On Mon, Sep 6, 2010 at 7:30 PM, Paul Mensonides <pmenso57@comcast.net> wrote:
On 9/6/2010 3:17 PM, Wolf Lammen wrote:
I wrote wrt to BOOST_PP_IS_EMPTY:
"I think it is not part of the API, because it does not conform to C90. You must not submit parameters to macros that expand to nothing, because C90 does not cover this situation"
The relevant portions of the standard makes it clear:
"If (before argument substitution) any argument consists of no preprocessing tokens, the behavior is undefined." ... "After the arguments for the invocation of a function-like macro have been identified, argument substitution takes place" ... "Before being substituted, each argument's preprocessing tokens are completely macro replaced as if they formed the rest of the source file"
Paul Mensonides is right, an argument must not be empty, when it is collected, but it may be at the moment when it is substituted for a parameter. I was mislead by the double mention and meaning of "substitute/substitution", but argument substitution (as opposed to parameter substitution) is the combined process of expanding the argument and substitution for a parameter, and checking for emptyness takes place prior to "argument substitution".
I apologize for having given a wrong interpretation and hope this have not lead to too much confusion.
Luckily, it no longer matters in C and will no longer matter in C++ shortly.
Thanks a lot to all for the replies and clarifications. I think I understand the situation. I use IS_EMPTY more or less like this: #define HAS_NO_OPTION(x) BOOST_PP_IS_EMPTY(x) #define NO_OPTION BOOST_PP_EMPTY #define XYZ_OPTION xyz BOOST_PP_EMPTY HAS_NO_OPTION(NO_OPTION()) // expand to 1 HAS_NO_OPTION(XYZ_OPTION()) // expand to 0 This seems fine because I never pass an empty macro parameter (but only parameters that eventually expand to empty, which is fine). Plus, by construction, `x` only contains literals or numbers with no dot. (MORE BACKGROUND: This is an implementation detail of my parenthesized syntax pp-parsers. For example, a member function might or no be `const`, so after parsing `const` is represented internally as either `BOOST_PP_EMPTY` or `const BOOST_PP_EMPTY` within an array of parsed function signature traits called `sign`. Then trait inspection macros can be applied to check `IS_CONST(sign)`. These inspection macros expand `BOOST_PP_EMPTY` and check for emptiness using `BOOST_PP_IS_EMPTY()`. This implementation works on both GCC and MSVC.) -- Lorenzo