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1<html>
2<head>
3        <title>techniques.html</title>
4        <link rel="stylesheet" type="text/css" href="../styles.css">
5        <style>
6                u { font-weight:  normal; text-decoration:  none; }
7        </style>
8</head>
9<body>
10<h4>Techniques</h4>
11<div>
12        The preprocessor metaprogramming techniques are presented in example format.
13</div>
14
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&lt;class T, int n&gt;                                          \
19   vec&lt;T, n&gt; operator op ## =(vec&lt;T, n&gt; lhs, const vec&lt;T, n&gt;& rhs) { \
20      for (int i = 0; i &lt; n; ++i) {                                  \
21         lhs(i) op ## = rhs(i);                                      \
22      }                                                              \
23   }                                                                 \
24   /**/
25
26BOOST_PP_DEF(+)
27BOOST_PP_DEF(-)
28BOOST_PP_DEF(*)
29BOOST_PP_DEF(/)
30
31#undef BOOST_PP_DEF
32</pre></div>
33<div>
34        <b>Tip:</b>&nbsp; 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.
36</div>
37<div>
38        <b>Tip:</b>&nbsp; It is easier to verify proper use of the line continuation operator when they are aligned.
39</div>
40<div>
41        <b>Notes:</b>&nbsp; You can extend this example by defining more and different kinds of operators.&nbsp;
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>).&nbsp;
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.&nbsp;
46        The resulting <i>categorical repetition</i> of tokens can be eliminated by using preprocessor metaprogramming.
47</div>
48
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&lt;class base&gt;            \
53   cv() typename implement_subscript_using_begin_subscript&lt;base&gt;::value_type& \
54   implement_subscript_using_begin_subscript&lt;base&gt;::operator[](index_type i) cv() { \
55      return base::begin()[i];     \
56   }                               \
57   /**/
58
59   BOOST_PP_DEF(BOOST_PP_EMPTY)
60   BOOST_PP_DEF(BOOST_PP_IDENTITY(const))
61</pre></div>
62<div>
63        <b>How:</b>&nbsp; BOOST_PP_EMPTY() expands to nothing and can be used as an unused parameter.
64</div>
65<div>
66        <b>Note:</b>&nbsp; BOOST_PP_EMPTY with the () never gets expanded.&nbsp;
67        The () is necessary to invoke a function-like macro.
68</div>
69<div>
70        <b>Caveat:</b>&nbsp; You cannot safely use concatenation while using BOOST_PP_EMPTY().
71</div>
72<div>
73        <b>Tip:</b>&nbsp; Occasionally, one or two lines are considerably longer than the rest.&nbsp;
74        It can often save some work to <i>not</i> align all the line continuation operators without making the code too unreadable.
75</div>
76<div>
77        <b>Tip:</b>&nbsp; Use syntax highlighting on preprocessor metaprogramming macro identifiers such as:
78        <ul>
79                <li>BOOST_PP_DEF</li>
80                <li>BOOST_PP_EMPTY</li>
81                <li>BOOST_PP_REPEAT</li>
82                <li>...</li>
83        </ul>
84        It can greatly improve readability.
85</div>
86
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 }; \
91   typedef char \
92      BOOST_PP_CAT(static_assert_, __LINE__)[BOOST_PP_CAT(static_check_, __LINE__)] \
93   /**/
94
95// ...
96
97STATIC_ASSERT(sizeof(int) &lt;= sizeof(long));
98</pre></div>
99<div>
100        <b>Why:</b>&nbsp; Macro expansion proceeds recursively in "layers."&nbsp;
101        Token pasting prevents the preprocessor from performing macro expansion,
102        therefore it is often necessary to delay token concatenation.
103</div>
104
105<h4>Example<u> - Use BOOST_PP_STRINGIZE instead of # whenever necessary.</u></h4>
106<div class="code"><pre>
107#define NOTE(str) \
108   message(__FILE__ "(" BOOST_PP_STRINGIZE(__LINE__) ") : " str) \
109   /**/
110
111// ...
112
113#pragma NOTE("TBD!")
114</pre></div>
115<div>
116        <b>Why:</b>&nbsp; Macro expansion proceeds recursively in "layers."&nbsp;
117        Stringization prevents the preprocessor from performing macro expansion, therefore it is often necessary to delay stringization.
118</div>
119
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>
122struct make_type_list_end;
123
124template&lt;
125   BOOST_PP_ENUM_PARAMS_WITH_A_DEFAULT(
126      MAKE_TYPE_LIST_MAX_LENGTH,
127      class T,
128      make_type_list_end
129   )
130&gt;
131struct make_type_list {
132   private:
133      enum { end = is_same&lt;T0, make_type_list_end&gt;::value };
134   public:
135      typedef typename type_if&lt;
136         end, type_cons_empty,
137         type_cons&lt;
138            T0,
139            typename type_inner_if&lt;
140               end, type_identity&lt;end&gt;,
141               make_type_list&lt;
142                  BOOST_PP_ENUM_SHIFTED_PARAMS(
143                     MAKE_TYPE_LIST_MAX_LENGTH,
144                     T
145                  )
146               &gt;
147            &gt;::type
148         &gt;
149      &gt;::type type;
150};
151</pre></div>
152<div>
153        <b>How:</b>&nbsp; BOOST_PP_REPEAT uses simulated recursion (pseudo code):
154</div>
155<div><pre>
156#define BOOST_PP_REPEAT(n, m, p) BOOST_PP_REPEAT ## n(m, p)
157
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)
163// ...
164</pre></div>
165<div>
166        <i>Note:&nbsp; This is no longer how BOOST_PP_REPEAT is implemented, so the code above is illustrative only!&nbsp;</i>
167</div>
168<div>
169        BOOST_PP_ENUM_PARAMS and its variations use BOOST_PP_REPEAT.&nbsp;
170        BOOST_PP_COMMA_IF(I) expands to a comma if I != 0.&nbsp;
171        BOOST_PP_INC(I) essentially expands to "I+1," and BOOST_PP_DEC(I) essentially expands to "I-1.".
172</div>
173
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
178#endif
179</pre></div>
180<div>
181        Now the user can configure the <code>make_type_list</code> primitive without modifying library code.
182</div>
183
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
188
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
202
203#define ARITHMETIC_TYPE_CNT 13
204
205// ...
206
207#define BOOST_PP_DEF(z, I, _) /* ... */ \
208   catch (ARITHMETIC_TYPE(I) t) {       \
209      report_typeid(t);                 \
210      report_value(t);                  \
211   }                                    \
212   /**/
213
214BOOST_PP_REPEAT(ARITHMETIC_TYPE_CNT, BOOST_PP_DEF, _)
215
216#undef BOOST_PP_DEF
217</pre></div>
218<div>
219        <b>Note:</b>&nbsp; The repetition of the above example can be eliminated using template metaprogramming <a href="../bibliography.html#czarnecki">[Czarnecki]</a> as well.&nbsp;
220        However categorical repetition of operator tokens cannot be completely eliminated by using template metaprogramming.
221</div>
222
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
227#endif
228
229// ...
230
231#define ARG_FUN(z, i, _) BOOST_PP_COMMA_IF(i) T a ## i
232#define ASSIGN_FUN(z, i, ) (*this)[i] = a ## i;
233
234#define DEF_VEC_CTOR_FUN(z, i, _) /* ... */ \
235   vec(BOOST_PP_REPEAT(i, ARG_FUN, _)) { \
236      BOOST_PP_REPEAT(i, ASSIGN_FUN, _)  \
237   }                                     \
238   /**/
239
240BOOST_PP_REPEAT(BOOST_PP_INC(MAX_VEC_ARG_CNT), DEF_VEC_CTOR_FUN, _)
241
242#undef ARG_FUN
243#undef ASSIGN_FUN
244#undef DEF_VEC_CTOR_FUN
245
246// ...
247</pre></div>
248<div>
249        <b>How:</b>&nbsp; BOOST_PP_REPEAT is implemented is a special way to enable <i>automatic recursion</i>.
250</div>
251
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)() \
256   /**/
257
258BOOST_PP_IF(0, true, false) == false;
259BOOST_PP_IF(1, true, false) == true;
260</pre></div>
261<div>
262        BOOST_PP_IF enables convenient generation of lists using BOOST_PP_REPEAT.
263</div>
264<div>
265        <b>Note:</b>&nbsp; <i>THEN</i> and <i>ELSE</i> don't have to be macros.&nbsp;
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.&nbsp;
268        This can often be done using BOOST_PP_IDENTITY.&nbsp;
269        Consider the following example (by Aleksey Gurtovoy):
270</div>
271<div><pre>
272#define NUMBERED_EXPRESSION(i, x) /* ... */ \
273   BOOST_PP_IF(                             \
274      i,                                    \
275      BOOST_PP_IDENTITY(x ## i)             \
276      BOOST_PP_EMPTY                        \
277   )()                                      \
278   /**/
279</pre></div>
280<div>
281        <b>Note:</b>&nbsp; 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.&nbsp;
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.
284</div>
285<div>
286        <b>How:</b>&nbsp; BOOST_PP_IF is defined for the entire repeat range (psuedo code):
287</div>
288<div><pre>
289#define BOOST_PP_IF(c, THEN, ELSE) BOOST_PP_IF ## c(THEN, ELSE)
290
291#define BOOST_PP_IF0(THEN, ELSE) ELSE
292#define BOOST_PP_IF1(THEN, ELSE) THEN
293#define BOOST_PP_IF1(THEN, ELSE) THEN
294// ...
295</pre></div>
296
297<h4>Example:<u>&nbsp; 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! \
303   )                                        \
304   BOOST_PP_ENUM_PARAMS(i, elem)            \
305   BOOST_PP_COMMA_IF(i) special             \
306   BOOST_PP_REPEAT(                         \
307      BOOST_PP_SUB(BOOST_PP_DEC(n), i),     \
308      SPECIAL_NUMBERED_LIST_HELPER,         \
309      (elem, i)                             \
310   )                                        \
311   /**/
312
313#define SPECIAL_NUMBERED_LIST_HELPER(z, i, elem_base) \
314   ,                                          \
315   BOOST_PP_CAT(                              \
316      BOOST_PP_TUPLE_ELEM(2, 0, elem_base),   \
317      BOOST_PP_ADD(                           \
318         i,                                   \
319         BOOST_PP_TUPLE_ELEM(2, 1, elem_base) \
320      )                                       \
321   )                                          \
322   /**/
323
324SPECIAL_NUMBERED_LIST(3, 0, E, S)
325SPECIAL_NUMBERED_LIST(3, 1, E, S)
326SPECIAL_NUMBERED_LIST(3, 2, E, S)
327SPECIAL_NUMBERED_LIST(3, 3, E, S)
328</pre></div>
329
330<hr size="1">
331<div style="margin-left: 0px;">
332        <i>© Copyright <a href="http://www.housemarque.com" target="_top">Housemarque Oy</a> 2002</i>
333</div>
334<div style="margin-left: 0px;">
335Permission to copy, use, modify, sell and distribute this document is granted provided this copyright notice appears in all copies.&nbsp;
336This document is provided "as is" without express or implied warranty and with no claim as to its suitability for any purpose.
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