Re: [Boost-users] Looping through variadic macro arguments

Subject: Re: [Boost-users] Looping through variadic macro arguments
From: Edward Diener (eldiener_at_[hidden])
Date: 2016-03-18 09:06:26

On 3/18/2016 6:19 AM, Florian Lindner wrote:
> Hello,
>
> I have this macro:
>
> #define vmacro(expr, ...)                                              \
>    cout << "n macro = " << BOOST_PP_VARIADIC_SIZE(__VA_ARGS__) << endl;  \
>    cout << BOOST_PP_VARIADIC_ELEM(0, __VA_ARGS__) << endl;               \
>    cout << BOOST_PP_VARIADIC_ELEM(1, __VA_ARGS__) << endl;               \
>    for (int i = 0; i < BOOST_PP_VARIADIC_SIZE(__VA_ARGS__); i++) {       \
>      cout << i << ": " << BOOST_PP_VARIADIC_ELEM(i, __VA_ARGS__) << endl; \
>    }
>
> The VARIADIC_SIZE is correct and I can access elements using BOOST_PP_VARIADIC_ELEM as long as I use a literal. The loop body produces error messages when compiling.
> BOOST_PP_VARIADICS is 1.

Your macro 'vmacro' gets expanded at preprocessor time when it is 
invoked as 'vmacro(4,5,6)'. At preprocessor time the expansion of 
'BOOST_PP_VARIADIC_ELEM(i, __VA_ARGS__)' fails because 'i' is not a 
valid Boost PP number.

>
> The goal is to concatenate all variadic macro arguments to a string. If
>   there a more clever solution, I'm very eager to hear them.

#include <boost/preprocessor/stringize.hpp>
#include <boost/preprocessor/seq/cat.hpp>
#include <boost/preprocessor/variadic/to_seq.hpp>

cout << 
BOOST_PP_STRINGIZE(BOOST_PP_SEQ_CAT(BOOST_PP_VARIADIC_TO_SEQ(__VA_ARGS__))) 
<< endl;

>
> Thanks!
> Florian
>
>
> ERROR MESSAGES:
>
> % clang++ -std=c++11 -g3 -rdynamic  -ldl test.cpp
> test.cpp:154:3: error: expected expression
>    vmacro(4, 5, 6);
>    ^
> test.cpp:120:26: note: expanded from macro 'vmacro'
>      cout << i << ": " << BOOST_PP_VARIADIC_ELEM(i, __VA_ARGS__) << endl;                                    \
>                           ^
> /usr/include/boost/preprocessor/variadic/elem.hpp:26:101: note: expanded from macro 'BOOST_PP_VARIADIC_ELEM'
> #        define BOOST_PP_VARIADIC_ELEM(n, ...) BOOST_PP_CAT(BOOST_PP_VARIADIC_ELEM_, n)(__VA_ARGS__,)
>                                                                                                      ^
> 1 error generated.
>
>
> g++ -std=c++11 -g3 -rdynamic  -ldl test.cpp
> In file included from test.cpp:9:0:
> test.cpp: In function 'void f(int)':
> test.cpp:120:26: error: expected primary-expression before ')' token
>       cout << i << ": " << BOOST_PP_VARIADIC_ELEM(i, __VA_ARGS__) << endl; \
>                            ^
> test.cpp:154:3: note: in expansion of macro 'vmacro'
>     vmacro(4, 5, 6);
>     ^
> test.cpp:120:26: error: 'BOOST_PP_VARIADIC_ELEM_i' was not declared in this scope
>       cout << i << ": " << BOOST_PP_VARIADIC_ELEM(i, __VA_ARGS__) << endl; \
>                            ^
> test.cpp:154:3: note: in expansion of macro 'vmacro'
>     vmacro(4, 5, 6);
>     ^
>