Re: [boost] PP: detecting a token?

From: Paul Mensonides (pmenso57_at_[hidden])
Date: 2003-04-11 16:43:41

Aleksey Gurtovoy wrote:

>> First, though, I want to make sure I understand exactly what you
>> mean.  Do you want everything to automatically pick up the default
>> value(s)?  Or do you just want a way to signal "use default" to the
>> implementation of the macro in question, such as "MY_MACRO" above?
>
> The latter. Basically, I want something like this (pseudo code):
>
> #define MY_MACRO(n, func) \
>     BOOST_PP_IF( \
>           BOOST_PP_TOKEN_EQUAL(func, default) \
>         , DO_SOMETHING \
>         , DO_SOMETHING_WITH_DEFAULT_FUNC \
>         )(n) \
> /**/

Without, once again, token-pasting of unrelated tokens, I cannot do this for
_arbitrary_ input.  However, I can under certain constraints.  If 'func' is +, I
cannot compare it to "default".  If func is MY_MACRO, then I can.  If you make
the following input constraint:  Input is not allowed to be parenthesized, then
you can do it like this:

#define MY_MACRO(n, func) \
    IF( \
        IS_UNARY(func), \
        DO_SOMETHING, \
        DO_SOMETHING_WITH_DEFAULT_FUNC \
    )(n) \
    /**/

Of course, you'd presumably want to actually *use* func for something.  Plus,
the above isn't the "best" solution.  E.g. this is better:

#define DEFAULT(x, def) \
    IIF( \
        IS_UNARY(x), \
        x,
        def \
    ) \
    /**/

#define MACRO(n, func) \
    DEFAULT(func, DEFAULT_FUNC)(n) \
    /**/
#define DEFAULT_FUNC(n) default

#define FUNC(n) user

MACRO(5, FUNC) // user
MACRO(5, (*) ) // default

P.S. it doesn't matter what is inside the unary parentheses.  It could be
(default) or whatever else you might want.

Paul Mensonides