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| <title>evaluated_slots.html</title> |
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| <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> |
