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

2014-05-28 17:30 GMT+03:00 TONGARI J :

2014-05-28 22:20 GMT+08:00 TONGARI J :

2014-05-28 21:53 GMT+08:00 Igor R :

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Hello,

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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!