Re: [boost] Review Request: Variadic Macro Data library

Subject: Re: [boost] Review Request: Variadic Macro Data library
From: Paul Mensonides (pmenso57_at_[hidden])
Date: 2011-02-21 13:03:04

On Mon, 21 Feb 2011 12:12:15 -0500, Lorenzo Caminiti wrote:

> On Mon, Feb 21, 2011 at 9:19 AM, Edward Diener <eldiener_at_[hidden]>
> wrote:

>>> However, DATA_SIZE() => Â 0 is ill-conceived. Â An empty argument is
>>> still an argument to the preprocessor. Â A better correlation is:
>>>
>>> DATA_SIZE(,,) => Â 3
>>> DATA_SIZE(,) => Â 2
>>> DATA_SIZE() => Â 1
>>
>> Thanks ! I will just have to further document that the data size
>> returned can never be 0, even when the variadic macro is invoked with
>> an empty argument.
> 
> IMO, that makes sense -- so documentation is a good option.
> 
> However, I still don't understand why MSVC accepts this DATA_SIZE()
> invocation:
> 
> #define DATA_SIZE(...)
> DATA_SIZE(1)
> DATA_SIZE() // No error -- why??
> 
> #define SIZE(x)
> SIZE(1)
> SIZE() // Error -- as it should!

Neither case should be an error.  According to the C99 standard (and C+
+0x), the sequence of tokens (and whitespace separations) that makes up a 
macro argument may be empty or contain no tokens.  If the sequence of 
tokens (and whitespace separations) contains no tokens, the formal 
parameters in the replacement list are replaced by a "placemarker" which 
is a sort of virtual token.

So, in the following scenarios:

#define MACRO(x) [x]

MACRO(123)     // [123]
MACRO( 123 )   // [ 123 ]
MACRO(  123  ) // [ 123 ]
        // adjacent whitespace is combined
        // in an earlier phase of translation

MACRO( ) // [<placemarker>] -> []
MACRO()  // [<placemarker>] -> []

For variadics, for the purposes of substitution, token-pasting, and 
stringizing, all of the variadic arguments act as one argument:

#define MACRO(...) [__VA_ARGS__]

MACRO()     // [<placemarker>] -> []
MACRO( )    // [<placemarker>] -> []
MACRO(a)    // [a]
MACRO(a,b)  // [a,b]
MACRO(a, b) // [a, b]
MACRO( a,b, c ) // [ a,b, c ]

* Note that I don't know of a single preprocessor that actually handles 
whitespace correctly in all cases.

Regards,
Paul Mensonides