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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; }
+ </style>
+</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>
+ </div>
+</body>
+</html>