Comma omitted in variadic function declaration in C++

Question

I am used to declaring variadic functions like this:

int f(int n, ...); 

When reading The C++ Programming Language I found that the declarations in the book omit the comma:

int f(int n...); // the comma has been omitted 

It seems like this syntax is C++ specific as I get this error when I try to compile it using a C compiler:

test.c:1:12: error: expected ‘;’, ‘,’ or ‘)’ before ‘...’ token int f(int n...);

Is there any difference between writing int f(int n, ...) and int f(int n...)?

Why was this syntax added C++?

Answer 1

According to § 8.3.5.4 of the C++ standard (current draft):

Where syntactically correct and where “...” is not part of an abstract-declarator, “, ...” is synonymous with “...”.

In short, in C++ ... (ellipsis) is an operator in its own right and so can be used without the comma, but use of the comma is retained for backwards compatibility.