Digit Separators

ISO/IEC JTC1 SC22 WG21 N2281 = 07-0141 - 2007-05-02

Lawrence Crowl

Problem

Numeric literals of more than a few digits are hard to read. For example, compare 237498123 with 237499123 for equality and 237499123 with 20249472 for relative magnitude.

Solution

We propose to add the underscore character as a digit separator in numeric literals. The pervious examples become clearer as 237_498_123 with 237_499_123 and 237_499_123 with 20_249_472.

Alternate Solutions

Bjarne Stroustrup has suggested using a space as a separator. While this approach is consistent with some presentation styles, it would likely make editing tools that grab "words" less reliable. Furthermore, the preprocessor syntax would need to change (section 2.9 below) with potential unforseen risk.

Unaddressed Issues

This proposal does not address binary literals or hexadecimal floating-point literals. We believe those should be addressed in a separate paper.

Implementation

This approach has been implemented in the Ada programming language.

Changes to the C++ Standard

2.9 [lex.ppnumber]

To the grammar, no changes are necessary. The preprocessing number tokens already admit an underscore via the non-terminal nondigit.

pp-number:

digit

. digit

pp-number digit

pp-number nondigit

pp-number e sign

pp-number E sign

pp-number .

2.10 Identifiers [lex.name]

To the grammar, no changes are necessary. This section defines non-terminals used elsewhere.

nondigit: one of

a b c d e f g h i j k l m

n o p q r s t u v w x y z

A B C D E F G H I J K L M

N O P Q R S T U V W X Y Z _

digit: one of

0 1 2 3 4 5 6 7 8 9

2.13.1 Integer literals [lex.icon]

To the grammar, edit as follows, permitting underscores between digits. (Note some renderings of HTML will overstrike the underscore with the markup for inserted text. Look for "more than usual" spacing in that case.)

integer-literal:

decimal-literal integer-suffix_opt

octal-literal integer-suffix_opt

hexadecimal-literal integer-suffix_opt

decimal-literal:

nonzero-digit

decimal-literal digit

decimal-literal _ digit

octal-literal:

0

octal-literal octal-digit

octal-literal _ octal-digit

hexadecimal-literal:

0x hexadecimal-digit

0X hexadecimal-digit

hexadecimal-literal hexadecimal-digit

hexadecimal-literal _ hexadecimal-digit

nonzero-digit: one of

1 2 3 4 5 6 7 8 9

octal-digit: one of

0 1 2 3 4 5 6 7

hexadecimal-digit: one of

0 1 2 3 4 5 6 7 8 9

a b c d e f

A B C D E F

2.13.3 Floating literals [lex.fcon]

To the grammar, edit as follows, permitting underscores between digits. (Note some renderings of HTML will overstrike the underscore with the markup for inserted text. Look for "more than usual" spacing in that case.)

floating-literal:

fractional-constant exponent-part_opt floating-suffix_opt

digit-sequence exponent-part floating-suffix_opt

fractional-constant:

digit-sequence_opt . digit-sequence

digit-sequence .

exponent-part:

e sign_opt digit-sequence

E sign_opt digit-sequence

sign: one of

+ -

digit-sequence:

digit

digit-sequence digit

digit-sequence _ digit