Why was the space character not chosen for C++14 digit separators?

Question

As of C++14, thanks to n3781 (which in itself does not answer this question) we may write code like the following:

const int x = 1'234; // one thousand two hundred and thirty four

The aim is to improve on code like this:

const int y = 100000000;

and make it more readable.

The underscore (_) character was already taken in C++11 by user-defined literals, and the comma (,) has localisation problems — many European countries bafflingly^† use this as the decimal separator — and conflicts with the comma operator, though I do wonder what real-world code could possibly have been broken by allowing e.g. 1,234,567.

Anyway, a better solution would seem to be the space character:

const int z = 1 000 000;

These adjacent numeric literal tokens could be concatenated by the preprocessor just as are string literals:

const char x[5] = "a" "bc" "d";

Instead, we get the apostrophe ('), not used by any writing system I'm aware of as a digit separator.

Is there a reason that the apostrophe was chosen instead of a simple space?

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_{^† It's baffling because all of those languages, within text, maintain the notion of a comma "breaking apart" an otherwise atomic sentence, with a period functioning to "terminate" the sentence — to me, at least, this is quite analogous to a comma "breaking apart" the integral part of a number and a period "terminating" it ready for the fractional input.}

Answer 1

There is a previous paper, n3499, which tell us that although Bjarne himself suggested spaces as separators:

While this approach is consistent with one common typeographic style, it suffers from some compatibility problems.

• It does not match the syntax for a pp-number, and would minimally require extending that syntax.
• More importantly, there would be some syntactic ambiguity when a hexadecimal digit in the range [a-f] follows a space. The preprocessor would not know whether to perform symbol substitution starting after the space.
• It would likely make editing tools that grab "words" less reliable.

I guess the following example is the main problem noted:

const int x = 0x123 a;

though in my opinion this rationale is fairly weak. I still can't think of a real-world example to break it.

The "editing tools" rationale is even worse, since 1'234 breaks basically every syntax highlighter known to mankind (e.g. that used by Markdown in the above question itself!) and makes updated versions of said highlighters much harder to implement.

Still, for better or worse, this is the rationale that led to the adoption of apostrophes instead.