What does this declare `void func(bool b ...)`?

Question

I answered this question How can I get my va_list arguments to repeat itself? and noticed the uncommon function declaration:

void ordered(int num1, double list ...);

First i thought the compiler would complain but clang 3.2 didn't and neither did g++ 4.7.2.

What does this declaration expand to? As what does it get interpreted?

Edit: I know about the ellipsis. But the normal form is <return type> <function-name>(<argument1-type> <arg-name>, ...); in the example the comma is missing.

This kind of reminds me of http://stackoverflow.com/questions/5625600/what-is-the-meaning-of-token — chris, Mar 27 '13 at 13:57

score 3 · Accepted Answer · answered Mar 27 '13 at 13:41

3

It's the same as:

void ordered(int num1, double list, ...);

answered Mar 27 '13 at 13:41

chris

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And i guess that that declaration does not work with `va_arg`? My tests at least show they don't. – RedX Mar 27 '13 at 13:47
@RedX, It should work the same as any other C variadic function. – chris Mar 27 '13 at 13:49

score 3 · Answer 2 · answered Mar 27 '13 at 13:46

3

This is a snippet of the grammar in the C++ standard:

parameter-declaration-clause:
  parameter-declaration-list[opt] ...[opt]
  parameter-declaration-list , ...

Basically the ellipsis can be preceded by , if the are other parameter declarations, but it need not be. The function declaration:

void f(int,double...);

really means:

void f(int,double,...);

answered Mar 27 '13 at 13:46

David Rodríguez - dribeas

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Interesting, didn't know the comma in this case was optional. Thank you. – RedX Mar 27 '13 at 13:48

Alok Save · Answer 3 · 2013-03-27T13:55:03.617

void ordered(int num1, double list ...);

is same as:

void ordered(int num1, double list, ...);

Reference:
Standard C++11 8.3.5.3/4:

parameter-declaration-clause:
parameter-declaration-listopt ...opt
parameter-declaration-list , ...

If the parameter-declaration-clause terminates with an ellipsis or a function parameter pack (14.5.3), the number of arguments shall be equal to or greater than the number of parameters that do not have a default argument and are not function parameter packs. Where syntactically correct and where “...” is not part of an abstract-declarator, “, ...” is synonymous with “...”.

score 1 · Answer 4 · answered Mar 27 '13 at 13:42

1

The three dots (...) are called "ellipses" and denote a variable argument list. So you can pass as many arguments as you like (there is an OS-specified limitation, though). In this way, printf works, for example.

See here for further explanation.

answered Mar 27 '13 at 13:42

bash.d

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score 1 · Answer 5 · answered Mar 27 '13 at 13:42

1

I guess you mean the "..." right?

For some functions it is not possible to specify the number and type of all arguments expected in a call. Such a function is declared by terminating the list of argument declarations with the ellipsis (...).

answered Mar 27 '13 at 13:42

Normal People Scare Me

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What does this declare `void func(bool b ...)`?

5 Answers5