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

23 Mar 2016

      Hello,

another question: I was expecting that the line

BOOST_PP_SEQ_FOR_EACH_I(PRINT_ARGUMENT,, BOOST_PP_VARIADIC_TO_SEQ(__VA_ARGS__));

in a macro #define vm(expr, ...) expands to nothing, when the the macro is called with just one argument vm(false), but it fails, only when called with just one argument:

% g++ -std=c++11 -g3 test.cpp

test.cpp: In function 'void f(int)':
test.cpp:109:51: error: expected primary-expression before '<<' token
   std::cerr << "  Argument " << i << ": " << elem << std::endl;
                                                   ^
/usr/include/boost/preprocessor/seq/for_each_i.hpp:85:66: note: in expansion of macro 'PRINT_ARGUMENT'
 # define BOOST_PP_SEQ_FOR_EACH_I_M_I(r, macro, data, seq, i, sz) macro(r, data, i, BOOST_PP_SEQ_HEAD(seq))
                                                                  ^
test.cpp:140:1: note: in expansion of macro 'assertion'
 assertion(!true);
 ^

Is there any way to make it work like that?

Thanks a lot!
Florian

The code looks like that:

#define PRINT_ARGUMENT(r, data, i, elem)                        \
  std::cerr << "  Argument " << i << ": " << elem << std::endl;

#define assertion(expr, ...) if (!(expr)) {                             \
    std::cerr << "Assertion in " << __FILE__ << ":" << __LINE__         \
              << ", failed expression: " << #expr <<  std::endl;        \
    std::cout << BOOST_PP_VARIADIC_SIZE(__VA_ARGS__) << std::endl;      \
    BOOST_PP_SEQ_FOR_EACH_I(PRINT_ARGUMENT,, BOOST_PP_VARIADIC_TO_SEQ(__VA_ARGS__)); \
    std::cerr.flush();                                                  \
    std::cout.flush();                                                  \
    assert(false);                                                      \
  }                                                                     \

On Tue, 22 Mar 2016 20:53:06 -0400
Edward Diener  wrote:
...
On 3/22/2016 3:35 PM, Florian Lindner wrote:
...
On Mon, 21 Mar 2016 10:39:52 -0400
Edward Diener  wrote:
...
On 3/21/2016 4:55 AM, Florian Lindner wrote:
...
Hey,
snipped ...
...
[...]
...
Any comments on that? How to do it better? Maybe without a helper
macro PRINT?
I'm a bit surprised I have to use r-1 to get 1 for the first
argument.
The PRINT macro is fine. Almost all Boost PP looping constructs
use a helper macro like that.
In real life code I would name all macros in uppercase and give
them a very distinct name to reduce clashes with other potential
C++ identifiers and macros.
Sure!
...
Look at BOOST_PP_SEQ_FOR_EACH_I to get the actual argument index
instead of using 'r'.
That 'r' in your code above is actually 2,3, and 4 is just luck.
The 'next available BOOST_PP_FOR repetition' as described in the
doc just happened to be 2,3, and 4 but could theoretically have
been any PP number each time and is not related to the index of
each argument.
Ok, try to use FOR_EACH_I, but ran into compilation errors:
#define PRINT(r, data, i, elem)                         \
   cout << "Argument " << i << ": " << elem << endl;
#define
vmacro(expr, ...)                                               \
cout << "Expression: " << #expr <<
endl;                              \
BOOST_PP_SEQ_FOR_EACH_I(PRINT, ,
BOOST_PP_VARIADIC_TO_SEQ(__VA_ARGS__)); \
What is that final '\' about ?
Please show the header files being included.
...
but:
% g++ -std=c++11 -g3 -rdynamic  -ldl test.cpp
&& ./a.out                                                                                                                                                                         :( test.cpp:
In function 'void f(int)': test.cpp:127:27: error: 'PRINT' was not
declared in this scope BOOST_PP_SEQ_FOR_EACH_I(PRINT, ,
BOOST_PP_VARIADIC_TO_SEQ(__VA_ARGS__)); \ ^
test.cpp:159:3: note: in expansion of macro 'vmacro'
    vmacro(1-2, 5, 6);
    ^
test.cpp:127:34: error: expected primary-expression before ',' token
    BOOST_PP_SEQ_FOR_EACH_I(PRINT, ,
BOOST_PP_VARIADIC_TO_SEQ(__VA_ARGS__)); \ ^
test.cpp:159:3: note: in expansion of macro 'vmacro'
    vmacro(1-2, 5, 6);
    ^
In file included
from /usr/include/boost/preprocessor/variadic/to_seq.hpp:17:0, from
test.cpp:11: test.cpp:127:36: error: expression cannot be used as a
function BOOST_PP_SEQ_FOR_EACH_I(PRINT, ,
BOOST_PP_VARIADIC_TO_SEQ(__VA_ARGS__)); \ ^
test.cpp:159:3: note: in expansion of macro 'vmacro'
    vmacro(1-2, 5, 6);
    ^
test.cpp:127:73: error: 'BOOST_PP_SEQ_FOR_EACH_I' was not declared
in this scope BOOST_PP_SEQ_FOR_EACH_I(PRINT, ,
BOOST_PP_VARIADIC_TO_SEQ(__VA_ARGS__)); \ ^
test.cpp:159:3: note: in expansion of macro 'vmacro'
    vmacro(1-2, 5, 6);
    ^
boost 1.60, gcc 5.3.0.
The same code, using FOR_EACH works (only the argumentens of PRINT
were altered.
Thanks!
Florian