Re: [Boost-users] [preprocessor] BOOST_PP_REPEAT with multiple "data"

Subject: Re: [Boost-users] [preprocessor] BOOST_PP_REPEAT with multiple "data"
From: Igor R (boost.lists_at_[hidden])
Date: 2014-05-28 13:39:02

2014-05-28 17:30 GMT+03:00 TONGARI J <tongari95_at_[hidden]>:

> 2014-05-28 22:20 GMT+08:00 TONGARI J <tongari95_at_[hidden]>:
>
> 2014-05-28 21:53 GMT+08:00 Igor R <boost.lists_at_[hidden]>:
>>
>> Hello,
>>>
>>> I'm trying to make a macro for the following case (essentially, I have
>>> to translate a run-time decision to a compile-time one):
>>>
>>> switch(i)
>>> {
>>> case 1: my_func(arg, offsetof(my_struct, arr[1].my_field))); break;
>>> case 2: my_func(arg, offsetof(my_struct, arr[2].my_field))); break;
>>> // etc. up to 10
>>> }
>>>
>>> If I could hard-code the whole function call into the macro, I could do
>>> something like this:
>>> #define EXPR(unused, x, text) my_func(arg, offsetof(my_struct,
>>> arr[x].my_field))); break;
>>>
>>> #define MY_MACRO(x) \
>>> switch(x)\
>>> {\
>>> BOOST_PP_REPEAT(x, EXPR, _)\
>>> }
>>>
>>> However, I'd like to pass "arg" and "my_field" as parameters to this
>>> macro.
>>> What would be the right way to do this?
>>>
>>
>> Perhaps:
>>
>>     #define EXPR(unused, x, data) \
>>     case x: my_func(BOOST_PP_TUPLE_ELEM(2, 0, data) , offsetof(my_struct,
>> arr[x].BOOST_PP_TUPLE_ELEM(2, 1, data)))); break;
>>
>>     #define MY_MACRO(x, arg, my_field) \
>>     switch(i)\
>>     {\
>>     BOOST_PP_REPEAT(x, EXPR, (arg, my_field))\
>>     }
>>
>> Or if you don't insist on a PP solution, lambda may help as well:
>>
>>     #define EXPR(unused, x, data) case x: f(x); break;
>>     #define MY_MACRO(x, arg, my_field) \
>>     auto f = [&](int i){ my_func(arg, offsetof(my_struct,
>> arr[i].my_field))); } \
>>     switch(i)\
>>     {\
>>     BOOST_PP_REPEAT(x, EXPR, (arg, my_field))\
>>     }
>>
>> (note: code not tested)
>>
>
> I guess your example is a contrived one, since you don't even need a
> switch...
>

The expression "offsetof(my_struct, arr[i].my_field))" requires
compile-time "i" - that's the point, so your 2nd suggestion won't work (and
certainly, my own example was wrong - I can't put "switch(x)" into the
macro).
I'll try to apply the 1st one.

Thanks!