It just makes it easier to read the magnitude of a number and get that extra visual reassurance that it's correct. It's not easy to (quickly) tell the difference between 1 billion and 100 million unless you count zeros (1000000000 vs 100000000). On the other hand it's very easy to discern 1'000'000'000 vs 100'000'000.
The standard can't use commas as a separator because using commas vs periods to demarcate components is locale specific. Apostrophes look like a reasonable compromise because lots of calculators already use apostrophes to separate digits:
I would've gone with spaces. Those are the official standard here in Canada as a pragmatic compromise between anglos and francos that have opposing usage of comma/dot.
So instead of 1,234.56789 (English) or 1.234,56789 (French) we do 1 234.56789 (international)
[0-9]+ [0-9]+ has no meaning in C++, so using space for magnitude decorations would have worked.
Briefly, but I think there are like three people in the world that fully grok C++ grammar, and I'm definitely not one of them. I just know that C++ generally has some kind of infix operator or delimiter between pairs of immediate data or variables, I'm pretty sure it's currently just a compiler error in every case.
User-defined literals can't generally start with a digit, so it should be perfectly fine to allow 1_000_000 syntax for 1000000. Or I'm forgetting something?
The "already an operator" can be a factor in the design reasoning but if they really really wanted to, they could have made the comma syntax for digit separators by modifying the parsing grammar: e.g. a special prefix and/or contextual parsing.
For example, the "->" was already an operator for pointer indirection but it was reused for lambda definitions. The "[]" was already used for array subscript indexes but was reused for lambda captures.
You can't do that for ",", because it will break the existing code.
For example, int x = 0; x = 1,2; is perfectly valid pre c++11 code. This will assign 1 to x. Now if you make "," as digit separator, it will assign 12 to x
I mentioned the possibility of a special prefix in my previous post. In other words, that parsing ambiguity goes away if the standards committee wanted to define a special prefix such as 'd', 'k', or '_' to inform the parser that the next comma is a "digit separator" instead of "comma operator" such as _1,000,000 . E.g. there's already intelligence about commas in the parser to disambiguate function calls with commas as in "repositionxyz(914,348,122)"
Or, they could have defined C++14 to simply invalidate your example syntax of "x=1,2". As an example, the "auto" keyword was made a "breaking change" such that "for (auto int i = 0;;)" no longer compiles.
The bottom line is that there are a myriad of ways to address (potential) parsing ambiguities when introducing new language features and syntax. (Whether or not a comma digit separator is worth the clumsier syntax of a special prefix or inflicting the pain of a breaking change is a separate concept.)
C++ doesn't have the friendliest grammar for addressing potential parsing ambiguities.
As an example, your proposed solutions
_1,000,000
k1,000,000
Both are already valid if variables or functions _1 or k1 are in scope, and evaluate to (octal) zero.
And I haven't checked, but it would guess that invalidating "x=1,2" in the parser would not be "simple". I cannot think of a reason, but it also might turn out to be more common then one would think due to macro expansions.
The standard can't use commas as a separator because using commas vs periods to demarcate components is locale specific. Apostrophes look like a reasonable compromise because lots of calculators already use apostrophes to separate digits:
https://www.google.com/search?q=14+digit+calculator&source=l...