Slides for P3776R1 — More trailing commas

Document number:: D3897R0
Date:: 2025-10-31
Audience:: EWG
Project:: ISO/IEC 14882 Programming Languages — C++, ISO/IEC JTC1/SC22/WG21
Reply-to:: Jan Schultke <janschultke@gmail.com>
Source:: github.com/Eisenwave/cpp-proposals/blob/master/src/more-trailing-commas-slides.cow

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More trailing commas
P3776R1

Jan Schultke | Slides for P3776R1 — More trailing commas | Kona 2025 | Slide 1

Introduction

void

(

int

← trailing comma should be allowed here

)

;

int

[

]

{

}

;

already supported in braced lists

trailing commas would be useful
[P0562R2] Trailing Commas in Base-clauses and Ctor-initializers

Jan Schultke | Slides for P3776R1 — More trailing commas | Kona 2025 | Slide 2

P0562R2

Poll: D0562R1 — forward […] to CWG for inclusion in C++26.

SF F N A SA

12 11 4 3 0

SF	F	N	A	SA
12	11	4	3	0

Forwarded to CWG, but currently on ice because of parsing ambiguity:

struct

{

}

;

struct

← OK, new trailing comma

{

(

)

(

)

← parsing ambiguity due to trailing comma

{

...

}

A, b, and c declared down here somewhere

}

;

Jan Schultke | Slides for P3776R1 — More trailing commas | Kona 2025 | Slide 3

Trailing commas in other languages

Many languages now support trailing commas in parenthesized lists:

Swift
2025

Kotlin
2020

JavaScript
2017

TypeScript
2017

Rust
always

Go
always

Python
always

Julia
always

Many more (including C++) support trailing commas in braced lists.

Jan Schultke | Slides for P3776R1 — More trailing commas | Kona 2025 | Slide 4

Improved text editing

Without trailing commas, we cannot always swap or cut/paste lines:

No trailing comma	With trailing comma
// before void f( int x, int y ); // after void f( int y // syntax error int x, );	// before void f( int x, int y, ); // after void f( int y, // OK int x, );

Jan Schultke | Slides for P3776R1 — More trailing commas | Kona 2025 | Slide 5

Improved version control

No trailing comma	With trailing comma
void f( int x, - int y + int y, + int z );	void f( int x, int y, + int z, );

trailing comma is much better for version control:
- smaller changes when viewing line differences
- no pollution of per-line history (git blame)

Jan Schultke | Slides for P3776R1 — More trailing commas | Kona 2025 | Slide 6

Code generation convenience for [P3294R2]

No trailing comma	With trailing comma
consteval meta::info gen_params( span<const Parameter> ps ) { meta::info result = ^^{}; bool first = true; for (const Parameter& p : ps) { result = ^^{ \tokens(result) \tokens(first ? ^^{} : ^^{,}) \tokens(gen_param(p)) }; first = false; } return result; }	consteval meta::info gen_params( span<const Parameter> ps, ) { meta::info result = ^^{}; for (const Parameter& p : ps) { result = ^^{ \tokens(result) \tokens(gen_param(p)) , }; } return result; }

Jan Schultke | Slides for P3776R1 — More trailing commas | Kona 2025 | Slide 7

Improved auto-formatter control

ClangFormat disables bin-packing when trailing comma is present
this allows fine-tuned stylistic control via comma

vector

int

numbers

{

LIST_ITEM_A

LIST_ITEM_B

LIST_ITEM_C

}

;

← column limit

vector

int

numbers

{

LIST_ITEM_A

LIST_ITEM_B

LIST_ITEM_C

← trailing comma here

}

;

Jan Schultke | Slides for P3776R1 — More trailing commas | Kona 2025 | Slide 8

Eliminating some uses of __VA_OPT__

No trailing comma	With trailing comma
#define F(...) \ f(0 __VA_OPT__(,) \ __VA_ARGS__) #define L(...) \ [& __VA_OPT__(,) \ __VA_ARGS__] {}	#define F(...) \ f(0 , __VA_ARGS__) #define L(...) \ [& , __VA_ARGS__] {}

__VA_OPT__ is necessary to avoid error on trailing comma
no longer necessary if trailing comma is OK

Jan Schultke | Slides for P3776R1 — More trailing commas | Kona 2025 | Slide 9

Preventing string joining bugs

No trailing comma	With trailing comma
emplace_strings( "fee", "fie", "foe" + "fum" );	emplace_strings( "fee", "fie", "foe", + "fum" );

no trailing comma: "foe" and "fum" are concatenated
missing commas happen (forgetfulness, key slip, etc.)

Jan Schultke | Slides for P3776R1 — More trailing commas | Kona 2025 | Slide 10

Where to add trailing commas

in summary: after all lists, except:
- [P0562R2]'s mem-initializer-list and base-specifier-list
- semicolon-terminated lists like using a, b,;
- non-lists like static_assert(true,"",), delete("",)
- after ellipsis parameter like void f(int, ...,)
- in macros like #define M(x,y,)
see proposal for examples and more details

Discuss with clang-format maintainers.

Jan Schultke | Slides for P3776R1 — More trailing commas | Kona 2025 | Slide 11

Addressing criticisms (1/2)

aesthetic objections
- valid, although trailing commas are 100% optional
asymmetry with C
- just don't use the trailing comma if you need C compatibility
- WG14 proposal will be published to synchronize languages
making f(0,) valid may invite "forgotten argument" bugs
- experience from other langs, {0,} in C++ tells us it's okay
- type system, compiler warnings, auto-formatting catches this

Jan Schultke | Slides for P3776R1 — More trailing commas | Kona 2025 | Slide 12

Addressing criticisms (2/2)

inconsistency with macros: M(1,2,) may be ill-formed
- does not apply to variadic, i.e. __VA_ARGS__ macros
- macros are already inconsistent, e.g. M({1,2})
multiple ways to do the same thing
- that's okay, same for indentation and other editorial choices
- similar to //... vs. /*...*/
syntax space is claimed
- no plausible alternative meaning for f(1,) exists anyway

Jan Schultke | Slides for P3776R1 — More trailing commas | Kona 2025 | Slide 13

Last but not least,

Implementation experience: Clang fork,
- approximately 40 LOC,
- naive implementation that ignores trailing comma,
- proper implementation may need changes to AST,
Impact on existing code: None,
Wording: Exists, briefly seen by CWG chair,

Jan Schultke | Slides for P3776R1 — More trailing commas | Kona 2025 | Slide 14

Jan Schultke | Slides for P3776R1 — More trailing commas | Kona 2025 | Slide 15

Questions

Agreement with overall design?
SIMD overloads in another proposal?
Forward to LEWG?

Jan Schultke | Slides for P3776R1 — More trailing commas | Kona 2025 | Slide 16

References

[P3776R1] Jan Schultke. More trailing commas 2025-09-09 https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2025/p3776r1.html

[P0562R2] Alan Talbot. Trailing Commas in Base-clauses and Ctor-initializers 2024-04-15 https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2024/p0562r2.pdf

[P3294R2] Andrei Alexandrescu, Barry Revzin, Daveed Vandevoorde. Code Injection with Token Sequences 2024-10-15 https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2024/p3294r2.html