Re: [Boost-users] [preprocessor] It works, but how can I make it better/more elegant?

Subject: Re: [Boost-users] [preprocessor] It works, but how can I make it better/more elegant?
From: Daniel Bradburn (moagstar_at_[hidden])
Date: 2010-07-29 03:27:11

The following is probably a cleaner way of achieving the same result:
// Pass the sequence in as the additional data - then access the nth element which // will be your tuple.  This can then be split into it's individual elements#define TEST_PP_GENERATE_SUPPORTED_TYPE_PAIR(z, n, seq)		\	boost::mpl::pair<									\	BOOST_PP_TUPLE_ELEM(2, 0, BOOST_PP_SEQ_ELEM(n, seq)),	\	BOOST_PP_TUPLE_ELEM(2, 1, BOOST_PP_SEQ_ELEM(n, seq))>// You don't need to separate the elements of a sequence with commas, which // is probably why it wasn't working with BOOST_PP_SEQ_SIZE#define TEST_PP_SUPPORTED_TYPES				\	((SomeIntArray, SomeInt))					\	((SomeDoubleArray, SomeDouble))			\	((SomBoolArray, SomeBool))				\	((SomeOtherClassArray, SomeOtherClass))
// Generate the pairstypedef boost::mpl::map<	BOOST_PP_ENUM(BOOST_PP_SEQ_SIZE(TEST_PP_SUPPORTED_TYPES),		TEST_PP_GENERATE_SUPPORTED_TYPE_PAIR, TEST_PP_SUPPORTED_TYPES)>supported_types_map;
"Was I correct in my assumption (which is why I handled it the way I did in my below code) that basically anytime a a comma appears, the two surrounding tokens must be treated as a tuple (and thus parenthesized?"
Yes otherwise the preprocessor thinks that they are the parameters to be passed to a macro.
If you can guarentee some continuity with the names of these types, then you could simplify it further, e.g.
// You can generate the second parameter from the element name#define TEST_PP_GENERATE_SUPPORTED_TYPE_PAIR(z, n, seq)		\	boost::mpl::pair<									\	BOOST_PP_CAT(BOOST_PP_SEQ_ELEM(n, seq), Array),		\	BOOST_PP_SEQ_ELEM(n, seq)>
#define TEST_PP_SUPPORTED_TYPES		\	(SomeInt)						\	(SomeDouble)					\	(SomeBool)					\	(SomeOtherClass)typedef boost::mpl::map<	BOOST_PP_ENUM(BOOST_PP_SEQ_SIZE(TEST_PP_SUPPORTED_TYPES),		TEST_PP_GENERATE_SUPPORTED_TYPE_PAIR, TEST_PP_SUPPORTED_TYPES)>supported_types_map;

> To: boost-users_at_[hidden]
> From: geoff.hilton_at_[hidden]
> Date: Wed, 28 Jul 2010 13:47:56 -0400
> Subject: [Boost-users] [preprocessor] It works,	but how can I make it better/more elegant?
> 
> Yay! I finally got my type map generating code to work! There has got to 
> be a better way though because it feels really kludgey. Opinions? If you 
> have a more elegant way of accomplishing the same thing (without 
> misusing BOOST_PP_APPLY as I think I did, because I only used it to 
> remove the extra pair of parens surrounding the tuple in 
> TEST_PP_get_type, something that, oddly, BOOST_PP_TUPLE_REM_CTOR doesn't 
> succeed at doing (at least not without breaking the current code).
> 
> Was I correct in my assumption (which is why I handled it the way I did 
> in my below code) that basically anytime a a comma appears, the two 
> surrounding tokens must be treated as a tuple (and thus parenthesized? 
> This is what I've gathered from the online documentation as well as the 
> "C++ Template Metaprogramming" book.
> 
> The purpose of this code is to create a type map between array types and 
> corresponding value types of 3rd party classes which don't conform at 
> all to STL usage practices, that which I'd like to use, so I'm writing 
> an array wrapper template class with associated iterator template 
> classes using iterator_facade & the MPL. The map generated by this code 
> is used to map between array and value types for type selection purposes.
> 
> The following (reformatted for readability) code should generate this 
> (also reformatted) preprocessed output, though I've only tested the real 
> code and not this that is mostly copy/pasted:
> 
> typedef bmpl::map<
>    bmpl::pair<SomeIntArray, SomeInt>,
>    bmpl::pair<SomeDoubleArray, SomeDouble>,
>    bmpl::pair<SomeBoolArray, SomeBool>,
>    bmpl::pair<SomeOtherClassArray, SomeOtherClass>
>  > supported_types_map;
> 
> --test.hpp--
> (omitting #inclusions)
> namespace blah {
> namespace bmpl = boost::mpl;
> 
> //must match size as below tuples in sequence for readability.
> #define TEST_PP_tuple_size 2
> 
> //index of array type inside tuple for readability.
> #define TEST_PP_array_descriptor 0
> 
> //index of value type inside tuple for readability.
> #define TEST_PP_value_descriptor 1
> //sequence of tuples of supported types
> #define TEST_PP_supported_types         \
> ((SomeIntArray, SomeInt))               \
> ,((SomeDoubleArray, SomeDouble))        \
> ,((SomBoolArray, SomeBool))             \
> ,((SomeOtherClassArray, SomeOtherClass))\
> /**/
> 
> //size of TEST_PP_supported_types (BOOST_PP_SEQ_SIZE doesn't work on it)
> #define TEST_PP_types_size 4
> 
> #define TEST_PP_get_type(index, type_descriptor)       \
>    BOOST_PP_TUPLE_ELEM(TEST_PP_types_tuple_size,        \
>      type_descriptor,                                   \
>      TEST_PP_get_tuple(index)                           \
>    )                                                    \
> /**/
> 
> //gets tuple from TEST_PP_supported_types
> #define TEST_PP_get_tuple(index)                          \
>    BOOST_PP_APPLY(                                         \
>      BOOST_PP_TUPLE_ELEM(                                  \
>        TEST_PP_types_size, index, (TEST_PP_supported_types)\
>      )                                                     \
>    )                                                       \
> /**/
> 
> //this returns a tuple containing the first and last part of the mpl::pair
> #define TEST_PP_bmpl_pair(index)                                      \
>    (bmpl::pair<TEST_PP_get_type(index, TEST_PP_array_descriptor),      \
>      TEST_PP_get_type(index, TEST_PP_value_descriptor)>)               \
> /**/
> 
> #define TEST_PP_pair_fwder(next_index, index, data) \
>    BOOST_PP_TUPLE_REM_CTOR(2,TEST_PP_bmpl_pair(index))    \
> /**/
> 
> typedef bmpl::map<
>    BOOST_PP_ENUM(TEST_PP_types_size, TEST_PP_pair_fwder, ~)
>  > supported_types_map;
> } //namespace blah
> 
> 
> 
> Thank you!
> Geoff
> 
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