Avoid multiple macro unfolding for same input

Question

I want my macro to be only unfolded once for each possible argument.

    #define PAX_CREATE_MEMBER_CHECKER_FOR_NAME(memberName) \
            template <typename T, typename = int> \
            struct HasMember_##memberName : std::false_type { }; \
            template <typename T> \
            struct HasMember_##memberName <T, decltype((void) T::memberName, 0)> : std::true_type { };

When I use the same memberName twice, I don't want the macro to be unfolded a second time, cause this would lead to errors, as the functions are now defined twice. I tried to use macros inside macros, but it is clear to me, that this won't work, as macros are unfolded only once (reasonably). Although it might clarify the question:

    #define PAX_CREATE_MEMBER_CHECKER_FOR_NAME(memberName) \
        #ifndef PAX_CREATE_MEMBER_CHECKER_FOR_NAME_##memberName \
        #define PAX_CREATE_MEMBER_CHECKER_FOR_NAME_##memberName \
            template <typename T, typename = int> \
            struct HasMember_##memberName : std::false_type { }; \
            template <typename T> \
            struct HasMember_##memberName <T, decltype((void) T::memberName, 0)> : std::true_type { }; \
        #endif

A template-based solution is fine, too!

This macro is a slightly modified version of DEFINE_MEMBER_CHECKER macro in

How to detect whether there is a specific member variable in class?

What about that support code requires it to exist in one place only? — HolyBlackCat, Jul 16 '17 at 13:55
There is no defining macros inside macros. The preprocessor is one pass only. — StoryTeller - Unslander Monica, Jul 16 '17 at 13:58
@HolyBlackCat: The support code consists of function with 'type' in their names. Therefore double macro would mean double code would mean compile failure. — PaulProgrammerNoob, Jul 16 '17 at 14:05
@StoryTeller: Did you even read my question? I said that by myself. — PaulProgrammerNoob, Jul 16 '17 at 14:05
Then what purpose is there to show code that attempts it, other than to obfuscate your question? Write your own preprocessor in python or perl if you need complex preprocessing, there is no magic solution. — StoryTeller - Unslander Monica, Jul 16 '17 at 14:07
Next, in what language you want your answer to be? C or C++ or both? — HolyBlackCat, Jul 16 '17 at 14:07
@StoryTeller: It was easier for me to formulate my question with this example, cause everybody knows include guards. — PaulProgrammerNoob, Jul 16 '17 at 14:10
Funny enough, include guards have nothing to do with this question. The title is quite misleading. — StoryTeller - Unslander Monica, Jul 16 '17 at 14:11
@StoryTeller I think the entire question is still somewhat misleading and looks like an XY problem. It seems that what OP *actually* seems to ask is how to dodge 'multiple definitions of function foo()' when redefining the same function with a macro. — HolyBlackCat, Jul 16 '17 at 14:14
@PaulProgrammerNoob Is there a reason you can't use templates for that instead of macros? — HolyBlackCat, Jul 16 '17 at 14:14
@HolyBlackCat: Templates are fine. I am using templates already, but I did not found a solution for this one. I will show you more code then. And yes, that's what I wanted to ask. — PaulProgrammerNoob, Jul 16 '17 at 14:16
@EdHeal: Because I dont' want do duplicate my function for each member name and I didn't find a template solution. If you have one, I am fine with that, too. — PaulProgrammerNoob, Jul 16 '17 at 14:27
I still don't understand what you're trying to do. Tell us exactly what do you want to achieve. Also, how are you using `DEFINE_MEMBER_CHECKER()` in `CREATE_SUPPORT_FOR(type)`? There are no parameters for `DEFINE_MEMBER_CHECKER()` that depend on type. — HolyBlackCat, Jul 16 '17 at 14:31
@PaulProgrammerNoob It seems that your question now is "I don't want to double my function decalarations, when using the same name twice"; maybe you should clarify what exactly is doubled. — anatolyg, Jul 16 '17 at 14:57

Avoid multiple macro unfolding for same input

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