p0476r2
Bit-casting object representations

This paper is a revision of [P0476r1], addressing LEWG comments from the 2017 Toronto meeting as well as comments from LEWG and LWG from the 2017 Albuquerque meeting. See §4 Revision History for details.

1. Background

Low-level code often seeks to interpret objects of one type as another: keep the same bits, but obtain an object of a different type. Doing so correctly is error-prone: using reinterpret_cast or union runs afoul of type-aliasing rules yet these are the intuitive solutions developers mistakenly turn to.

Attuned developers use aligned_storage with memcpy, avoiding alignment pitfalls and allowing them to bit-cast non-default-constructible types.

This proposal uses appropriate concepts to prevent misuse. As the sample implementation demonstrates we could as well use static_assert or template SFINAE, but the timing of this library feature will likely coincide with concept’s standardization.

Furthermore, it is currently impossible to implement a constexpr bit-cast function, as memcpy itself isn’t constexpr. Marking the proposed function as constexpr doesn’t require or prevent memcpy from becoming constexpr, but requires compiler support. This leaves implementations free to use their own internal solution (e.g. LLVM has a bitcast opcode).

We should standardize this oft-used idiom, and avoid the pitfalls once and for all.

2. Proposed Wording

Below, substitute the � character with a number or name the editor finds appropriate for the sub-section.

In 20.5.1.2 [headers] add the header <bit> to:

• Table 16 — C++ library headers

• Table 19 — C++ headers for freestanding implementations

In the numerics section, add the following:

namespace std {
  
  // 29.�.3 bit_cast
  template<typename To, typename From>
  constexpr To bit_cast(const From& from) noexcept;
  
}

template<typename To, typename From>
constexpr To bit_cast(const From& from) noexcept;

2.5. Feature testing

The __cpp_lib_bit_cast feature test macro should be added.

3. Appendix

The Standard’s [basic.types] section explicitly blesses memcpy:

For any trivially copyable type T, if two pointers to T point to distinct T objects obj1 and obj2, where neither obj1 nor obj2 is a base-class subobject, if the underlying bytes (1.7) making up obj1 are copied into obj2, obj2 shall subsequently hold the same value as obj1.

[Example:

    T* t1p;
    T* t2p;
    // provided that t2p points to an initialized object ...
    std::memcpy(t1p, t2p, sizeof(T));
    // at this point, every subobject of trivially copyable type in *t1p contains
    // the same value as the corresponding subobject in *t2p

— end example]

Whereas section [class.union] says:

In a union, at most one of the non-static data members can be active at any time, that is, the value of at most one of the non-static data members can be stored in a union at any time.

4. Revision History

4.1. r1 ➡ r2

The paper was reviewed by LEWG at the 2017 Toronto meeting and feedback was provided. In the 2017 Albuquerque meeting LEWG provided feedback regarding usage of concepts while discussing [P0802r0], and EWG reviewed the paper:

• Use "shall not participate in overload resolution" wording instead of a requires clause.

• The author was asked to explore naming. LEWG took a poll in Albuquerque and voted to keep bit_cast.

• There was strong sentiment that this facility should be available in freestanding implementations. LEWG is changing its guidance regarding freestanding header granularity, but until guidance is actually changed it was decided that a currently freestanding header should be used. LEWG took a poll in Albuquerque, and the new <bit> header was chosen instead of <cstddef>.

• Call out that constexpr requires compiler support.

• Make constexpr conditional, similar to variant’s [variant.ctor] wording, based on an EWG straw poll in Albuquerque.

• LWG review made the constexpr remark recursive, and tuned the return wording, asking CWG to review the changes.

• LWG review requested that this paper also add the <bit> header, and let the editor resolve races if multiple papers add the header concurrently.

• CWG substantially tuned the wording.

4.2. r0 ➡ r1

The paper was reviewed by LEWG at the 2016 Issaquah meeting:

• Remove the standard layout requirement—trivially copyable suffices for the memcpy requirement.

• We discussed removing constexpr, but there was no consent either way. There was some suggestion that it’ll be hard for implementers, but there’s also some desire (by the same implementers) to have those features available in order to support things like constexpr instances of std::variant.

• The pointer-forbidding logic was removed. It was initially there to help developers when a better tool is available, but it’s easily worked around (e.g. with a struct containing a pointer). Note that this doesn’t prevent constexpr versions of bit_cast: the implementation is allowed to error out on bit_cast of pointer.

• Some discussion about concepts-usage, but it seems like mostly an LWG issue and we’re reasonably sure that concepts will land before this or in a compatible vehicle.

Straw polls:

• Do we want to see [P0476r0] again? unanimous consent.

• bit_cast should allow pointer types in To and From. SF F N A SA 4 5 4 2 1

• bit_cast should be constexpr? SF F N A SA 4 3 7 2 3

5. Acknowledgement

Thanks to Saam Barati, Jeffrey Yasskin, and Sam Benzaquen for their early review and suggested improvements.