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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_TOKEN_EQUAL(func, default) \
> )(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, \
    )(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), \
        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

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