devn00b
/
EQ2EMu


			
				
					
						
						
							123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960616263646566676869707172737475767778798081828384858687888990919293949596979899100101102103104105106107108109110111112113114115116117118119120121122123124125126127128129130131132133134135136137138139140141142143144145146147148149150151152153154155156157
							<html>
<head>
	<title>evaluated_slots.html</title>
	<link rel="stylesheet" type="text/css" href="../styles.css">
</head>
<body>
<h4>Evaluated Slots</h4>
<div>
	The evaluated slot mechanism is a tool to fully evaluate a constant integral expression and avoid the lazy evaluation normally performed by the preprocessor.
</div>
<h4>Tutorial</h4>
<div>
	In order to understand the use of such a mechanism, I will start with a simple file-iteration example.&nbsp;
	Consider the following scenario....
</div>
<div class ="code"><pre>
for (int i = 0; i < 10; ++i) {
   for (int j = 0; j < i; ++j) {
      // ... use i and j
   }
}
</pre></div>
<div>
	The above is a simple runtime model of the following multidimensional file-iteration....
</div>
<div class="code"><pre>
// file.hpp
#if !BOOST_PP_IS_ITERATING
   #ifndef FILE_HPP_
   #define FILE_HPP_

   #include &lt;boost/preprocessor/iteration/iterate.hpp&gt;

   #define BOOST_PP_ITERATION_PARAMS_1 (3, (0, 9, "file.hpp"))
   #include BOOST_PP_ITERATE()

   #endif // FILE_HPP_
#elif BOOST_PP_ITERATION_DEPTH() == 1
   #define I BOOST_PP_ITERATION()

   #define BOOST_PP_ITERATION_PARAMS_2 (3, (0, I, "file.hpp"))
   #include BOOST_PP_ITERATE()

   #undef I
#elif BOOST_PP_ITERATION_DEPTH() == 2
   #define J BOOST_PP_ITERATION()

   // use I and J

   #undef J
#endif
</pre></div>
<div>
	There is a problem with the code above.&nbsp;
	The writer expected <i>I</i> to refer the previous iteration frame.&nbsp;
	However, that is not the case.&nbsp;
	When the user refers to <i>I</i>, he is actually referring to <b>BOOST_PP_ITERATION</b>(),
	not the value of <b>BOOST_PP_ITERATION</b>() at the point of definition.&nbsp;
	Instead, it refers to exactly the same value to which <i>J</i> refers.
</div>
<div>
	The problem is that the preprocessor always evaluates everything with lazy evaluation.&nbsp;
	To solve the problem, we need <i>I</i> to be <i>evaluated</i> here:
</div>
<div class="code"><pre>
// ...
#elif BOOST_PP_ITERATION_DEPTH() == 1
   #define I BOOST_PP_ITERATION()
// ...
</pre></div>
<div>
	Fortunately, the library offers a mechanism to do just that:&nbsp; evaluated slots.&nbsp;
	The following code uses this mechanism to "fix" the example above...
</div>
<div class="code"><pre>
// ...
#elif BOOST_PP_ITERATION_DEPTH() == 1
   #define BOOST_PP_VALUE BOOST_PP_ITERATION()
   #include BOOST_PP_ASSIGN_SLOT(1)
   #define I BOOST_PP_SLOT(1)
// ...
</pre></div>
<div>
	There are two steps to the assignment of an evaluated slot.&nbsp;
	First, the user must define the <i>named external argument</i> <b>BOOST_PP_VALUE</b>.&nbsp;
	This value must be an integral constant expression.&nbsp;
	Second, the user must <i>include</i> <b>BOOST_PP_ASSIGN_SLOT</b>(<i>x</i>), where <i>x</i> is the particular slot to be assigned to (<i>1</i> to <b>BOOST_PP_LIMIT_SLOT_COUNT</b>).&nbsp;
	This will evaluate <b>BOOST_PP_VALUE</b> and assign the result to the slot at index <i>x</i>.
</div>
<div>
	To retrieve a slot's value, the user must use <b>BOOST_PP_SLOT</b>(<i>x</i>).
</div>
<div>
	In the case above, <i>I</i> is <i>still</i> lazily evaluated.&nbsp;
	However, it now evaluates to <b>BOOST_PP_SLOT</b>(<i>1</i>).&nbsp;
	This value <i>will not change</i> unless there is a subsequent call to <b>BOOST_PP_ASSIGN_SLOT</b>(<i>1</i>).
</div>
<h4>Advanced Techniques</h4>
<div>
	The slot mechanism can also be used to perform calculations:
</div>
<div class="code"><pre>
#include &lt;iostream&gt;

#include &lt;boost/preprocessor/slot/slot.hpp&gt;
#include &lt;boost/preprocessor/stringize.hpp&gt;

#define X() 4

#define BOOST_PP_VALUE 1 + 2 + 3 + X()
#include BOOST_PP_ASSIGN_SLOT(1)

#undef X

int main(void) {
   std::cout
      &lt;&lt; BOOST_PP_STRINGIZE(BOOST_PP_SLOT(1))
      &lt;&lt; &amp;std::endl;
   return 0;
}
</pre></div>
<div>
	In essence, anything that can be evaluated in an #if (or #elif) preprocessor directive is available <i>except</i> the <i>defined</i> operator.
</div>
<div>
	It is even possible to use a particular slot itself while reassigning it:
</div>
<div class="code"><pre>
#define BOOST_PP_VALUE 20
#include BOOST_PP_ASSIGN_SLOT(1)

#define BOOST_PP_VALUE 2 * BOOST_PP_SLOT(1)
#include BOOST_PP_ASSIGN_SLOT(1)

BOOST_PP_SLOT(1) // 40
</pre></div>
<h4>See Also</h4>
<ul>
	<li><a href="../ref/assign_slot.html">BOOST_PP_ASSIGN_SLOT</a></li>
	<li><a href="../ref/limit_slot_count.html">BOOST_PP_LIMIT_SLOT_COUNT</a></li>
	<li><a href="../ref/slot.html">BOOST_PP_SLOT</a></li>
	<li><a href="../ref/value.html">BOOST_PP_VALUE</a></li>
</ul>
<div class="sig">- Paul Mensonides</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>