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[diff.expr] Unmentioned incompatibility with C about pointer indirection CWG2875 #5506

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frederick-vs-ja opened this issue Jun 4, 2022 · 4 comments
Open

[diff.expr] Unmentioned incompatibility with C about pointer indirection CWG2875 #5506

frederick-vs-ja opened this issue Jun 4, 2022 · 4 comments
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cwg Issue must be reviewed by CWG. not-editorial Issue is not deemed editorial; the editorial issue is kept open for tracking.

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@frederick-vs-ja
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frederick-vs-ja commented Jun 4, 2022 •
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These usages of pointer indirection are explicitly allowed in C via WG14 N721, but seemly not allowed in C++.

int* p = &(*((int*)0)); // undefined behavior in C++, well-defined in C (DR076/N721)
int a[10];
int* q = &a[10];        // undefined behavior in C++, well-defined in C (DR076/N721)

Should we mention them in [diff.expr]?
@xmh0511
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xmh0511 commented Jun 6, 2022 •
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This is CWG232. A good question located at https://stackoverflow.com/questions/47227977/is-dereferencing-invalid-pointers-legal-if-no-lvalue-to-rvalue-conversion-occurs/47229011#47229011

@jensmaurer jensmaurer changed the title [diff.expr] Unmentioned incompatibility with C about pointer indirection [diff.expr] Unmentioned incompatibility with C about pointer indirection CWG232 Jun 6, 2022
@jensmaurer jensmaurer added cwg Issue must be reviewed by CWG. not-editorial Issue is not deemed editorial; the editorial issue is kept open for tracking. labels Jun 6, 2022
@frederick-vs-ja
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This is CWG232. A good question located at https://stackoverflow.com/questions/47227977/is-dereferencing-invalid-pointers-legal-if-no-lvalue-to-rvalue-conversion-occurs/47229011#47229011

Thank you!

I find that this change is essentially a DR in C (DR076). And the proposed resolution of CWG232 is significantly more permissive than that of C.

@frederick-vs-ja
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Reopening as CWG232 is closed as NAD.

Perhaps such incompatibility is intended.

@frederick-vs-ja frederick-vs-ja changed the title [diff.expr] Unmentioned incompatibility with C about pointer indirection CWG232 [diff.expr] Unmentioned incompatibility with C about pointer indirection CWG2875 Mar 20, 2024
@frederick-vs-ja
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Addressed by CWG2875.

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