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254 lines
10 KiB
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<title>reentrancy.html</title>
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<link rel="stylesheet" type="text/css" href="../styles.css">
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<h4>Reentrancy</h4>
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<div>
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Macro expansion in the preprocessor is entirely functional.
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Therefore, there is no iteration.
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Unfortunately, the preprocessor also disallows recursion.
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This means that the library must fake iteration or recursion by
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defining sets of macros that are implemented similarly.
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</div>
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<div>
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To illustrate, here is a simple concatenation macro:
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</div>
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<div class="code"><pre>
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#define CONCAT(a, b) CONCAT_D(a, b)
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#define CONCAT_D(a, b) a ## b
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CONCAT(a, CONCAT(b, c)) // abc
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</pre></div>
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<div>
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This is fine for a simple case like the above, but what happens in a scenario like the following:
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</div>
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<div class="code"><pre>
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#define AB(x, y) CONCAT(x, y)
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CONCAT(A, B(p, q)) // CONCAT(p, q)
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</pre></div>
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<div>
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Because there is no recursion, the example above expands to <code>CONCAT(p, q)</code> rather than <code>pq</code>.
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</div>
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<div>
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There are only two ways to "fix" the above.
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First, it can be documented that <code>AB</code> uses <code>CONCAT</code> and disallow usage similar to the above.
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Second, multiple concatenation macros can be provided....
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</div>
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<div class="code"><pre>
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#define CONCAT_1(a, b) CONCAT_1_D(a, b)
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#define CONCAT_1_D(a, b) a ## b
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#define CONCAT_2(a, b) CONCAT_2_D(a, b)
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#define CONCAT_2_D(a, b) a ## b
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#define AB(x, y) CONCAT_2(x, y)
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CONCAT_1(A, B(p, q)) // pq
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</pre></div>
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<div>
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This solves the problem.
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However, it is now necessary to know that <code>AB</code> uses, not only <i>a</i> concatenation macro,
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but <code>CONCAT_2</code> specifically.
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</div>
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<div>
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A better solution is to abstract <i>which</i> concatenation macro is used....
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</div>
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<div class="code"><pre>
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#define AB(c, x, y) CONCAT_ ## c(x, y)
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CONCAT_1(A, B(2, p, q)) // pq
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</pre></div>
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<div>
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This is an example of <i>generic reentrance</i>, in this case, into a fictional set of concatenation macros.
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The <code>c</code> parameter represents the "state" of the concatenation construct,
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and as long as the user keeps track of this state, <code>AB</code> can be used inside of a concatenation macro.
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</div>
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<div>
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The library has the same choices.
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It either has to disallow a construct being inside itself or provide multiple, equivalent definitions of a construct
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and provide a uniform way to <i>reenter</i> that construct.
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There are several contructs that <i>require</i> recursion (such as <b>BOOST_PP_WHILE</b>).
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Consequently, the library chooses to provide several sets of macros with mechanisms to reenter the set at a macro
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that has not already been used.
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</div>
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<div>
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In particular, the library must provide reentrance for <b>BOOST_PP_FOR</b>, <b>BOOST_PP_REPEAT</b>, and <b>BOOST_PP_WHILE</b>.
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There are two mechanisms that are used to accomplish this: state parameters (like the above concatenation example) and <i>automatic recursion</i>.
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</div>
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<h4>State Parameters</h4>
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<div>
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Each of the above constructs (<b>BOOST_PP_FOR</b>, <b>BOOST_PP_REPEAT</b>, and <b>BOOST_PP_WHILE</b>) has an associated state.
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This state provides the means to reenter the respective construct.
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</div>
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<div>
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Several user-defined macros are passed to each of these constructs (for use as predicates, operations, etc.).
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Every time a user-defined macro is invoked, it is passed the current state of the construct that invoked it so that the macro can reenter
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the respective set if necessary.
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</div>
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<div>
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These states are used in one of two ways--either by concatenating to or passing to another macro.
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</div>
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<div>
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There are three types of macros that use these state parameters.
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First, the set itself which is reentered through concatenation.
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Second, corresponding sets that act like they are a part of the the primary set.
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These are also reentered through concatenation.
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And third, macros that internally use the first or second type of macro.
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These macros take the state as an additional argument.
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</div>
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<div>
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The state of <b>BOOST_PP_WHILE</b> is symbolized by the letter <i>D</i>.
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Two user-defined macros are passed to <b>BOOST_PP_WHILE</b>--a predicate and an operation.
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When <b>BOOST_PP_WHILE</b> expands these macros, it passes along its state so that these macros
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can reenter the <b>BOOST_PP_WHILE</b> set.
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</div>
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<div>
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Consider the following multiplication implementation that illustrates this technique:
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</div>
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<div class="code"><pre>
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// The addition interface macro.
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// The _D signifies that it reenters
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// BOOST_PP_WHILE with concatenation.
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#define ADD_D(d, x, y) \
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BOOST_PP_TUPLE_ELEM( \
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2, 0, \
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BOOST_PP_WHILE_ ## d(ADD_P, ADD_O, (x, y)) \
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) \
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/**/
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// The predicate that is passed to BOOST_PP_WHILE.
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// It returns "true" until "y" becomes zero.
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#define ADD_P(d, xy) BOOST_PP_TUPLE_ELEM(2, 1, xy)
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// The operation that is passed to BOOST_PP_WHILE.
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// It increments "x" and decrements "y" which will
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// eventually cause "y" to equal zero and therefore
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// cause the predicate to return "false."
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#define ADD_O(d, xy) \
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( \
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BOOST_PP_INC( \
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BOOST_PP_TUPLE_ELEM(2, 0, xy) \
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), \
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BOOST_PP_DEC( \
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BOOST_PP_TUPLE_ELEM(2, 1, xy) \
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) \
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) \
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/**/
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// The multiplication interface macro.
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#define MUL(x, y) \
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BOOST_PP_TUPLE_ELEM( \
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3, 0, \
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BOOST_PP_WHILE(MUL_P, MUL_O, (0, x, y)) \
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) \
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/**/
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// The predicate that is passed to BOOST_PP_WHILE.
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// It returns "true" until "y" becomes zero.
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#define MUL_P(d, rxy) BOOST_PP_TUPLE_ELEM(3, 2, rxy)
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// The operation that is passed to BOOST_PP_WHILE.
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// It adds "x" to "r" and decrements "y" which will
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// eventually cause "y" to equal zero and therefore
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// cause the predicate to return "false."
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#define MUL_O(d, rxy) \
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( \
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ADD_D( \
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d, /* pass the state on to ADD_D */ \
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BOOST_PP_TUPLE_ELEM(3, 0, rxy), \
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BOOST_PP_TUPLE_ELEM(3, 1, rxy) \
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), \
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BOOST_PP_TUPLE_ELEM(3, 1, rxy), \
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BOOST_PP_DEC( \
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BOOST_PP_TUPLE_ELEM(3, 2, rxy) \
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) \
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) \
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/**/
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MUL(3, 2) // expands to 6
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</pre></div>
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<div>
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There are a couple things to note in the above implementation.
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First, note how <code>ADD_D</code> reenters <b>BOOST_PP_WHILE</b> using the <i>d</i> state parameter.
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Second, note how <code>MUL</code>'s operation, which is expanded by <b>BOOST_PP_WHILE</b>, passes the state
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on to <code>ADD_D</code>.
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This illustrates state reentrance by both argument and concatenation.
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</div>
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<div>
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For every macro in the library that uses <b>BOOST_PP_WHILE</b>,
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there is a state reentrant variant.
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If that variant uses an argument rather than concatenation, it is suffixed by <code>_D</code> to symbolize its
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method of reentrance.
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Examples or this include the library's own <b>BOOST_PP_ADD_D</b> and <b>BOOST_PP_MUL_D</b>.
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If the variant uses concatenation, it is suffixed by an underscore.
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It is completed by concatenation of the state.
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This includes <b>BOOST_PP_WHILE</b> itself with <b>BOOST_PP_WHILE_</b> ## <i>d</i> and, for example,
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<b>BOOST_PP_LIST_FOLD_LEFT</b> with <b>BOOST_PP_LIST_FOLD_LEFT_</b> ## <i>d</i>.
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</div>
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<div>
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The same set of conventions are used for <b>BOOST_PP_FOR</b> and <b>BOOST_PP_REPEAT</b>, but with the letters
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<i>R</i> and <i>Z</i>, respectively, to symbolize their states.
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</div>
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<div>
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Also note that the above <code>MUL</code> implementation, though not immediately obvious, is using <i>all three</i>
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types of reentrance.
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Not only is it using both types of <i>state</i> reentrance, it is also using <i>automatic recursion</i>....
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</div>
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<h4>Automatic Recursion</h4>
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<div>
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Automatic recursion is a technique that vastly simplifies the use of reentrant constructs.
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It is used by simply <i>not</i> using any state parameters at all.
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</div>
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<div>
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The <code>MUL</code> example above uses automatic recursion when it uses <b>BOOST_PP_WHILE</b> by itself.
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In other words, <code>MUL</code> can <i>still</i> be used inside <b>BOOST_PP_WHILE</b> even though it doesn't
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reenter <b>BOOST_PP_WHILE</b> by concatenating the state to <b>BOOST_PP_WHILE_</b>.
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</div>
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<div>
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To accomplish this, the library uses a "trick."
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Despite what it looks like, the macro <b>BOOST_PP_WHILE</b> does not take three arguments.
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In fact, it takes no arguments at all.
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Instead, the <b>BOOST_PP_WHILE</b> macro expands <i>to</i> a macro that takes three arguments.
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It simply detects what the next available <b>BOOST_PP_WHILE_</b> ## <i>d</i> macro is and returns it.
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This detection process is somewhat involved, so I won't go into <i>how</i> it works here,
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but suffice to say it <i>does</i> work.
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</div>
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<div>
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Using automatic recursion to reenter various sets of macros is obviously much simpler.
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It completely hides the underlying implementation details.
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So, if it is so much easier to use, why do the state parameters still exist?
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The reason is simple as well.
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When state parameters are used, the state is <i>known</i> at all times.
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This is not the case when automatic recursion is used.
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The automatic recursion mechanism has to <i>deduce</i> the state at each point that it is used.
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This implies a cost in macro complexity that in some situations--notably at deep macro depths--will slow
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some preprocessors to a crawl.
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</div>
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<h4>Conclusion</h4>
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<div>
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It is really a tradeoff whether to use state parameters or automatic recursion for reentrancy.
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The strengths of automatic recursion are ease of use and implementation encapsulation.
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These come at a performance cost on some preprocessors in some situations.
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The primary strength of state parameters, on the other hand, is efficiency.
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Use of the state parameters is the only way to achieve <i>maximum</i> efficiency.
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This efficiency comes at the cost of both code complexity and exposition of implementation.
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</div>
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<h4>See Also</h4>
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<ul>
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<li><a href="../ref/for.html">BOOST_PP_FOR</a></li>
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<li><a href="../ref/repeat.html">BOOST_PP_REPEAT</a></li>
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<li><a href="../ref/while.html">BOOST_PP_WHILE</a></li>
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</ul>
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<div class="sig">- Paul Mensonides</div>
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</body>
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</html>
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