P0476r1
Bit-casting object representations

This paper is a revision of [P0476r0], addressing LEWG comments from the 2016 Issaquah 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. 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 the editor finds appropriate for the sub-section.

2.1. Synopsis

Under 20.2 Header <utility> synopsis [utility]:

namespace std {
  // ...
  
  // 20.2.� bit-casting:
  template<typename To, typename From>
  requires
    sizeof(To) == sizeof(From) &&
    is_trivially_copyable_v<To> &&
    is_trivially_copyable_v<From>
  constexpr To bit_cast(const From& from) noexcept;
  
  // ...
}

2.2. Details

Under 20.2.� Bit-casting [utility.bitcast]:

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

1. Requires: sizeof(To) == sizeof(From), is_trivially_copyable_v<To> is true, is_trivially_copyable_v<From> is true.

2. Returns: an object of type To whose object representation is equal to the object representation of From. If multiple object representations could represent the value representation of From, then it is unspecified which To value is returned. If no value representation corresponds to To's object representation then the returned value is unspecified.

2.3. 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. 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.