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| <h4>Techniques</h4> |
| <div> |
| The preprocessor metaprogramming techniques are presented in example format. |
| </div> |
| |
| <h4>Example<u> - Use a local macro to avoid small scale repetition.</u></h4> |
| <div class="code"><pre> |
| #define BOOST_PP_DEF(op) /* ..................................... */ \ |
| template<class T, int n> \ |
| vec<T, n> operator op ## =(vec<T, n> lhs, const vec<T, n>& rhs) { \ |
| for (int i = 0; i < n; ++i) { \ |
| lhs(i) op ## = rhs(i); \ |
| } \ |
| } \ |
| /**/ |
| |
| BOOST_PP_DEF(+) |
| BOOST_PP_DEF(-) |
| BOOST_PP_DEF(*) |
| BOOST_PP_DEF(/) |
| |
| #undef BOOST_PP_DEF |
| </pre></div> |
| <div> |
| <b>Tip:</b> It is usually okay to use a standard macro name like <code>BOOST_PP_DEF</code> for this kind of code |
| because the macro is both defined and undefined in the immediate site of its use. |
| </div> |
| <div> |
| <b>Tip:</b> It is easier to verify proper use of the line continuation operator when they are aligned. |
| </div> |
| <div> |
| <b>Notes:</b> You can extend this example by defining more and different kinds of operators. |
| Before doing so, consider using the <i>algebraic categories</i> technique introduced in <a href="../bibliography.html#barton">[Barton]</a> |
| or a <i>layered architecture</i> (see for instance <a href="../bibliography.html#czarnecki">[Czarnecki]</a>). |
| However, at some point you must type the operator tokens <code>*</code>, <code>/</code>, <code>+</code>, <code>-</code>, etc., |
| because it is impossible to generate them using templates. |
| The resulting <i>categorical repetition</i> of tokens can be eliminated by using preprocessor metaprogramming. |
| </div> |
| |
| <h4>Example<u> - Use BOOST_PP_EMPTY as an unused parameter in local macro instantiations.</u></h4> |
| <div class="code"><pre> |
| #define BOOST_PP_DEF(cv) /* ... */ \ |
| template<class base> \ |
| cv() typename implement_subscript_using_begin_subscript<base>::value_type& \ |
| implement_subscript_using_begin_subscript<base>::operator[](index_type i) cv() { \ |
| return base::begin()[i]; \ |
| } \ |
| /**/ |
| |
| BOOST_PP_DEF(BOOST_PP_EMPTY) |
| BOOST_PP_DEF(BOOST_PP_IDENTITY(const)) |
| </pre></div> |
| <div> |
| <b>How:</b> BOOST_PP_EMPTY() expands to nothing and can be used as an unused parameter. |
| </div> |
| <div> |
| <b>Note:</b> BOOST_PP_EMPTY with the () never gets expanded. |
| The () is necessary to invoke a function-like macro. |
| </div> |
| <div> |
| <b>Caveat:</b> You cannot safely use concatenation while using BOOST_PP_EMPTY(). |
| </div> |
| <div> |
| <b>Tip:</b> Occasionally, one or two lines are considerably longer than the rest. |
| It can often save some work to <i>not</i> align all the line continuation operators without making the code too unreadable. |
| </div> |
| <div> |
| <b>Tip:</b> Use syntax highlighting on preprocessor metaprogramming macro identifiers such as: |
| <ul> |
| <li>BOOST_PP_DEF</li> |
| <li>BOOST_PP_EMPTY</li> |
| <li>BOOST_PP_REPEAT</li> |
| <li>...</li> |
| </ul> |
| It can greatly improve readability. |
| </div> |
| |
| <h4>Example<u> - Use BOOST_PP_CAT instead of ## when necessary.</u></h4> |
| <div class="code"><pre> |
| #define STATIC_ASSERT(expr) \ |
| enum { BOOST_PP_CAT(static_check_, __LINE__) = (expr) ? 1 : -1 }; \ |
| typedef char \ |
| BOOST_PP_CAT(static_assert_, __LINE__)[BOOST_PP_CAT(static_check_, __LINE__)] \ |
| /**/ |
| |
| // ... |
| |
| STATIC_ASSERT(sizeof(int) <= sizeof(long)); |
| </pre></div> |
| <div> |
| <b>Why:</b> Macro expansion proceeds recursively in "layers." |
| Token pasting prevents the preprocessor from performing macro expansion, |
| therefore it is often necessary to delay token concatenation. |
| </div> |
| |
| <h4>Example<u> - Use BOOST_PP_STRINGIZE instead of # whenever necessary.</u></h4> |
| <div class="code"><pre> |
| #define NOTE(str) \ |
| message(__FILE__ "(" BOOST_PP_STRINGIZE(__LINE__) ") : " str) \ |
| /**/ |
| |
| // ... |
| |
| #pragma NOTE("TBD!") |
| </pre></div> |
| <div> |
| <b>Why:</b> Macro expansion proceeds recursively in "layers." |
| Stringization prevents the preprocessor from performing macro expansion, therefore it is often necessary to delay stringization. |
| </div> |
| |
| <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> |
| <div class="code"><pre> |
| struct make_type_list_end; |
| |
| template< |
| BOOST_PP_ENUM_PARAMS_WITH_A_DEFAULT( |
| MAKE_TYPE_LIST_MAX_LENGTH, |
| class T, |
| make_type_list_end |
| ) |
| > |
| struct make_type_list { |
| private: |
| enum { end = is_same<T0, make_type_list_end>::value }; |
| public: |
| typedef typename type_if< |
| end, type_cons_empty, |
| type_cons< |
| T0, |
| typename type_inner_if< |
| end, type_identity<end>, |
| make_type_list< |
| BOOST_PP_ENUM_SHIFTED_PARAMS( |
| MAKE_TYPE_LIST_MAX_LENGTH, |
| T |
| ) |
| > |
| >::type |
| > |
| >::type type; |
| }; |
| </pre></div> |
| <div> |
| <b>How:</b> BOOST_PP_REPEAT uses simulated recursion (pseudo code): |
| </div> |
| <div><pre> |
| #define BOOST_PP_REPEAT(n, m, p) BOOST_PP_REPEAT ## n(m, p) |
| |
| #define BOOST_PP_REPEAT0(m, p) |
| #define BOOST_PP_REPEAT1(m, p) m(0, p) |
| #define BOOST_PP_REPEAT2(m, p) m(0, p) m(1, p) |
| #define BOOST_PP_REPEAT3(m, p) BOOST_PP_REPEAT2(m, p) m(2, p) |
| #define BOOST_PP_REPEAT4(m, p) BOOST_PP_REPEAT3(m, p) m(3, p) |
| // ... |
| </pre></div> |
| <div> |
| <i>Note: This is no longer how BOOST_PP_REPEAT is implemented, so the code above is illustrative only! </i> |
| </div> |
| <div> |
| BOOST_PP_ENUM_PARAMS and its variations use BOOST_PP_REPEAT. |
| BOOST_PP_COMMA_IF(I) expands to a comma if I != 0. |
| BOOST_PP_INC(I) essentially expands to "I+1," and BOOST_PP_DEC(I) essentially expands to "I-1.". |
| </div> |
| |
| <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> |
| <div class="code"><pre> |
| #ifndef MAKE_TYPE_LIST_MAX_LENGTH |
| #define MAKE_TYPE_LIST_MAX_LENGTH 8 |
| #endif |
| </pre></div> |
| <div> |
| Now the user can configure the <code>make_type_list</code> primitive without modifying library code. |
| </div> |
| |
| <h4>Example<u> - Use BOOST_PP_REPEAT and a <i>token look-up function</i> to eliminate categorical repetition.</u></h4> |
| <div class="code"><pre> |
| // CAVEAT: My compiler is not standard on arithmetic types. |
| #define ARITHMETIC_TYPE(I) ARITHMETIC_TYPE ## I |
| |
| #define ARITHMETIC_TYPE0 bool |
| #define ARITHMETIC_TYPE1 char |
| #define ARITHMETIC_TYPE2 signed char |
| #define ARITHMETIC_TYPE3 unsigned char |
| #define ARITHMETIC_TYPE4 short |
| #define ARITHMETIC_TYPE5 unsigned short |
| #define ARITHMETIC_TYPE6 int |
| #define ARITHMETIC_TYPE7 unsigned int |
| #define ARITHMETIC_TYPE8 long |
| #define ARITHMETIC_TYPE9 unsigned long |
| #define ARITHMETIC_TYPE10 float |
| #define ARITHMETIC_TYPE11 double |
| #define ARITHMETIC_TYPE12 long double |
| |
| #define ARITHMETIC_TYPE_CNT 13 |
| |
| // ... |
| |
| #define BOOST_PP_DEF(z, I, _) /* ... */ \ |
| catch (ARITHMETIC_TYPE(I) t) { \ |
| report_typeid(t); \ |
| report_value(t); \ |
| } \ |
| /**/ |
| |
| BOOST_PP_REPEAT(ARITHMETIC_TYPE_CNT, BOOST_PP_DEF, _) |
| |
| #undef BOOST_PP_DEF |
| </pre></div> |
| <div> |
| <b>Note:</b> The repetition of the above example can be eliminated using template metaprogramming <a href="../bibliography.html#czarnecki">[Czarnecki]</a> as well. |
| However categorical repetition of operator tokens cannot be completely eliminated by using template metaprogramming. |
| </div> |
| |
| <h4>Example<u> - Use BOOST_PP_REPEAT to avoid <i>O</i>(<i>n</i>*<i>n</i>) repetition.</u></h4> |
| <div class="code"><pre> |
| #ifndef MAX_VEC_ARG_CNT |
| #define MAX_VEC_ARG_CNT 8 |
| #endif |
| |
| // ... |
| |
| #define ARG_FUN(z, i, _) BOOST_PP_COMMA_IF(i) T a ## i |
| #define ASSIGN_FUN(z, i, ) (*this)[i] = a ## i; |
| |
| #define DEF_VEC_CTOR_FUN(z, i, _) /* ... */ \ |
| vec(BOOST_PP_REPEAT(i, ARG_FUN, _)) { \ |
| BOOST_PP_REPEAT(i, ASSIGN_FUN, _) \ |
| } \ |
| /**/ |
| |
| BOOST_PP_REPEAT(BOOST_PP_INC(MAX_VEC_ARG_CNT), DEF_VEC_CTOR_FUN, _) |
| |
| #undef ARG_FUN |
| #undef ASSIGN_FUN |
| #undef DEF_VEC_CTOR_FUN |
| |
| // ... |
| </pre></div> |
| <div> |
| <b>How:</b> BOOST_PP_REPEAT is implemented is a special way to enable <i>automatic recursion</i>. |
| </div> |
| |
| <h4>Example<u> - Use BOOST_PP_IF to implement special case for the first element.</u></h4> |
| <div class="code"><pre> |
| #define COMMA_IF(c) \ |
| BOOST_PP_IF(c, BOOST_PP_COMMA, BOOST_PP_EMPTY)() \ |
| /**/ |
| |
| BOOST_PP_IF(0, true, false) == false; |
| BOOST_PP_IF(1, true, false) == true; |
| </pre></div> |
| <div> |
| BOOST_PP_IF enables convenient generation of lists using BOOST_PP_REPEAT. |
| </div> |
| <div> |
| <b>Note:</b> <i>THEN</i> and <i>ELSE</i> don't have to be macros. |
| However, if at least one of them is a function-like macro, and you want it be expanded conditionally, |
| you have to make the other parameter a function-like macro too. |
| This can often be done using BOOST_PP_IDENTITY. |
| Consider the following example (by Aleksey Gurtovoy): |
| </div> |
| <div><pre> |
| #define NUMBERED_EXPRESSION(i, x) /* ... */ \ |
| BOOST_PP_IF( \ |
| i, \ |
| BOOST_PP_IDENTITY(x ## i) \ |
| BOOST_PP_EMPTY \ |
| )() \ |
| /**/ |
| </pre></div> |
| <div> |
| <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. |
| If the parameters were invoked, the code would not expand correctly, because the BOOST_PP_EMPTY parameter would get expanded |
| to nothing before the <b>BOOST_PP_IF</b> would be properly expanded. |
| </div> |
| <div> |
| <b>How:</b> BOOST_PP_IF is defined for the entire repeat range (psuedo code): |
| </div> |
| <div><pre> |
| #define BOOST_PP_IF(c, THEN, ELSE) BOOST_PP_IF ## c(THEN, ELSE) |
| |
| #define BOOST_PP_IF0(THEN, ELSE) ELSE |
| #define BOOST_PP_IF1(THEN, ELSE) THEN |
| #define BOOST_PP_IF1(THEN, ELSE) THEN |
| // ... |
| </pre></div> |
| |
| <h4>Example:<u> Use arithmetic, logical, and comparison operations when necessary.</u></h4> |
| <div class="code"><pre> |
| #define SPECIAL_NUMBERED_LIST(n, i, elem, special) \ |
| BOOST_PP_ASSERT_MSG( \ |
| BOOST_PP_LESS(i, n), \ |
| bad params for SPECIAL_NUMBERED_LIST! \ |
| ) \ |
| BOOST_PP_ENUM_PARAMS(i, elem) \ |
| BOOST_PP_COMMA_IF(i) special \ |
| BOOST_PP_REPEAT( \ |
| BOOST_PP_SUB(BOOST_PP_DEC(n), i), \ |
| SPECIAL_NUMBERED_LIST_HELPER, \ |
| (elem, i) \ |
| ) \ |
| /**/ |
| |
| #define SPECIAL_NUMBERED_LIST_HELPER(z, i, elem_base) \ |
| , \ |
| BOOST_PP_CAT( \ |
| BOOST_PP_TUPLE_ELEM(2, 0, elem_base), \ |
| BOOST_PP_ADD( \ |
| i, \ |
| BOOST_PP_TUPLE_ELEM(2, 1, elem_base) \ |
| ) \ |
| ) \ |
| /**/ |
| |
| SPECIAL_NUMBERED_LIST(3, 0, E, S) |
| SPECIAL_NUMBERED_LIST(3, 1, E, S) |
| SPECIAL_NUMBERED_LIST(3, 2, E, S) |
| SPECIAL_NUMBERED_LIST(3, 3, E, S) |
| </pre></div> |
| |
| <hr size="1"> |
| <div style="margin-left: 0px;"> |
| <i>© Copyright <a href="http://www.housemarque.com" target="_top">Housemarque Oy</a> 2002</i> |
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| <div style="margin-left: 0px;"> |
| Permission to copy, use, modify, sell and distribute this document is granted provided this copyright notice appears in all copies. |
| This document is provided "as is" without express or implied warranty and with no claim as to its suitability for any purpose. |
| </div> |
| <hr size="1"> |
| <div style="margin-left: 0px;"> |
| <i>© Copyright <a href="http://www.housemarque.com" target="_top">Housemarque Oy</a> 2002</i> |
| </br><i>© Copyright Paul Mensonides 2002</i> |
| </div> |
| <div style="margin-left: 0px;"> |
| <p><small>Distributed under the Boost Software License, Version 1.0. (See |
| accompanying file <a href="../../../../LICENSE_1_0.txt">LICENSE_1_0.txt</a> or |
| copy at <a href= |
| "http://www.boost.org/LICENSE_1_0.txt">www.boost.org/LICENSE_1_0.txt</a>)</small></p> |
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