299 lines
11 KiB
HTML
299 lines
11 KiB
HTML
<html>
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<head>
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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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</head>
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<body>
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<h4>
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Reentrancy
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</h4>
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<div>
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Macro expansion in the preprocessor is entirely functional. Therefore,
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there is no iteration. Unfortunately, the preprocessor also disallows
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recursion. This means that the library must fake iteration or recursion
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by 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">
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<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>
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</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
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like the following:
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</div>
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<div class="code">
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<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>
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</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>
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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. First, it can be documented
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that <code>AB</code> uses <code>CONCAT</code> and disallow usage similar to the
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above. Second, multiple concatenation macros can be provided....
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</div>
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<div class="code">
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<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>
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</div>
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<div>
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This solves the problem. However, it is now necessary to know that <code>AB</code>
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uses, not only <i>a</i> concatenation macro, 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">
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<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>
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</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
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set of concatenation macros. The <code>c</code> parameter represents the
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"state" of the concatenation construct, and as long as the user keeps track of
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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. It either has to disallow a construct
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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. There are
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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
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mechanisms to reenter the set at a macro 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>,
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and <b>BOOST_PP_WHILE</b>. There are two mechanisms that are used to
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accomplish this: state parameters (like the above concatenation example)
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and <i>automatic recursion</i>.
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</div>
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<h4>
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State Parameters
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</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>)
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has an associated state. This state provides the means to reenter the
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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
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predicates, operations, etc.). Every time a user-defined macro is
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invoked, it is passed the current state of the construct that invoked it so
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that the macro can reenter 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
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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. First,
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the set itself which is reentered through concatenation. Second,
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corresponding sets that act like they are a part of the the primary set.
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These are also reentered through concatenation. And third, macros that
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internally use the first or second type of macro. These macros take the
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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
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operation. When <b>BOOST_PP_WHILE</b> expands these macros, it passes
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along its state so that these macros 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
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technique:
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</div>
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<div class="code">
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<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>
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</div>
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<div>
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There are a couple things to note in the above implementation. First,
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note how <code>ADD_D</code> reenters <b>BOOST_PP_WHILE</b> using the <i>d</i> state
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parameter. Second, note how <code>MUL</code>'s operation, which is
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expanded by <b>BOOST_PP_WHILE</b>, passes the state 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>, there is a
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state reentrant variant. If that variant uses an argument rather than
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concatenation, it is suffixed by <code>_D</code> to symbolize its method of
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reentrance. Examples or this include the library's own <b>BOOST_PP_ADD_D</b>
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and <b>BOOST_PP_MUL_D</b>. If the variant uses concatenation, it is
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suffixed by an underscore. It is completed by concatenation of the
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state. This includes <b>BOOST_PP_WHILE</b> itself with <b>BOOST_PP_WHILE_</b>
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## <i>d</i> and, for example, <b>BOOST_PP_LIST_FOLD_LEFT</b> with <b>BOOST_PP_LIST_FOLD_LEFT_</b>
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## <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>,
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but with the letters <i>R</i> and <i>Z</i>, respectively, to symbolize their
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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
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immediately obvious, is using <i>all three</i> types of reentrance. Not
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only is it using both types of <i>state</i> reentrance, it is also using <i>automatic
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recursion</i>....
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</div>
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<h4>
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Automatic Recursion
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</h4>
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<div>
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Automatic recursion is a technique that vastly simplifies the use of reentrant
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constructs. It is used by simply <i>not</i> using any state parameters at
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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>
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by itself. In other words, <code>MUL</code> can <i>still</i> be used
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inside <b>BOOST_PP_WHILE</b> even though it doesn't reenter <b>BOOST_PP_WHILE</b>
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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." Despite what it looks
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like, the macro <b>BOOST_PP_WHILE</b> does not take three arguments. In
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fact, it takes no arguments at all. Instead, the <b>BOOST_PP_WHILE</b> macro
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expands <i>to</i> a macro that takes three arguments. It simply detects
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what the next available <b>BOOST_PP_WHILE_</b> ## <i>d</i> macro is and returns
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it. This detection process is somewhat involved, so I won't go into <i>how</i>
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it works here, 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
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simpler. 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
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exist? The reason is simple as well. When state parameters are
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used, the state is <i>known</i> at all times. This is not the case when
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automatic recursion is used. The automatic recursion mechanism has to <i>deduce</i>
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the state at each point that it is used. This implies a cost in macro
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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>
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Conclusion
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</h4>
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<div>
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It is really a tradeoff whether to use state parameters or automatic recursion
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for reentrancy. The strengths of automatic recursion are ease of use and
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implementation encapsulation. These come at a performance cost on some
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preprocessors in some situations. The primary strength of state
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parameters, on the other hand, is efficiency. Use of the state parameters
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is the only way to achieve <i>maximum</i> efficiency. This efficiency
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comes at the cost of both code complexity and exposition of implementation.
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</div>
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<h4>
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See Also
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</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">
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- Paul Mensonides
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</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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