Boost Preprocessor Recursion - c

Question

I'm trying to create a 'C' macro (not C++) that will define and initialize static data.
For example:
STATIC_CONST_STRUCT
(
A, a,
MEMBER_DATA(CONST_STR, a, "Hello, a")
MEMBER_DATA(CONST_STR, b, "Hello, b")
MEMBER_STRUCT
(
C, c,
MEMBER_DATA(CONST_STR, d, "Hello, d")
MEMBER_DATA(CONST_INT, e, 1)
)
);
Would cause the 'C' preprocessor to create:
static const struct A
{
CONST_STR a;
CONST_STR b;
struct C
{
CONST_STR d;
CONST_INT e;
} c;
} =
{"Hello, a", "Hello, b", {"Hello, d", 1}};
I've tried to use the Boost Preprocessor
http://www.boost.org/doc/libs/1_54_0/libs/preprocessor/doc/
but I can't quite figure out how to make this work. My macros stop expanding. I suspect the recursive nature of the problem having an arbitrarily deep nesting is why.
The solutions I read regarding getting the preprocessor to recurse either don't work, or the description of how to get recursion to work isn't clear enough to implement a working solution.
Here's what I have so far:
#define MEMBER_DATA_TAG 0
#define MEMBER_STRUCT_TAG 1
#define MEMBER_TAG(MEMBER) BOOST_PP_SEQ_ELEM(0, MEMBER)
#define MEMBER_DATA_TYPE(MEMBER_DATA) BOOST_PP_SEQ_ELEM(1, MEMBER_DATA)
#define MEMBER_DATA_NAME(MEMBER_DATA) BOOST_PP_SEQ_ELEM(2, MEMBER_DATA)
#define MEMBER_DATA_VALUE(MEMBER_DATA) BOOST_PP_SEQ_ELEM(3, MEMBER_DATA)
#define MEMBER_STRUCT_TYPE(MEMBER_STRUCT) BOOST_PP_SEQ_ELEM(1, MEMBER_STRUCT)
#define MEMBER_STRUCT_NAME(MEMBER_STRUCT) BOOST_PP_SEQ_ELEM(2, MEMBER_STRUCT)
#define MEMBER_STRUCT_MEMBER_SEQ(MEMBER_STRUCT) BOOST_PP_SEQ_ELEM(3, MEMBER_STRUCT)
#define MEMBER_DATA(TYPE, NAME, VALUE) ((MEMBER_DATA_TAG)(TYPE)(NAME)(VALUE))
#define MEMBER_STRUCT(TYPE, NAME, MEMBER_SEQ) ((MEMBER_STRUCT_TAG)(TYPE)(NAME)(MEMBER_SEQ))
#define IS_MEMBER_STRUCT(MEMBER_SEQ_ELEM) BOOST_PP_EQUAL(MEMBER_TAG(MEMBER_SEQ_ELEM), MEMBER_STRUCT_TAG)
#define MEMBER_STRUCT_DECLARE(TYPE, NAME, MEMBER_SEQ) \
struct TYPE \
{ \
BOOST_PP_SEQ_FOR_EACH(MEMBER_ELEM_DECLARE, BOOST_PP_EMPTY(), MEMBER_SEQ) \
} NAME
#define MEMBER_ELEM_DECLARE(_r, _data, MEMBER_SEQ_ELEM) \
BOOST_PP_IIF \
( \
IS_MEMBER_STRUCT(MEMBER_SEQ_ELEM), \
MEMBER_STRUCT_DECLARE \
( \
MEMBER_STRUCT_TYPE(MEMBER_SEQ_ELEM), \
MEMBER_STRUCT_NAME(MEMBER_SEQ_ELEM), \
MEMBER_STRUCT_MEMBER_SEQ(MEMBER_SEQ_ELEM) \
), \
MEMBER_DATA_DECLARE \
( \
MEMBER_DATA_TYPE(MEMBER_SEQ_ELEM), \
MEMBER_DATA_NAME(MEMBER_SEQ_ELEM), \
MEMBER_DATA_VALUE(MEMBER_SEQ_ELEM) \
) \
);
#define MEMBER_DATA_DECLARE(TYPE, NAME, VALUE) TYPE NAME
#define MEMBER_VALUE_INIT(MEMBER_SEQ) \
BOOST_PP_SEQ_FOR_EACH_I(MEMBER_VALUE_INIT_DECLARE, BOOST_PP_EMPTY(), MEMBER_SEQ);
#define MEMBER_VALUE_INIT_DECLARE(_r, _data, i, MEMBER_SEQ_ELEM) \
BOOST_PP_COMMA_IF(i) \
BOOST_PP_IIF \
( \
IS_MEMBER_STRUCT(MEMBER_SEQ_ELEM), \
{MEMBER_VALUE_INIT(MEMBER_SEQ_ELEM)}, \
MEMBER_DATA_VALUE(MEMBER_SEQ_ELEM) \
)
#define STATIC_CONST_STRUCT(TYPE, NAME, MEMBER_SEQ) \
static const MEMBER_STRUCT_DECLARE(TYPE, NAME, MEMBER_SEQ) = \
{ \
MEMBER_VALUE_INIT(MEMBER_SEQ) \
}
Thanks.

Answer 1

It can be done without boost-preprocessor.
You don't actually need recursion. Just a loop to iterate over the macro arguments twice.
I took liberty of modifying the syntax a bit, to allow for commas in types and initializers, in case someone decides to use it with C++.
STATIC_CONST_STRUCT
(
A, a,
MEMBER_DATA(a, const char *) "Hello, a"
MEMBER_DATA(b, const char *) "Hello, b"
MEMBER_STRUCT
(
C, c,
MEMBER_DATA(d, const char *) "Hello, d"
MEMBER_DATA(e, int) 42
)
)
This expands to:
static const struct A
{
const char *a;
const char *b;
struct C
{
const char *d;
int e;
} c;
} a =
{
"Hello, a",
"Hello, b",
{
"Hello, d",
42,
},
};
Implementation:
#define STATIC_CONST_STRUCT(type_, name_, ...) \
static const struct type_ { \
END( LOOP_DECL_0 (__VA_ARGS__) ) \
} name_ = { \
END( LOOP_INIT_0 (__VA_ARGS__) ) \
};
#define MEMBER_DATA(name_, ...) )(var,name_,(__VA_ARGS__),
#define MEMBER_STRUCT(type_, name_, ...) )(open,type_ __VA_ARGS__ )(close,name_
#define IDENTITY(...) __VA_ARGS__
#define CAT(x, y) CAT_(x, y)
#define CAT_(x, y) x##y
#define END(...) END_(__VA_ARGS__)
#define END_(...) __VA_ARGS__##_END
#define LOOP_DECL_0() LOOP_DECL_A
#define LOOP_DECL_A(...) LOOP_DECL_BODY(__VA_ARGS__) LOOP_DECL_B
#define LOOP_DECL_B(...) LOOP_DECL_BODY(__VA_ARGS__) LOOP_DECL_A
#define LOOP_DECL_0_END
#define LOOP_DECL_A_END
#define LOOP_DECL_B_END
#define LOOP_DECL_BODY(action_, ...) CAT(LOOP_DECL_BODY_, action_)(__VA_ARGS__)
#define LOOP_DECL_BODY_var(name_, type_, ...) IDENTITY type_ name_;
#define LOOP_DECL_BODY_open(type_) struct type_ {
#define LOOP_DECL_BODY_close(name_) } name_;
#define LOOP_INIT_0() LOOP_INIT_A
#define LOOP_INIT_A(...) LOOP_INIT_BODY(__VA_ARGS__) LOOP_INIT_B
#define LOOP_INIT_B(...) LOOP_INIT_BODY(__VA_ARGS__) LOOP_INIT_A
#define LOOP_INIT_0_END
#define LOOP_INIT_A_END
#define LOOP_INIT_B_END
#define LOOP_INIT_BODY(action_, ...) CAT(LOOP_INIT_BODY_, action_)(__VA_ARGS__)
#define LOOP_INIT_BODY_var(name_, type_, ...) __VA_ARGS__,
#define LOOP_INIT_BODY_open(type_) {
#define LOOP_INIT_BODY_close(name_) },
If used with C++, IDENTITY type_ should be wrapped in std::type_identity_t<...> to allow using types such as function pointers without typedefing them. void (*)() foo; is illegal, while std::type_identity_t<void (*)()> foo; is ok.