `[1, 2, 3]` as an In-Out argument to function `f()`
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While `f()` is executing, outside the function, `a` gets modified to add another value.
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At this point `a` is `[1, 2, 3, 4]`
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When `f()` finishes executing the copy of `a` copied back to the calling context, losing the last value 4.
That's the expected behavior according to the documentation. I have this test case showing it doesn't work that way. Down below the documentation mentions an "optimization" that "satisfies all of the requirements of the copy-in copy-out model while removing the overhead of copying." That last part is misleading. This "optimization" explains the behavior I'm seeing, but if I were to "Write your code using the model given by copy-in copy-out, without depending on the call-by-reference optimization" I would be surprised the system is not working as expected.
Attached is the test case
PS: The double quotes above are to show what's in the documentation verbatim.
