Home > Uncategorized > Single-quote as a digit separator soon to be in C++

Single-quote as a digit separator soon to be in C++

At the C++ Standard’s meeting in Chicago last week agreement was finally reached on what somebody in the language standards world referred to as one of the longest bike-shed controversies; the C++14 draft that goes out for voting real-soon-now will include support for single-quotation-mark as a digit separator. Assuming the draft makes it through ISO voting you could soon be writing (Compiler support assumed) 32'767 and 0.000'001 and even 1'2'3'4'5'6'7'8'9 if you so fancied, in your conforming C++ programs.

Why use single-quote? Wouldn’t underscore have been better? This issue has been on the go since 2007 and if you feel really strongly about it the next bike-shed C++ Standard’s meeting is in Issaquah, WA at the start of next year.

Changing the lexical grammar of a language is fraught with danger; will there be a change in the behavior of existing code? If the answer is Yes, then the next question is how many people will be affected and how badly? Let’s investigate; here are the lexical details of the proposed change:

pp-number:
    digit
    . digit
    pp-number digit
    pp-number ' digit
    pp-number ' nondigit
    pp-number identifier-nondigit
    pp-number e sign
    pp-number E sign
    pp-number .

Ideally the change of behavior should cause the compiler to generate a diagnostic, when code containing it is encountered, so the developer gets to see the problem and do something about it. The following conforming C++ code will upset a C++14 compiler (when I write C++ I mean the C++ Standard as it exists in 2013, i.e., what was called C++11 before it was ratified):

#define M(x) #x   // stringize the macro argument
 
char *p=M(1'2,3'4);

At the moment the call to the macro M contains one argument, the sequence of three tokens {1}, {'2,3'} and {4} (the usual convention is to bracket the characters making up one token with matching curly braces).

In C++14 the call to M will contain the two arguments {1'2} and {3,4}. conforming compiler is required to complain when the number of arguments to a macro invocation don’t match the definition…. Unless the macro is defined to accept a variable number of arguments:

#define M(x, ...) __VA_ARGS__
 
          int x[2] = { M(1'2,3'4) };
// C++11: int x[2] = {};
// C++14: int x[2] = { 3'4 };

This is the worst kind of change in behavior, known as a silent change, the existing code compiles without complaint but has different behavior.

How much existing code contains either of these constructs? I suspect very very little human written code, maybe even none. This is the sort of stuff that is more likely to be produced by automatic code generators. But how much more likely? I have no idea.

How much benefit does the new feature provide? It certainly looks useful, but coming up with a number for the benefit is hard. I guess it has the potential to shave a fraction of a second off of the attention a developer has to pay when reading code, after they have invested in learning about the construct (which is lots of seconds). Multiplied over many developers and not that many instances (the majority of numeric literals contain a single digit), we could be talking a man year or two per year of worldwide development effort?

All of the examples I have seen require the ‘assistance’ of macros, here is another (courtesy of Jeff Snyer):

#define M(x) A ## x
#define A0xb
 
int operator "" _de(char);
int x = M(0xb'c'_de);

Are there any examples of a silent change that don’t involve the preprocessor?

  1. October 1st, 2013 at 07:13 | #1

    I wonder whether I’ll ever need to use these digit separators in case they come into existence… by the way does JAVA allows anything similar?

  2. October 27th, 2013 at 22:12 | #2

    The examples (3rd grey pane) contain some curved quotes “’” U+2019 instead of single quotes “‘”.

  3. October 27th, 2013 at 22:45 | #3

    @Neil Hodgson
    Thanks, fixed.

  4. December 11th, 2013 at 11:38 | #4

    Cool. I hadn’t thought of using the apostrophe before for this, but it actually seems visually natural (like an upside down comma), and I see that an upper mark is even used in some locales in writing (Germany, Italy, Romania, Switzerland).

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