Why was the space character not chosen for C++14 d

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

It has to do with how the language is parsed. It would have been difficult for the compiler authors to rewrite their products to accept space delimited literals.

Also, I don't think seperating digits with spaces is very common. That i've seen, it's always non-whitespace characters, even in different countries.