https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103238
Bug ID: 103238
Summary: c++ Feature: array-copy experimental extensions
implementing P1997
Product: gcc
Version: 12.0
Status: UNCONFIRMED
Keywords: patch
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: wjwray at gmail dot com
Target Milestone: ---
This is a feature-tracking PR for an incoming patch to implement
C++ proposal P1997 "Relaxing restrictions on array", that adds:
C array copy semantics:
* array-array initializations
* array-array assignments
* array return-by-value from functions
(array formal parameters are unchanged; there's no pass-by-value).
Plus, C++ specific:
* array pseudo-destructors
* array element type deduction
(i.e. admitting placeholder auto in array variable declarations).
Implementation strategy
The features are implemented as an experimental extension, disabled by default.
The patch should have no effect until enabled by the new option:
-farray-copy (flag_array_copy)
This single flag enables all features, copy-semantic or not.
The features are enabled as an extension in all -std modes as appropriate
(i.e. placeholder auto in C++11 and up).
The extension is documented as experimental with no guarantee of stability;
features may be added, removed or changed in detail.
In particular, there's no guarantee of ABI stability;
allowing array as a function return type has ABI impact:
* Calling conventions for the array return slot must be agreed
* On the C++ side, name-mangling conventions must be defined.
The plan is to first merge array-copy as experimental, with ABI defined as
'what the code does', and then to go ahead with ABI discussions, decide on
a written specification, and then do any required implementation work.
C and C++ proposal status
P1997R1 was seen by SG22, the ISO Joint C and C++ Liaison Study Group,
on Aug 6th 2021 with positive feedback from both C and C++ sides of the room.
It was voted forward to C++ EWG, with the strong recommendation to proceed
only once implementation experience has been gained. This work is the result.
The EWG chair prefers that "it be reviewed and tentatively approved by ...gcc
code owners" as an upstreamable contribution. The aim is to merge array-copy
extensions on trunk for wide availability. Likewise for other compilers.
The C compatible subset of the copy-semantic changes apply to C language,
and could be made to accommodate array extensions such as VLAs and VM types.
The plan is to follow up on the C side once this patch is accepted into g++:
(1) to assist in writing the C proposal based on this experience, and
(2) to contribute to the gcc implementation, with support of C code owners.
The C++ proposal will be revised based on experience and feedback as the
implementation is honed via issues, bug fixes or further opportunities.
An interesting suggestion is that these array extensions be incorporated into
C and C++ standards as a DR, applying as they do to the C subset and all -std,
with DR standing for Dennis Ritchie, great language designer in the sky.
