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

Subject: Re: [Boost-users] [preprocessor] BOOST_PP_REPEAT with multiple "data"
From: TONGARI J (tongari95_at_[hidden])
Date: 2014-05-28 10:30:00

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...