Containers should not be using memset to value-initialize POD types

[Giuseppe D'Angelo]

(Apologies if this is the Nth bug report on the matter, I couldn't find any other.)

Boost.Containers use memset(0) to value-initialize certain types, as documented here. Users have certain macro-based opt-outs in case they don't want this to happen, although the defaults are sane (integers and floating points are zero-initialized via zero-filling on any common implementation).

The problem is that this detection and zero-filling is extended to POD types by default:

If neither BOOST_CONTAINER_MEMZEROED_FLOATING_POINT_IS_NOT_ZERO nor BOOST_CONTAINER_MEMZEROED_POINTER_IS_NOT_ZERO is defined Boost.Container also considers POD types to be value initializable via std::memset with value zero.

This is wrong.

Consider a type like struct S { int S::*pmd; }; , this type is a POD, but zero-filling would not necessarily give a result equivalent to value initialization. On Itanium, pointers to data members are zero-initialized to -1u (source). Zero-filling instead results in code such as

   boost::container::vector<S> v(10);

not correctly value-initializing its contents on Itanium. A complete testcase is here: https://gcc.godbolt.org/z/Ehdefvdro

Unfortunately there's no solution / type trait in the Standard Library actually usable to perform the optimization you want to do here. You need ABI-specific codepaths for where this is safe / allowed, and workarounds for broken compilers. There's some discussion in this draft proposal https://github.com/ecatmur/constexpr-bit-cast-nullptr/blob/main/paper.md (many thanks to Ed Catmur).

In Qt we ended up with an ad-hoc detection (no opt-outs or opt-ins), std::is_scalar_v<T> && !std::is_member_object_pointer_v<T>;, cf. qt/qtbase@e8f5f20

[Giuseppe D'Angelo]

For the record: this was part of some research sponsored by my employer, stemming from the bug(s) in Qt that I've mentioned.

The ultimate goal is to propose to add a type trait to the Standard Library, so that Boost.Container and others can use such a trait avoid ad-hoc detections.

The latest draft at the time of this writing is here https://isocpp.org/files/papers/P2782R0.html , once mailed you'll find the most up-to-date version here https://wg21.link/P2782

[Ion Gaztañaga]

Thanks for the info. If possible, I would thank if R1 can say Boost.Container bug was fixed for Boost 1.82 ;-)

[Giuseppe D'Angelo]

Already done! :-)