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