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LWG4148: unique_ptr::operator* should not allow dangling references #118362

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ldionne opened this issue Dec 2, 2024 · 3 comments · May be fixed by #128213
Open

LWG4148: unique_ptr::operator* should not allow dangling references #118362

ldionne opened this issue Dec 2, 2024 · 3 comments · May be fixed by #128213
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@elhewaty
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c++26 libc++ libc++ C++ Standard Library. Not GNU libstdc++. Not libc++abi. lwg-issue this is an issue that was filed to the Library Working Group

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@ldionne
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ldionne commented Dec 2, 2024

Link: https://wg21.link/LWG4148
@ldionne ldionne added c++26 libc++ libc++ C++ Standard Library. Not GNU libstdc++. Not libc++abi. lwg-issue this is an issue that was filed to the Library Working Group labels Dec 2, 2024
@elhewaty
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I think this is already implemented: https://godbolt.org/z/Y8KEY5GK9

when I test on my local machine I get this error

./bin/clang++ -std=c++26 -stdlib=libc++  -nostdinc++ -I ./include/c++/v1   -L ./lib/libc++ -Wl,-rpath,./lib p.cpp
In file included from p.cpp:2:
In file included from ./include/c++/v1/iostream:46:
In file included from ./include/c++/v1/istream:170:
In file included from ./include/c++/v1/__ostream/basic_ostream.h:18:
./include/c++/v1/__memory/unique_ptr.h:265:12: error: returning reference to local temporary object
  265 |     return *__ptr_;
      |            ^~~~~~~
p.cpp:15:11: note: in instantiation of member function 'std::unique_ptr<const int, deleter>::operator*' requested here
   15 |   int i = *p; // undefined
      |           ^
1 error generated.

@frederick-vs-ja
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I think this is already implemented: https://godbolt.org/z/Y8KEY5GK9

This is due to C++26 core language change P2748R5 - Disallow Binding a Returned Glvalue to a Temporary.

But the resolution makes sense in C++23. Perhaps we should add static_assert in C++23 (or an even earlier) mode using __reference_converts_from_temporary (as reference_converts_from_temporary_v isn't implemented yet).

@elhewaty
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Thanks, I added it but the error message is
error: indirection requires pointer operand ('std::unique_ptr<const int, deleter>' invalid)
Is this correct?

I added this to operator*()

requires (!__reference_converts_from_temporary(__add_lvalue_reference_t<_Tp>, decltype(*declval<pointer>())))

@elhewaty elhewaty linked a pull request Feb 21, 2025 that will close this issue
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@elhewaty elhewaty

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c++26 libc++ libc++ C++ Standard Library. Not GNU libstdc++. Not libc++abi. lwg-issue this is an issue that was filed to the Library Working Group
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libc++ Standards Conformance
Status: Todo
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@ldionne @elhewaty @frederick-vs-ja