考虑下面的代码。
int value1 = BOOST_BINARY( 100 111000 01 1 110 );
有人可以解释 BOOST_BINARY 如何计算那里的 int 值吗？
问问题
1271 次
2 回答
5
使用来源，卢克！
The following code works by converting the input bit pattern into a
Boost.Preprocessor sequence, then converting groupings of 3 bits each into
the corresponding octal digit, and finally concatenating all of the digits
together along with a leading zero. This yields a standard octal literal
with the desired value as specified in bits.
这很容易。我们只需要定义一些宏。紧紧抓住。
#define BOOST_BINARY( bit_groupings ) \
BOOST_BINARY_LITERAL_D( BOOST_PP_DEDUCE_D(), bit_groupings )
// ...
#define BOOST_BINARY_LITERAL_D( d, bit_groupings ) \
BOOST_PP_SEQ_CAT \
( (0) BOOST_DETAIL_CREATE_BINARY_LITERAL_OCTAL_SEQUENCE( d, bit_groupings ) \
)
#define BOOST_DETAIL_CREATE_BINARY_LITERAL_OCTAL_SEQUENCE( d, bit_groupings ) \
BOOST_PP_SEQ_TRANSFORM \
( BOOST_DETAIL_TRIPLE_TO_OCTAL_OPERATION \
, BOOST_PP_NIL \
, BOOST_PP_IDENTITY( BOOST_DETAIL_CONVERT_BIT_SEQUENCE_TO_TRIPLE_SEQUENCE )()\
( BOOST_DETAIL_COMPLETE_TRIPLE_SEQUENCE \
( \
d \
, BOOST_DETAIL_CREATE_BINARY_LITERAL_BIT_SEQUENCE( d, bit_groupings ) \
) \
) \
)
#define BOOST_DETAIL_CONVERT_BIT_SEQUENCE_TO_TRIPLE_SEQUENCE( bit_sequence ) \
BOOST_PP_CAT \
( BOOST_DETAIL_CONVERT_BIT_SEQUENCE_TO_PARENTHETIC_TUPLE_1 bit_sequence \
, END_BIT \
)
#define BOOST_DETAIL_BITS_PER_OCTIT 3
#define BOOST_DETAIL_COMPLETE_TRIPLE_SEQUENCE( d, incomplete_nibble_sequence ) \
BOOST_PP_CAT \
( BOOST_DETAIL_CREATE_TRIPLE_COMPLETION_SEQUENCE_ \
, BOOST_PP_MOD_D( d \
, BOOST_PP_SEQ_SIZE( incomplete_nibble_sequence ) \
, BOOST_DETAIL_BITS_PER_OCTIT \
) \
) \
incomplete_nibble_sequence
#define BOOST_DETAIL_FIXED_COMPL( bit ) \
BOOST_PP_CAT( BOOST_DETAIL_FIXED_COMPL_, bit )
#define BOOST_DETAIL_FIXED_COMPL_0 1
#define BOOST_DETAIL_FIXED_COMPL_1 0
#define BOOST_DETAIL_CREATE_BINARY_LITERAL_BIT_SEQUENCE( d, bit_groupings ) \
BOOST_PP_EMPTY \
BOOST_PP_CAT( BOOST_PP_WHILE_, d ) \
( BOOST_DETAIL_BINARY_LITERAL_PREDICATE \
, BOOST_DETAIL_BINARY_LITERAL_OPERATION \
, bit_groupings () \
)
#define BOOST_DETAIL_BINARY_LITERAL_PREDICATE( d, state ) \
BOOST_DETAIL_FIXED_COMPL( BOOST_DETAIL_IS_NULLARY_ARGS( state ) )
#define BOOST_DETAIL_BINARY_LITERAL_OPERATION( d, state ) \
BOOST_DETAIL_SPLIT_AND_SWAP \
( BOOST_PP_CAT( BOOST_DETAIL_BINARY_LITERAL_ELEMENT_, state ) )
#define BOOST_DETAIL_TRIPLE_TO_OCTAL_OPERATION( s, dummy_param, tuple ) \
BOOST_DETAIL_TERNARY_TRIPLE_TO_OCTAL tuple
#define BOOST_DETAIL_TERNARY_TRIPLE_TO_OCTAL( bit2, bit1, bit0 ) \
BOOST_DETAIL_TRIPLE_TO_OCTAL_ ## bit2 ## bit1 ## bit0
#define BOOST_DETAIL_CREATE_TRIPLE_COMPLETION_SEQUENCE_1 (0)(0)
#define BOOST_DETAIL_CREATE_TRIPLE_COMPLETION_SEQUENCE_2 (0)
#define BOOST_DETAIL_CREATE_TRIPLE_COMPLETION_SEQUENCE_0
#define BOOST_DETAIL_CONVERT_BIT_SEQUENCE_TO_PARENTHETIC_TUPLE_1END_BIT
#define BOOST_DETAIL_CONVERT_BIT_SEQUENCE_TO_PARENTHETIC_TUPLE_1( bit ) \
( ( bit, BOOST_DETAIL_CONVERT_BIT_SEQUENCE_TO_PARENTHETIC_TUPLE_2
#define BOOST_DETAIL_CONVERT_BIT_SEQUENCE_TO_PARENTHETIC_TUPLE_2( bit ) \
bit, BOOST_DETAIL_CONVERT_BIT_SEQUENCE_TO_PARENTHETIC_TUPLE_3
#define BOOST_DETAIL_CONVERT_BIT_SEQUENCE_TO_PARENTHETIC_TUPLE_3( bit ) \
bit ) ) BOOST_DETAIL_CONVERT_BIT_SEQUENCE_TO_PARENTHETIC_TUPLE_1
#define BOOST_DETAIL_SPLIT_AND_SWAP( params ) \
BOOST_PP_IDENTITY( BOOST_DETAIL_SPLIT_AND_SWAP_PARAMS )()( params )
#define BOOST_DETAIL_SPLIT_AND_SWAP_PARAMS( first_param, second_param ) \
second_param first_param
#define BOOST_DETAIL_LEFT_OF_COMMA( params ) \
BOOST_PP_IDENTITY( BOOST_DETAIL_FIRST_MACRO_PARAM )()( params )
#define BOOST_DETAIL_FIRST_MACRO_PARAM( first_param, second_param ) \
first_param
/* Begin derived concepts from Chaos by Paul Mensonides */
#define BOOST_DETAIL_IS_NULLARY_ARGS( param ) \
BOOST_DETAIL_LEFT_OF_COMMA \
( BOOST_PP_CAT( BOOST_DETAIL_IS_NULLARY_ARGS_R_ \
, BOOST_DETAIL_IS_NULLARY_ARGS_C param \
) \
)
#define BOOST_DETAIL_IS_NULLARY_ARGS_C() \
1
#define BOOST_DETAIL_IS_NULLARY_ARGS_R_1 \
1, BOOST_PP_NIL
#define BOOST_DETAIL_IS_NULLARY_ARGS_R_BOOST_DETAIL_IS_NULLARY_ARGS_C \
0, BOOST_PP_NIL
/* End derived concepts from Chaos by Paul Mensonides */
#define BOOST_DETAIL_TRIPLE_TO_OCTAL_000 0
#define BOOST_DETAIL_TRIPLE_TO_OCTAL_001 1
#define BOOST_DETAIL_TRIPLE_TO_OCTAL_010 2
#define BOOST_DETAIL_TRIPLE_TO_OCTAL_011 3
#define BOOST_DETAIL_TRIPLE_TO_OCTAL_100 4
#define BOOST_DETAIL_TRIPLE_TO_OCTAL_101 5
#define BOOST_DETAIL_TRIPLE_TO_OCTAL_110 6
#define BOOST_DETAIL_TRIPLE_TO_OCTAL_111 7
#define BOOST_DETAIL_BINARY_LITERAL_ELEMENT_0 (0),
#define BOOST_DETAIL_BINARY_LITERAL_ELEMENT_1 (1),
#define BOOST_DETAIL_BINARY_LITERAL_ELEMENT_00 (0)(0),
#define BOOST_DETAIL_BINARY_LITERAL_ELEMENT_01 (0)(1),
// ... and so on, until ...
#define BOOST_DETAIL_BINARY_LITERAL_ELEMENT_11111110 (1)(1)(1)(1)(1)(1)(1)(0),
#define BOOST_DETAIL_BINARY_LITERAL_ELEMENT_11111111 (1)(1)(1)(1)(1)(1)(1)(1),
这会大量使用 中的宏boost/preprocessor/，但我不会在这里重复。它是数千行代码。
我知道这不是一个很有帮助的深入答案，但无论如何我都会发布它，因为它总比没有好。希望有人有勇气涉足这一点并提供更有见地的东西！
于 2012-11-02T18:13:57.437 回答
5
这是非常非常复杂的。这个 BOOST 库使用由其他宏构造的宏。
我将向您展示一些预处理器编程的简化示例：
简化的问题：
如何使宏在休息开始时添加 0 或 1 - 就像在这个例子中：
CAT ( 0 1 ) --> 01
CAT ( 1 100 ) --> 1100
所以 -,必须以某种方式将它们按顺序放在两个参数之间。好吧，我这样做了：
// start point - this is our desired format
#define CATS(a,b) a##b
// helper macro to invoke preprocessor on result of a
#define CALLP(a) a
// some helper symbols to build CATS()
#define CATB (
#define CATE )
#define CATSN CATS
// I use that only 0 or 1 is expected at the beginning:
// CATS ( 0,
#define CAT_B0 CATSN CATB 0,
// CATS ( 1,
#define CAT_B1 CATSN CATB 1,
// the final macro:
// Example:
// CAT(0 XXX) --> CALLP( CAT_B ## 0 XXX ) )
// --> CALLP(CAT_B0 XXX ) --> CALLP(CATSN ( 0, XXX ) )
// --> CALLP( CATS(0, XXX) ) --> CATS(0,XXX) --> 0##XXX -> 0XXX
#define CAT(tt) CALLP (CAT_B ## tt CATE)
它确实有效（至少在 gcc 上）：http: //ideone.com/EKlTGt
现在想象一下 BOOST 人在他们的图书馆里所做的工作有多复杂？是的，它非常复杂 - 但使用我提出的这种预处理器技巧，也许还有许多其他技巧 - 我查看了这个 BOOST 库的来源http://www.boost.org/doc/libs/1_46_1/boost/utility/binary .hpp今天第一次（信不信由你）。
于 2012-11-02T18:58:32.527 回答