Brian Silverman | f27e085 | 2018-08-04 23:56:45 -0700 | [diff] [blame^] | 1 | <html> |
| 2 | <head> |
| 3 | <title>techniques.html</title> |
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| 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 | <i>© Copyright <a href="http://www.housemarque.com" target="_top">Housemarque Oy</a> 2002</i> |
| 333 | </div> |
| 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. |
| 337 | </div> |
| 338 | <hr size="1"> |
| 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> |
| 342 | </div> |
| 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 |
| 346 | copy at <a href= |
| 347 | "http://www.boost.org/LICENSE_1_0.txt">www.boost.org/LICENSE_1_0.txt</a>)</small></p> |
| 348 | </div> |
| 349 | </body> |
| 350 | </html> |