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- <html>
- <head>
- <title>techniques.html</title>
- <link rel="stylesheet" type="text/css" href="../styles.css">
- <style>
- u { font-weight: normal; text-decoration: none; }
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- </head>
- <body>
- <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>
- </div>
- <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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