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Vesa Karvonen
2002-01-31 21:52:46 +00:00
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@ -33,61 +33,23 @@
D suffix (e.g. BOOST_PP_ADD_D()), that accepts an additional recursion
depth as the first parameter. This technique is necessary to avoid
recursively expanding the same macro again, which is not permitted by the
C++ preprocessor.
C++ preprocessor. Note that the value of the D parameter may exceed
<a href="limits.htm#BOOST_PP_LIMIT_MAG">BOOST_PP_LIMIT_MAG</a>.
</UL>
<P>NOTE: The value of the D parameter may exceed <a href="limits.htm#BOOST_PP_LIMIT_MAG">BOOST_PP_LIMIT_MAG</a>.</P>
<H3>Caveat</H3>
<H3>Note</H3>
<P>Using <a href="while.htm#BOOST_PP_WHILE">BOOST_PP_WHILE</a>() is a bit tricky. This is due to the C++ preprocessor
limitations. It is recommended to take a look at the implementations of the
various PREPROCESSOR library primitives such as <a href="arithmetic_add.htm#BOOST_PP_ADD">BOOST_PP_ADD</a>() for additional
examples.</P>
various PREPROCESSOR library primitives such as <a href="arithmetic_add.htm#BOOST_PP_ADD">BOOST_PP_ADD</a>() for
additional examples.</P>
<H3>Example</H3>
<UL>
<LI><a href="../../example/count_down.c">count_down.c</a>
<LI><a hreF="../../example/linear_fib.c">linear_fib.c</a>
</UL>
<P>For a more complex example, let's take a look at an implementation of
<a href="arithmetic_mul.htm#BOOST_PP_MUL">BOOST_PP_MUL</a>().</P>
<PRE>
#define <a href="arithmetic_mul.htm#BOOST_PP_MUL">BOOST_PP_MUL</a>(X,Y) BOOST_PP_MUL_D(0,X,Y)
// Since the macro is implemented using WHILE, the actual implementation
// takes a depth as a parameter so that it can be called inside a WHILE.
// The above easy-to-use version simply uses 0 as the depth and can not be
// called inside a WHILE.
#define BOOST_PP_MUL_D(D,X,Y)\
<a href="tuple_elem.htm#BOOST_PP_TUPLE_ELEM">BOOST_PP_TUPLE_ELEM</a>(3,0,<a href="while.htm#BOOST_PP_WHILE">BOOST_PP_WHILE</a>##D(BOOST_PP_MUL_C,BOOST_PP_MUL_F,(0,X,Y)))
// ^^^ ^^^ ^^ ^^ ^^^^^^^
// #1 #2 #3 #3 #1
//
// #1) The state is a 3-tuple. After the iteration is finished, the first
// element of the tuple is the result.
//
// #2) The WHILE primitive is "invoked" directly. <a href="while.htm#BOOST_PP_WHILE">BOOST_PP_WHILE</a>(D,...)
// can't be used because it would not be expanded by the C++ preprocessor.
//
// #3) ???_C is the condition and ???_F is the iteration macro.
#define BOOST_PP_MUL_C(D,P)\
<a href="tuple_elem.htm#BOOST_PP_TUPLE_ELEM">BOOST_PP_TUPLE_ELEM</a>(3,2,P)
// Iteration is finished when the counter reaches 0.
#define BOOST_PP_MUL_F(D,P)\
( BOOST_PP_ADD_D(D,<a href="tuple_elem.htm#BOOST_PP_TUPLE_ELEM">BOOST_PP_TUPLE_ELEM</a>(3,0,P),<a href="tuple_elem.htm#BOOST_PP_TUPLE_ELEM">BOOST_PP_TUPLE_ELEM</a>(3,1,P))\
, <a href="tuple_elem.htm#BOOST_PP_TUPLE_ELEM">BOOST_PP_TUPLE_ELEM</a>(3,1,P)\
, <a href="dec.htm#BOOST_PP_DEC">BOOST_PP_DEC</a>(<a href="tuple_elem.htm#BOOST_PP_TUPLE_ELEM">BOOST_PP_TUPLE_ELEM</a>(3,2,P))\
)
// ( The result is increased by the multiplier.
// , The multiplier is retained without change.
// , The counter is decreased.
// )
</PRE>
<H3>Implementation rationale</H3>
<UL>
<LI>The maximum iteration depth is greater than 2*<a href="limits.htm#BOOST_PP_LIMIT_MAG">BOOST_PP_LIMIT_MAG</a> to make