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</title>
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12 The preprocessor metaprogramming techniques are presented in example format.
15 <h4>Example
<u> - Use a local macro to avoid small scale repetition.
</u></h4>
16 <div class=
"code"><pre>
17 #define BOOST_PP_DEF(op) /* ..................................... */ \
18 template
<class T, int n
> \
19 vec
<T, n
> operator op ## =(vec
<T, n
> lhs, const vec
<T, n
>& rhs) { \
20 for (int i =
0; i
< n; ++i) { \
21 lhs(i) op ## = rhs(i); \
34 <b>Tip:
</b> It is usually okay to use a standard macro name like
<code>BOOST_PP_DEF
</code> for this kind of code
35 because the macro is both defined and undefined in the immediate site of its use.
38 <b>Tip:
</b> It is easier to verify proper use of the line continuation operator when they are aligned.
41 <b>Notes:
</b> You can extend this example by defining more and different kinds of operators.
42 Before doing so, consider using the
<i>algebraic categories
</i> technique introduced in
<a href=
"../bibliography.html#barton">[Barton]
</a>
43 or a
<i>layered architecture
</i> (see for instance
<a href=
"../bibliography.html#czarnecki">[Czarnecki]
</a>).
44 However, at some point you must type the operator tokens
<code>*
</code>,
<code>/
</code>,
<code>+
</code>,
<code>-
</code>, etc.,
45 because it is impossible to generate them using templates.
46 The resulting
<i>categorical repetition
</i> of tokens can be eliminated by using preprocessor metaprogramming.
49 <h4>Example
<u> - Use BOOST_PP_EMPTY as an unused parameter in local macro instantiations.
</u></h4>
50 <div class=
"code"><pre>
51 #define BOOST_PP_DEF(cv) /* ... */ \
52 template
<class base
> \
53 cv() typename implement_subscript_using_begin_subscript
<base
>::value_type& \
54 implement_subscript_using_begin_subscript
<base
>::operator[](index_type i) cv() { \
55 return base::begin()[i]; \
59 BOOST_PP_DEF(BOOST_PP_EMPTY)
60 BOOST_PP_DEF(BOOST_PP_IDENTITY(const))
63 <b>How:
</b> BOOST_PP_EMPTY() expands to nothing and can be used as an unused parameter.
66 <b>Note:
</b> BOOST_PP_EMPTY with the () never gets expanded.
67 The () is necessary to invoke a function-like macro.
70 <b>Caveat:
</b> You cannot safely use concatenation while using BOOST_PP_EMPTY().
73 <b>Tip:
</b> Occasionally, one or two lines are considerably longer than the rest.
74 It can often save some work to
<i>not
</i> align all the line continuation operators without making the code too unreadable.
77 <b>Tip:
</b> Use syntax highlighting on preprocessor metaprogramming macro identifiers such as:
80 <li>BOOST_PP_EMPTY
</li>
81 <li>BOOST_PP_REPEAT
</li>
84 It can greatly improve readability.
87 <h4>Example
<u> - Use BOOST_PP_CAT instead of ## when necessary.
</u></h4>
88 <div class=
"code"><pre>
89 #define STATIC_ASSERT(expr) \
90 enum { BOOST_PP_CAT(static_check_, __LINE__) = (expr) ?
1 : -
1 }; \
92 BOOST_PP_CAT(static_assert_, __LINE__)[BOOST_PP_CAT(static_check_, __LINE__)] \
97 STATIC_ASSERT(sizeof(int)
<= sizeof(long));
100 <b>Why:
</b> Macro expansion proceeds recursively in
"layers."
101 Token pasting prevents the preprocessor from performing macro expansion,
102 therefore it is often necessary to delay token concatenation.
105 <h4>Example
<u> - Use BOOST_PP_STRINGIZE instead of # whenever necessary.
</u></h4>
106 <div class=
"code"><pre>
108 message(__FILE__
"(" BOOST_PP_STRINGIZE(__LINE__)
") : " str) \
116 <b>Why:
</b> Macro expansion proceeds recursively in
"layers."
117 Stringization prevents the preprocessor from performing macro expansion, therefore it is often necessary to delay stringization.
120 <h4>Example
<u> - Use BOOST_PP_ENUM_PARAMS (and its variants) or BOOST_PP_REPEAT and BOOST_PP_COMMA_IF to avoid
<i>O
</i>(
<i>n
</i>) repetition on lists in general.
</u></h4>
121 <div class=
"code"><pre>
122 struct make_type_list_end;
125 BOOST_PP_ENUM_PARAMS_WITH_A_DEFAULT(
126 MAKE_TYPE_LIST_MAX_LENGTH,
131 struct make_type_list {
133 enum { end = is_same
<T0, make_type_list_end
>::value };
135 typedef typename type_if
<
136 end, type_cons_empty,
139 typename type_inner_if
<
140 end, type_identity
<end
>,
142 BOOST_PP_ENUM_SHIFTED_PARAMS(
143 MAKE_TYPE_LIST_MAX_LENGTH,
153 <b>How:
</b> BOOST_PP_REPEAT uses simulated recursion (pseudo code):
156 #define BOOST_PP_REPEAT(n, m, p) BOOST_PP_REPEAT ## n(m, p)
158 #define BOOST_PP_REPEAT0(m, p)
159 #define BOOST_PP_REPEAT1(m, p) m(
0, p)
160 #define BOOST_PP_REPEAT2(m, p) m(
0, p) m(
1, p)
161 #define BOOST_PP_REPEAT3(m, p) BOOST_PP_REPEAT2(m, p) m(
2, p)
162 #define BOOST_PP_REPEAT4(m, p) BOOST_PP_REPEAT3(m, p) m(
3, p)
166 <i>Note:
This is no longer how BOOST_PP_REPEAT is implemented, so the code above is illustrative only!
</i>
169 BOOST_PP_ENUM_PARAMS and its variations use BOOST_PP_REPEAT.
170 BOOST_PP_COMMA_IF(I) expands to a comma if I !=
0.
171 BOOST_PP_INC(I) essentially expands to
"I+1," and BOOST_PP_DEC(I) essentially expands to
"I-1.".
174 <h4>Example
<u> - Use a
<i>conditional macro definition
</i> to enable user configuration of code repetition based on need rather than some
"reasonable" upper limit.
</u></h4>
175 <div class=
"code"><pre>
176 #ifndef MAKE_TYPE_LIST_MAX_LENGTH
177 #define MAKE_TYPE_LIST_MAX_LENGTH
8
181 Now the user can configure the
<code>make_type_list
</code> primitive without modifying library code.
184 <h4>Example
<u> - Use BOOST_PP_REPEAT and a
<i>token look-up function
</i> to eliminate categorical repetition.
</u></h4>
185 <div class=
"code"><pre>
186 // CAVEAT: My compiler is not standard on arithmetic types.
187 #define ARITHMETIC_TYPE(I) ARITHMETIC_TYPE ## I
189 #define ARITHMETIC_TYPE0 bool
190 #define ARITHMETIC_TYPE1 char
191 #define ARITHMETIC_TYPE2 signed char
192 #define ARITHMETIC_TYPE3 unsigned char
193 #define ARITHMETIC_TYPE4 short
194 #define ARITHMETIC_TYPE5 unsigned short
195 #define ARITHMETIC_TYPE6 int
196 #define ARITHMETIC_TYPE7 unsigned int
197 #define ARITHMETIC_TYPE8 long
198 #define ARITHMETIC_TYPE9 unsigned long
199 #define ARITHMETIC_TYPE10 float
200 #define ARITHMETIC_TYPE11 double
201 #define ARITHMETIC_TYPE12 long double
203 #define ARITHMETIC_TYPE_CNT
13
207 #define BOOST_PP_DEF(z, I, _) /* ... */ \
208 catch (ARITHMETIC_TYPE(I) t) { \
214 BOOST_PP_REPEAT(ARITHMETIC_TYPE_CNT, BOOST_PP_DEF, _)
219 <b>Note:
</b> The repetition of the above example can be eliminated using template metaprogramming
<a href=
"../bibliography.html#czarnecki">[Czarnecki]
</a> as well.
220 However categorical repetition of operator tokens cannot be completely eliminated by using template metaprogramming.
223 <h4>Example
<u> - Use BOOST_PP_REPEAT to avoid
<i>O
</i>(
<i>n
</i>*
<i>n
</i>) repetition.
</u></h4>
224 <div class=
"code"><pre>
225 #ifndef MAX_VEC_ARG_CNT
226 #define MAX_VEC_ARG_CNT
8
231 #define ARG_FUN(z, i, _) BOOST_PP_COMMA_IF(i) T a ## i
232 #define ASSIGN_FUN(z, i, ) (*this)[i] = a ## i;
234 #define DEF_VEC_CTOR_FUN(z, i, _) /* ... */ \
235 vec(BOOST_PP_REPEAT(i, ARG_FUN, _)) { \
236 BOOST_PP_REPEAT(i, ASSIGN_FUN, _) \
240 BOOST_PP_REPEAT(BOOST_PP_INC(MAX_VEC_ARG_CNT), DEF_VEC_CTOR_FUN, _)
244 #undef DEF_VEC_CTOR_FUN
249 <b>How:
</b> BOOST_PP_REPEAT is implemented is a special way to enable
<i>automatic recursion
</i>.
252 <h4>Example
<u> - Use BOOST_PP_IF to implement special case for the first element.
</u></h4>
253 <div class=
"code"><pre>
254 #define COMMA_IF(c) \
255 BOOST_PP_IF(c, BOOST_PP_COMMA, BOOST_PP_EMPTY)() \
258 BOOST_PP_IF(
0, true, false) == false;
259 BOOST_PP_IF(
1, true, false) == true;
262 BOOST_PP_IF enables convenient generation of lists using BOOST_PP_REPEAT.
265 <b>Note:
</b> <i>THEN
</i> and
<i>ELSE
</i> don't have to be macros.
266 However, if at least one of them is a function-like macro, and you want it be expanded conditionally,
267 you have to make the other parameter a function-like macro too.
268 This can often be done using BOOST_PP_IDENTITY.
269 Consider the following example (by Aleksey Gurtovoy):
272 #define NUMBERED_EXPRESSION(i, x) /* ... */ \
275 BOOST_PP_IDENTITY(x ## i) \
281 <b>Note:
</b> Like in the above implementation of COMMA_IF, the result of BOOST_PP_IF is often invoked and not the
<i>THEN
</i> and
<i>ELSE
</i> parameters.
282 If the parameters were invoked, the code would not expand correctly, because the BOOST_PP_EMPTY parameter would get expanded
283 to nothing before the
<b>BOOST_PP_IF
</b> would be properly expanded.
286 <b>How:
</b> BOOST_PP_IF is defined for the entire repeat range (psuedo code):
289 #define BOOST_PP_IF(c, THEN, ELSE) BOOST_PP_IF ## c(THEN, ELSE)
291 #define BOOST_PP_IF0(THEN, ELSE) ELSE
292 #define BOOST_PP_IF1(THEN, ELSE) THEN
293 #define BOOST_PP_IF1(THEN, ELSE) THEN
297 <h4>Example:
<u> Use arithmetic, logical, and comparison operations when necessary.
</u></h4>
298 <div class=
"code"><pre>
299 #define SPECIAL_NUMBERED_LIST(n, i, elem, special) \
300 BOOST_PP_ASSERT_MSG( \
301 BOOST_PP_LESS(i, n), \
302 bad params for SPECIAL_NUMBERED_LIST! \
304 BOOST_PP_ENUM_PARAMS(i, elem) \
305 BOOST_PP_COMMA_IF(i) special \
307 BOOST_PP_SUB(BOOST_PP_DEC(n), i), \
308 SPECIAL_NUMBERED_LIST_HELPER, \
313 #define SPECIAL_NUMBERED_LIST_HELPER(z, i, elem_base) \
316 BOOST_PP_TUPLE_ELEM(
2,
0, elem_base), \
319 BOOST_PP_TUPLE_ELEM(
2,
1, elem_base) \
324 SPECIAL_NUMBERED_LIST(
3,
0, E, S)
325 SPECIAL_NUMBERED_LIST(
3,
1, E, S)
326 SPECIAL_NUMBERED_LIST(
3,
2, E, S)
327 SPECIAL_NUMBERED_LIST(
3,
3, E, S)
331 <div style=
"margin-left: 0px;">
332 <i>© Copyright
<a href=
"http://www.housemarque.com" target=
"_top">Housemarque Oy
</a> 2002</i>
334 <div style=
"margin-left: 0px;">
335 Permission to copy, use, modify, sell and distribute this document is granted provided this copyright notice appears in all copies.
336 This document is provided
"as is" without express or implied warranty and with no claim as to its suitability for any purpose.
339 <div style=
"margin-left: 0px;">
340 <i>© Copyright
<a href=
"http://www.housemarque.com" target=
"_top">Housemarque Oy
</a> 2002</i>
341 </br><i>© Copyright Paul Mensonides
2002</i>
343 <div style=
"margin-left: 0px;">
344 <p><small>Distributed under the Boost Software License, Version
1.0. (See
345 accompanying file
<a href=
"../../../../LICENSE_1_0.txt">LICENSE_1_0.txt
</a> or
347 "http://www.boost.org/LICENSE_1_0.txt">www.boost.org/LICENSE_1_0.txt
</a>)
</small></p>