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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<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); \ | |
22 | } \ | |
23 | } \ | |
24 | /**/ | |
25 | ||
26 | BOOST_PP_DEF(+) | |
27 | BOOST_PP_DEF(-) | |
28 | BOOST_PP_DEF(*) | |
29 | BOOST_PP_DEF(/) | |
30 | ||
31 | #undef BOOST_PP_DEF | |
32 | </pre></div> | |
33 | <div> | |
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. | |
36 | </div> | |
37 | <div> | |
38 | <b>Tip:</b> It is easier to verify proper use of the line continuation operator when they are aligned. | |
39 | </div> | |
40 | <div> | |
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. | |
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<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]; \ | |
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> BOOST_PP_EMPTY() expands to nothing and can be used as an unused parameter. | |
64 | </div> | |
65 | <div> | |
66 | <b>Note:</b> BOOST_PP_EMPTY with the () never gets expanded. | |
67 | The () is necessary to invoke a function-like macro. | |
68 | </div> | |
69 | <div> | |
70 | <b>Caveat:</b> You cannot safely use concatenation while using BOOST_PP_EMPTY(). | |
71 | </div> | |
72 | <div> | |
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. | |
75 | </div> | |
76 | <div> | |
77 | <b>Tip:</b> 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 | ||
97 | STATIC_ASSERT(sizeof(int) <= sizeof(long)); | |
98 | </pre></div> | |
99 | <div> | |
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. | |
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> Macro expansion proceeds recursively in "layers." | |
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> | |
122 | struct make_type_list_end; | |
123 | ||
124 | template< | |
125 | BOOST_PP_ENUM_PARAMS_WITH_A_DEFAULT( | |
126 | MAKE_TYPE_LIST_MAX_LENGTH, | |
127 | class T, | |
128 | make_type_list_end | |
129 | ) | |
130 | > | |
131 | struct make_type_list { | |
132 | private: | |
133 | enum { end = is_same<T0, make_type_list_end>::value }; | |
134 | public: | |
135 | typedef typename type_if< | |
136 | end, type_cons_empty, | |
137 | type_cons< | |
138 | T0, | |
139 | typename type_inner_if< | |
140 | end, type_identity<end>, | |
141 | make_type_list< | |
142 | BOOST_PP_ENUM_SHIFTED_PARAMS( | |
143 | MAKE_TYPE_LIST_MAX_LENGTH, | |
144 | T | |
145 | ) | |
146 | > | |
147 | >::type | |
148 | > | |
149 | >::type type; | |
150 | }; | |
151 | </pre></div> | |
152 | <div> | |
153 | <b>How:</b> 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: This is no longer how BOOST_PP_REPEAT is implemented, so the code above is illustrative only! </i> | |
167 | </div> | |
168 | <div> | |
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.". | |
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 | ||
214 | BOOST_PP_REPEAT(ARITHMETIC_TYPE_CNT, BOOST_PP_DEF, _) | |
215 | ||
216 | #undef BOOST_PP_DEF | |
217 | </pre></div> | |
218 | <div> | |
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. | |
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 | ||
240 | BOOST_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> 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 | ||
258 | BOOST_PP_IF(0, true, false) == false; | |
259 | BOOST_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> <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): | |
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> 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. | |
284 | </div> | |
285 | <div> | |
286 | <b>How:</b> 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> 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 | ||
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) | |
328 | </pre></div> | |
329 | ||
330 | <hr size="1"> | |
331 | <div style="margin-left: 0px;"> | |
332 |