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
or
runs afoul of type-aliasing
rules yet these are the intuitive solutions developers mistakenly turn to.
Attuned developers use
with
, 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
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
bit-cast
function, as
itself isn’t
. Marking the proposed function as
doesn’t require or prevent
from becoming
, but
requires compiler support. This leaves implementations free to use their own
internal solution (e.g. LLVM has a
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
to:
-
Table 16 — C++ library headers
-
Table 19 — C++ headers for freestanding implementations
In the numerics section, add the following:
2.1. 29.� Bit manipulation library [bit]
2.2. 29.�.1 General [bit.general]
The header
provides components to access, manipulate and process both
individual bits and bit sequences.
2.3. 29.�.2 Header < bit >
synopsis [bit.syn]
namespace std { // 29.�.3 bit_cast template < typename To , typename From > constexpr To bit_cast ( const From & from ) noexcept ; }
2.4. 29.�.3 Function template bit_cast
[bit.cast]
template < typename To , typename From > constexpr To bit_cast ( const From & from ) noexcept ;
-
Remarks:
This function shall not participate in overload resolution unless:
issizeof ( To ) == sizeof ( From ) true
;
isis_trivially_copyable_v < To > true
; and
isis_trivially_copyable_v < From > true
.
This function shall be
if and only ifconstexpr
,To
, and the types of all subobjects ofFrom
andTo
are typesFrom
such that:T
isis_union_v < T > false
;
isis_pointer_v < T > false
;
isis_member_pointer_v < T > false
;
isis_volatile_v < T > false
; and
has no non-static data members of reference type.T
-
Returns:
An object of type
. Each bit of the value representation of the result is equal to the corresponding bit in the object representation ofTo
. Padding bits of thefrom
object are unspecified. If there is no value of typeTo
corresponding to the value representation produced, the behavior is undefined. If there are multiple such values, which value is produced is unspecified.To
2.5. Feature testing
The
feature test macro should be added.
3. Appendix
The Standard’s [basic.types] section explicitly blesses
:
For any trivially copyable type
, if two pointers to
T point to distinct
T objects
T and
obj1 , where neither
obj2 nor
obj1 is a base-class subobject, if the underlying bytes (1.7) making up
obj2 are copied into
obj1 ,
obj2 shall subsequently hold the same value as
obj2 .
obj1 [Example:
— end 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
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
header was chosen instead of< bit >
.< cstddef > -
Call out that
requires compiler support.constexpr -
Make
conditional, similar to variant’s [variant.ctor] wording, based on an EWG straw poll in Albuquerque.constexpr -
LWG review made the
remark recursive, and tuned the return wording, asking CWG to review the changes.constexpr -
LWG review requested that this paper also add the
header, and let the editor resolve races if multiple papers add the header concurrently.< bit > -
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
requirement.memcpy -
We discussed removing
, 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 likeconstexpr
instances ofconstexpr
.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
containing a pointer). Note that this doesn’t preventstruct
versions ofconstexpr
: the implementation is allowed to error out onbit_cast
of pointer.bit_cast -
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.
-
should allow pointer types inbit_cast
andTo
. SF F N A SA 4 5 4 2 1From -
should bebit_cast
? SF F N A SA 4 3 7 2 3constexpr
5. Acknowledgement
Thanks to Saam Barati, Jeffrey Yasskin, and Sam Benzaquen for their early review and suggested improvements.