Preprocessor initialize array

Question

How, if necessary, using boost preprocessor, initialize the array as follows:

INIT_ARRAY(plus,minus)
//extract to 
std::array<std::pair<char const *,std::string>, 2> array{{
{"plus", std::string("plus")}, {"minus", std::string("minus")} }};

INIT_ARRAY(plus,minus,multiply)
//extract to 
std::array<std::pair<char const *,std::string>, 3> array{{
{"plus", std::string("plus")}, {"minus", std::string("minus")}, {"multiply", std::string("multiply")} }};

P.S. Couldn't solve the problem with the last comma in the initializer (...{n-1} , {n} ,) and counting the number of arguments to pass to std::array<...,n>. I use the preprocessor, since the names passed to the macro will be used later for code generation. Used c++20

If you're using C++17 or later, you can just let std::array deduce its arguments from the initializer, no counting required. If not, please tag the appropriate C++ version. — Quentin
– Quentin, Commented Oct 21, 2020 at 15:58
@Quentin I would like to see the full declaration of the macro that implements the initializer — Spaghetti Man
– Spaghetti Man, Commented Oct 21, 2020 at 16:40

Artyer · Accepted Answer · 2020-10-21 16:48:15Z

1

You can use BOOST_PP_COMMA_IF for conditional commas, and BOOST_PP_VARIADIC_SIZE to work out the size beforehand.

This should work (for non-empty arrays):

#include <boost/preprocessor/punctuation/comma_if.hpp>
#include <boost/preprocessor/seq/for_each_i.hpp>
#include <boost/preprocessor/stringize.hpp>
#include <boost/preprocessor/variadic/size.hpp>
#include <boost/preprocessor/variadic/to_seq.hpp>


#define INIT_ARRAY_INITIALIZER(r, _, i, e) BOOST_PP_COMMA_IF(i) {BOOST_PP_STRINGIZE(e), std::string(BOOST_PP_STRINGIZE(e))}

#define INIT_ARRAY(...) \
std::array<std::pair<char const *, std::string>, \
BOOST_PP_VARIADIC_SIZE(__VA_ARGS__)> array{{ \
    BOOST_PP_SEQ_FOR_EACH_I(INIT_ARRAY_INITIALIZER, _, BOOST_PP_VARIADIC_TO_SEQ(__VA_ARGS__)) \
}};

edited Oct 21, 2020 at 16:48

answered Oct 21, 2020 at 16:43

Artyer

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Spaghetti Man Over a year ago

Can you recommend preprocessor tutorials? Thanks for the answer. Working.

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