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[expr.reinterpret.cast] Fix a note #4080

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merged 1 commit into from Jul 16, 2020
Merged

[expr.reinterpret.cast] Fix a note #4080

merged 1 commit into from Jul 16, 2020

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languagelawyer
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struct S { int i; } s;

auto fp = reinterpret_cast<float*>(&s); // the value of `fp` is  "pointer to s"

reinterpret_cast<float*>(reinterpret_cast<int*>(fp));
// converting to `int*` and back to `float*` does NOT yield the original pointer value,
// because the value became "pointer to s.i" when casting to `int*`.

@hubert-reinterpretcast

@hubert-reinterpretcast
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Would it make sense to try to salvage the note by indicating that the resulting pointer could also be a pointer to an object that is pointer-interconvertible with the object (if any) to which the original pointer points?

@jensmaurer
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It sounds reasonable to highlight the round-trip guarantee, but that needs a bit of wording surgery. Maybe just avoiding phrasing in terms of types would help. "If a pointer to a type T1 is converted to type "pointer to T2" and back to "pointer to T1" and (the stars are aligned correctly), you get the original pointer value."

@hubert-reinterpretcast
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I think just making the original pointer value something more specific works:
Converting a prvalue of type "pointer to T1" that points to an object of type T1 [ ... ]

@languagelawyer languagelawyer changed the title [expr.reinterpret.cast] Remove an incorrect note [expr.reinterpret.cast] Fix a note Jul 16, 2020
@languagelawyer languagelawyer force-pushed the reinterpretcast branch 3 times, most recently from b4ff6ae to 29a242a Compare July 16, 2020 14:26
@languagelawyer
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I didn't like "a prvalue that points" and rewrote as "a pointer that points", also, since T1 became known to be an object type, I had to change the "where" part. I also decided to remove parens and reflow the sentence.

@hubert-reinterpretcast
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LGTM, but I have no commit access.

@jensmaurer
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@hubert-reinterpretcast , thanks for the review.

@jensmaurer jensmaurer merged commit b1df284 into cplusplus:master Jul 16, 2020
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@languagelawyer @hubert-reinterpretcast @jensmaurer