[jasode]

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:

https://www.google.com/search?q=14+digit+calculator&source=l...

[Pxtl]

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.

[repsilat]

Nice, it fits well with the "adjacent strings are automatically concatenated by the compiler" rule, too.

[detrino]

Have you considered if this would fit with C++'s grammar?

[Pxtl]

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.

[nightcracker]

Except this is not C++ grammar, this is simple lexical analysis. And separating by spaces would work fine.

[dallos]

What about semantics? Probably 0.

[ufo]

The bigger reason why you can't use commas is because commas are already an operator in c++. 1,2 is an expression that evaluates to 2.

Dunno why they didn't use underscores though.

[flebron]

Underscores are already being used for user defined literals.

[drdaeman]

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?

[jeorgun]

Hexadecimal numbers, probably. 0x001_f05 would be ambiguous.

[AnimalMuppet]

I don't think that's it. 0x001_f05 starts with 0, which is still not something an identifier can do.

[plorkyeran]

0x001_f05 could be a UDL named f05 which should be passed 0x001.

[nightcracker]

Where do you get this information from, because it's entirely incorrect.

[jasode]

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.

[chappar]

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

[jasode]

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.)

[Someone]

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.