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

Subject: Re: [Boost-users] Looping through variadic macro arguments
From: Florian Lindner (mailinglists_at_[hidden])
Date: 2016-03-23 04:44:03

On Tue, 22 Mar 2016 20:53:06 -0400
Edward Diener <eldiener_at_[hidden]> wrote:

> On 3/22/2016 3:35 PM, Florian Lindner wrote:
> > On Mon, 21 Mar 2016 10:39:52 -0400
> > Edward Diener <eldiener_at_[hidden]> 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 ?

That is obviously unneeded, but not the cause.

> Please show the header files being included.

Sorry, my fault. I included the Header for for_each not for for_each_i.

Best,
Florian

> 
> >
> >
> > 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
> >