Substitute expression by value?

Question

I have the following macro:

#define REFLECTABLE(...) \
REFLECTABLE_CONST(BOOST_PP_VARIADIC_SIZE(__VA_ARGS__)==0, __VA_ARGS__)

I ran the pre-processor. It passes to REFLECTABLE_CONST 3==0 instead of 0 or false. In REFLECTABLE_CONST, I am using that value to simulate a conditional as described in this post. So, I need the pre-processor to pass a value. Is there a way to make pre-processor substitute things like 3==0 by false or 0?

No, the preprocessor is a text substitution mechanism. – molbdnilo Dec 11 '15 at 09:22 — molbdnilo, Dec 11 '15 at 09:22

score 2 · Accepted Answer · answered Dec 11 '15 at 09:23

I couldn't reproduce BOOST_PP_VARIADIC_SIZE being able to have a result of 0, but if you can, this should work:

#define IS_EMPTY_IMPL0 1
#define IS_EMPTY_IMPL1 0
#define IS_EMPTY_IMPL2 0
#define IS_EMPTY_IMPL3 0
#define IS_EMPTY_IMPL4 0
#define IS_EMPTY_IMPL5 0
#define IS_EMPTY_IMPL6 0
#define IS_EMPTY_IMPL7 0
#define IS_EMPTY_IMPL8 0
#define IS_EMPTY_IMPL9 0

#define IS_EMPTY(...) BOOST_PP_CAT(IS_EMPTY_IMPL, BOOST_PP_VARIADIC_SIZE(__VA_ARGS__))

#define REFLECTABLE(...) \
REFLECTABLE_CONST(IS_EMPTY(__VA_ARGS__), __VA_ARGS__)

**For a future visitor:** I accepted this reply, because it handles the question as is. However, see my own answer, which provides a simpler solution to the actual problem. — AlwaysLearning, Dec 11 '15 at 10:10

score 2 · Answer 2 · answered Dec 11 '15 at 09:44

2

Stumbled upon this solution. Just use this instead:

#define REFLECTABLE(...) \
REFLECTABLE_CONST(BOOST_PP_IS_EMPTY(__VA_ARGS__), __VA_ARGS__)

answered Dec 11 '15 at 09:44

AlwaysLearning

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This macro doesn't seem to be documented on the Boost website. – Simple Dec 11 '15 at 09:50
**For a future visitor**: Look up http://stackoverflow.com/a/34220519/2725810 for an application of this solution. – AlwaysLearning Dec 11 '15 at 10:13

Substitute expression by value?

2 Answers2