The C++ Trailing Comma Issue and Style Guide

[Ultrafeel]

abseil-cpp Trailing Comma

A trailing comma has been found in the C++ code , which is unusual for two reasons:

1. It's the only instance of a trailing comma in the entire codebase. P.S. It seems so, at least among this kind of syntax.
2. While C++ allows trailing commas in certain syntactic constructs, it's not immediately clear which construct this particular line belongs to.

Questions and Concerns

1. Style Guide: Is there an official style guide that addresses the use of trailing commas in this project?

2. Compilation Issues: The presence of this trailing comma has caused compilation problems across different projects that use this code. This suggests it might be more problematic than beneficial.

3. Removal Request: Given the compilation issues, would it be possible to remove this trailing comma?

4. Original Rationale: What was the reasoning behind including this trailing comma in the first place?

[derekmauro]

It is not the only trailing comma in the codebase. For example, see

abseil-cpp/absl/strings/escaping.cc

Lines 848 to 865 in 917bfee

	constexpr char kHexValueLenient[256] = {
	0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
	0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
	0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
	0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 0, 0, 0, 0, 0, 0, // '0'..'9'
	0, 10, 11, 12, 13, 14, 15, 0, 0, 0, 0, 0, 0, 0, 0, 0, // 'A'..'F'
	0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
	0, 10, 11, 12, 13, 14, 15, 0, 0, 0, 0, 0, 0, 0, 0, 0, // 'a'..'f'
	0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
	0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
	0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
	0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
	0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
	0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
	0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
	0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
	0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
	};

.

Compilation Issues: The presence of this trailing comma has caused compilation problems across different projects that use this code. This suggests it might be more problematic than beneficial.

Can you please give an example error? It is possible that we can remove it if there is a significant problem.

[Ultrafeel]

The example with kHexValueLenient you provide is immediately clear which certain syntactic construct this particular line belongs to - the one, where C++ allows trailing commas.
Actually , the project I know uses abseil indirectly through protobuf library.
So here is the error:

_bullseye/_packages/_protobuf/28.2.0.28___/include/absl/container/internal/raw_hash_set.h: In static member function 'static const absl::lts_20240116::container_internal::PolicyFunctions& absl::lts_::container_internal::raw_hash_set<Policy, Hash, Eq, Alloc>::GetPolicyFunctions()':
_bullseye/_packages/_protobuf/28.2.0.28___/include/absl/container/internal/raw_hash_set.h:3246:2: error: expected primary-expression before '}' token
 3246 |     };
      |  ^
_bullseye/_packages/_protobuf/28.2.0.28___/include/absl/container/internal/raw_hash_set.h:3246:2: error: expected ';' before '}' token
BullseyeCoverage error: status 1: /opt/rh/gcc-toolset-12/root/usr/bin/g++
BullseyeCoverage Compile 8.25.31 Linux-x64
Copyright (c) Bullseye Testing Technology

Although the compiler isn't fresh.

[Ultrafeel]

Any updates on this?

[derekmauro]

I'm submitting a change to remove the comma.