devn00b
/
EQ2EMu


			
				
					
						
						
							123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960616263646566676869707172737475767778798081828384858687888990919293949596979899100101102103104105106107108109110111112113114115116117118119
							<html>
<head>
	<title>motivation.html</title>
	<link rel="stylesheet" type="text/css" href="../styles.css">
</head>
<body>
<h4>Motivation</h4>
<div>
	The C++ function and template parameter lists are special syntactic constructs, and it is impossible to directly
	manipulate or generate them using C++ constructs.&nbsp;
	This leads to unnecessary code repetition.
</div>
<div>
	Consider the implementation of the <code>is_function&lt;&gt;</code> metafunction is Boost.&nbsp;
	The implementation uses an overloaded <code>is_function_tester()</code> function that is used for testing if a type is convertible
	to a pointer to a function.&nbsp;
	Because of the special treatment of parameter lists, it is not possible to directly match a function with an arbitrary parameter list.&nbsp;
	Instead, the <code>is_function_tester()</code> must be overloaded for every distinct number of parameters that is to be supported.&nbsp;
	For example:
</div>
<div class="code"><pre>
template&lt;class R&gt;
yes_type is_function_tester(R (*)());

template&lt;class R, class A0&gt;
yes_type is_function_tester(R (*)(A0));

template&lt;class R, class A0, class A1&gt;
yes_type is_function_tester(R (*)(A0, A1));

template&lt;class R, class A0, class A1, class A2&gt;
yes_type is_function_tester(R (*)(A0, A1, A2));

// ...
</pre></div>
<div>
	The need for this kind of repetition occurs particularly frequently while implementing generic components or metaprogramming facilities,
	but the need also manifests itself in many far simpler situations.
</div>
<h4>Typical Solutions</h4>
<div>
	Typically the repetition is done manually.&nbsp;
	Manual code repetition is highly unproductive, but sometimes more readable to the untrained eye.
</div>
<div>
	Another solution is to write an external program for generating the repeated code or use some other extra linguistic means such as a smart editor.&nbsp;
	Unfortunately, using external code generators has many disadvantages:
	<ul>
		<li>Writing the generator takes time.&nbsp; (This could be helped by using a standard generator.)</li>
		<li>It is no longer productive to manipulate C++ code directly.</li>
		<li>Invoking the generator may be difficult.</li>
		<li>Automating the invocation of the generator can be difficult in certain environments.&nbsp; (Automatic invocation is desirable for active libraries.)</li>
		<li>Porting and distributing the generator may be difficult or simply takes precious time.</li>
	</ul>
</div>
<h4>What about the preprocessor?</h4>
<div>
	Because C++ comes with a preprocessor, one would assume that it would support these kinds of needs directly.&nbsp;
	Using the preprocessor in this case is highly desirable because:
	<ul>
		<li>The preprocessor is highly portable.</li>
		<li>The preprocessor is automatically invoked as part of the compilation process.</li>
		<li>Preprocessor metacode can be directly embedded into the C++ source code.</li>
		<li>Compilers generally allow viewing or outputting the preprocessed code, which can be used for debugging or to copy and paste the generated code.</li>
	</ul>
</div>
<div>
	Most unfortunately, the preprocessor is a very low level preprocessor that specifically does not support repetition or recursive macros.&nbsp;
	Library support is needed!
</div>
<div>
	<i>For detailed information on the capabilities and limitations of the preprocessor, please refer to the C++ standard <a href="../bibliography.html#std">[Std]</a>.</i>
</div>
<h4>The Motivation Example Revisited</h4>
<div>
	Using the primitives of the preprocessor library, the <code>is_function_tester()</code>'s could be implemented like this:
</div>
<div class="code"><pre>
#include &lt;boost/preprocessor/arithmetic/inc.hpp&gt;
#include &lt;boost/preprocessor/punctuation/comma_if.hpp&gt;
#include &lt;boost/preprocessor/repetition.hpp&gt;

#ifndef MAX_IS_FUNCTION_TESTER_PARAMS
#define MAX_IS_FUNCTION_TESTER_PARAMS 15
#endif

#define IS_FUNCTION_TESTER(Z, N, _) \
   template&lt;class R BOOST_PP_COMMA_IF(N) BOOST_PP_ENUM_PARAMS(N, class A)&gt; \
   yes_type is_function_tester(R (*)(BOOST_PP_ENUM_PARAMS(N, A))); \
   /**/

BOOST_PP_REPEAT(BOOST_PP_INC(MAX_IS_FUNCTION_TESTER_PARAMS), IS_FUNCTION_TESTER, _)

#undef IS_FUNCTION_TESTER
</pre></div>
<div>
	In order to change the maximum number of function parameters supported, you now simply change the <code>MAX_IS_FUNCTION_TESTER_PARAMS</code> definition and recompile.
</div>
<hr size="1">
<div style="margin-left: 0px;">
	<i>© Copyright <a href="http://www.housemarque.com" target="_top">Housemarque Oy</a> 2002</i>
</div>
<div style="margin-left: 0px;">
Permission to copy, use, modify, sell and distribute this document is granted provided this copyright notice appears in all copies.&nbsp;
This document is provided "as is" without express or implied warranty and with no claim as to its suitability for any purpose.
</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>