158 lines
5.0 KiB
HTML
158 lines
5.0 KiB
HTML
<html>
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<head>
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<title>evaluated_slots.html</title>
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<link rel="stylesheet" type="text/css" href="../styles.css">
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</head>
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<body>
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<h4>Evaluated Slots</h4>
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<div>
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The evaluated slot mechanism is a tool to fully evaluate a constant integral expression and avoid the lazy evaluation normally performed by the preprocessor.
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</div>
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<h4>Tutorial</h4>
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<div>
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In order to understand the use of such a mechanism, I will start with a simple file-iteration example.
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Consider the following scenario....
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</div>
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<div class ="code"><pre>
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for (int i = 0; i < 10; ++i) {
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for (int j = 0; j < i; ++j) {
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// ... use i and j
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}
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}
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</pre></div>
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<div>
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The above is a simple runtime model of the following multidimensional file-iteration....
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</div>
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<div class="code"><pre>
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// file.hpp
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#if !BOOST_PP_IS_ITERATING
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#ifndef FILE_HPP_
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#define FILE_HPP_
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#include <boost/preprocessor/iteration/iterate.hpp>
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#define BOOST_PP_ITERATION_PARAMS_1 (3, (0, 9, "file.hpp"))
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#include BOOST_PP_ITERATE()
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#endif // FILE_HPP_
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#elif BOOST_PP_ITERATION_DEPTH() == 1
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#define I BOOST_PP_ITERATION()
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#define BOOST_PP_ITERATION_PARAMS_2 (3, (0, I, "file.hpp"))
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#include BOOST_PP_ITERATE()
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#undef I
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#elif BOOST_PP_ITERATION_DEPTH() == 2
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#define J BOOST_PP_ITERATION()
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// use I and J
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#undef J
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#endif
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</pre></div>
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<div>
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There is a problem with the code above.
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The writer expected <i>I</i> to refer the previous iteration frame.
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However, that is not the case.
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When the user refers to <i>I</i>, he is actually referring to <b>BOOST_PP_ITERATION</b>(),
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not the value of <b>BOOST_PP_ITERATION</b>() at the point of definition.
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Instead, it refers to exactly the same value to which <i>J</i> refers.
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</div>
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<div>
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The problem is that the preprocessor always evaluates everything with lazy evaluation.
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To solve the problem, we need <i>I</i> to be <i>evaluated</i> here:
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</div>
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<div class="code"><pre>
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// ...
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#elif BOOST_PP_ITERATION_DEPTH() == 1
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#define I BOOST_PP_ITERATION()
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// ...
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</pre></div>
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<div>
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Fortunately, the library offers a mechanism to do just that: evaluated slots.
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The following code uses this mechanism to "fix" the example above...
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</div>
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<div class="code"><pre>
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// ...
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#elif BOOST_PP_ITERATION_DEPTH() == 1
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#define BOOST_PP_VALUE BOOST_PP_ITERATION()
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#include BOOST_PP_ASSIGN_SLOT(1)
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#define I BOOST_PP_SLOT(1)
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// ...
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</pre></div>
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<div>
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There are two steps to the assignment of an evaluated slot.
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First, the user must define the <i>named external argument</i> <b>BOOST_PP_VALUE</b>.
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This value must be an integral constant expression.
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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>).
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This will evaluate <b>BOOST_PP_VALUE</b> and assign the result to the slot at index <i>x</i>.
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</div>
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<div>
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To retrieve a slot's value, the user must use <b>BOOST_PP_SLOT</b>(<i>x</i>).
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</div>
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<div>
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In the case above, <i>I</i> is <i>still</i> lazily evaluated.
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However, it now evaluates to <b>BOOST_PP_SLOT</b>(<i>1</i>).
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This value <i>will not change</i> unless there is a subsequent call to <b>BOOST_PP_ASSIGN_SLOT</b>(<i>1</i>).
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</div>
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<h4>Advanced Techniques</h4>
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<div>
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The slot mechanism can also be used to perform calculations:
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</div>
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<div class="code"><pre>
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#include <iostream>
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#include <boost/preprocessor/slot/slot.hpp>
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#include <boost/preprocessor/stringize.hpp>
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#define X() 4
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#define BOOST_PP_VALUE 1 + 2 + 3 + X()
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#include BOOST_PP_ASSIGN_SLOT(1)
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#undef X
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int main(void) {
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std::cout
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<< BOOST_PP_STRINGIZE(BOOST_PP_SLOT(1))
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<< &std::endl;
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return 0;
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}
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</pre></div>
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<div>
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In essence, anything that can be evaluated in an #if (or #elif) preprocessor directive is available <i>except</i> the <i>defined</i> operator.
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</div>
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<div>
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It is even possible to use a particular slot itself while reassigning it:
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</div>
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<div class="code"><pre>
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#define BOOST_PP_VALUE 20
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#include BOOST_PP_ASSIGN_SLOT(1)
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#define BOOST_PP_VALUE 2 * BOOST_PP_SLOT(1)
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#include BOOST_PP_ASSIGN_SLOT(1)
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BOOST_PP_SLOT(1) // 40
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</pre></div>
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<h4>See Also</h4>
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<ul>
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<li><a href="../ref/assign_slot.html">BOOST_PP_ASSIGN_SLOT</a></li>
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<li><a href="../ref/limit_slot_count.html">BOOST_PP_LIMIT_SLOT_COUNT</a></li>
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<li><a href="../ref/slot.html">BOOST_PP_SLOT</a></li>
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<li><a href="../ref/value.html">BOOST_PP_VALUE</a></li>
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</ul>
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<div class="sig">- Paul Mensonides</div>
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<hr size="1">
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<div style="margin-left: 0px;">
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<i><EFBFBD> Copyright <a href="http://www.housemarque.com" target="_top">Housemarque Oy</a> 2002</i>
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</br><i><EFBFBD> Copyright Paul Mensonides 2002</i>
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</div>
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<div style="margin-left: 0px;">
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<p><small>Distributed under the Boost Software License, Version 1.0. (See
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accompanying file <a href="../../../../LICENSE_1_0.txt">LICENSE_1_0.txt</a> or
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copy at <a href=
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"http://www.boost.org/LICENSE_1_0.txt">www.boost.org/LICENSE_1_0.txt</a>)</small></p>
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</div>
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</body>
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</html>
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