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<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.  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 <boost/preprocessor/iteration/iterate.hpp> #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.  The writer expected <i>I</i> to refer the previous iteration frame.  However, that is not the case.  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.  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.  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:  evaluated slots.  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.  First, the user must define the <i>named external argument</i> <b>BOOST_PP_VALUE</b>.  This value must be an integral constant expression.  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>).  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.  However, it now evaluates to <b>BOOST_PP_SLOT</b>(<i>1</i>).  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 <iostream> #include <boost/preprocessor/slot/slot.hpp> #include <boost/preprocessor/stringize.hpp> #define X() 4 #define BOOST_PP_VALUE 1 + 2 + 3 + X() #include BOOST_PP_ASSIGN_SLOT(1) #undef X int main(void) { std::cout << BOOST_PP_STRINGIZE(BOOST_PP_SLOT(1)) << &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>