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.
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
union runs afoul of type-aliasing
rules yet these are the intuitive solutions developers mistakenly turn to.
Attuned developers use
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
Furthermore, it is currently impossible to implement a
memcpy itself isn’t
constexpr. Marking the proposed function as
constexpr doesn’t require or prevent
memcpy from becoming
requires compiler support. This leaves implementations free to use their own
internal solution (e.g. LLVM has a
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 126.96.36.199 [headers] add the header
Table 16 — C++ library headers
Table 19 — C++ headers for freestanding implementations
In the numerics section, add the following:
2.5. Feature testing
__cpp_lib_bit_cast feature test macro should be added.
The Standard’s [basic.types] section explicitly blesses
For any trivially copyable type
T, if two pointers to
Tpoint to distinct
obj2, where neither
obj2is a base-class subobject, if the underlying bytes (1.7) making up
obj1are copied into
obj2shall subsequently hold the same value as
[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
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
Call out that
constexprrequires compiler support.
constexprconditional, similar to variant’s [variant.ctor] wording, based on an EWG straw poll in Albuquerque.
LWG review made the
constexprremark 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
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
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
structcontaining a pointer). Note that this doesn’t prevent
bit_cast: the implementation is allowed to error out on
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.
Do we want to see [P0476r0] again? unanimous consent.
bit_castshould allow pointer types in
From. SF F N A SA 4 5 4 2 1
constexpr? SF F N A SA 4 3 7 2 3
Thanks to Saam Barati, Jeffrey Yasskin, and Sam Benzaquen for their early review and suggested improvements.