120 lines
5.3 KiB
HTML
120 lines
5.3 KiB
HTML
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<html>
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<head>
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<title>motivation.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>Motivation</h4>
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<div>
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The C++ function and template parameter lists are special syntactic constructs, and it is impossible to directly
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manipulate or generate them using C++ constructs.
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This leads to unnecessary code repetition.
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</div>
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<div>
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Consider the implementation of the <code>is_function<></code> metafunction is Boost.
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The implementation uses an overloaded <code>is_function_tester()</code> function that is used for testing if a type is convertible
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to a pointer to a function.
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Because of the special treatment of parameter lists, it is not possible to directly match a function with an arbitrary parameter list.
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Instead, the <code>is_function_tester()</code> must be overloaded for every distinct number of parameters that is to be supported.
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For example:
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</div>
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<div class="code"><pre>
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template<class R>
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yes_type is_function_tester(R (*)());
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template<class R, class A0>
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yes_type is_function_tester(R (*)(A0));
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template<class R, class A0, class A1>
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yes_type is_function_tester(R (*)(A0, A1));
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template<class R, class A0, class A1, class A2>
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yes_type is_function_tester(R (*)(A0, A1, A2));
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// ...
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</pre></div>
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<div>
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The need for this kind of repetition occurs particularly frequently while implementing generic components or metaprogramming facilities,
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but the need also manifests itself in many far simpler situations.
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</div>
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<h4>Typical Solutions</h4>
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<div>
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Typically the repetition is done manually.
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Manual code repetition is highly unproductive, but sometimes more readable to the untrained eye.
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</div>
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<div>
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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.
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Unfortunately, using external code generators has many disadvantages:
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<ul>
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<li>Writing the generator takes time. (This could be helped by using a standard generator.)</li>
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<li>It is no longer productive to manipulate C++ code directly.</li>
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<li>Invoking the generator may be difficult.</li>
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<li>Automating the invocation of the generator can be difficult in certain environments. (Automatic invocation is desirable for active libraries.)</li>
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<li>Porting and distributing the generator may be difficult or simply takes precious time.</li>
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</ul>
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</div>
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<h4>What about the preprocessor?</h4>
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<div>
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Because C++ comes with a preprocessor, one would assume that it would support these kinds of needs directly.
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Using the preprocessor in this case is highly desirable because:
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<ul>
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<li>The preprocessor is highly portable.</li>
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<li>The preprocessor is automatically invoked as part of the compilation process.</li>
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<li>Preprocessor metacode can be directly embedded into the C++ source code.</li>
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<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>
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</ul>
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</div>
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<div>
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Most unfortunately, the preprocessor is a very low level preprocessor that specifically does not support repetition or recursive macros.
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Library support is needed!
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</div>
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<div>
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<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>
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</div>
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<h4>The Motivation Example Revisited</h4>
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<div>
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Using the primitives of the preprocessor library, the <code>is_function_tester()</code>'s could be implemented like this:
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</div>
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<div class="code"><pre>
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#include <boost/preprocessor/arithmetic/inc.hpp>
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#include <boost/preprocessor/punctuation/comma_if.hpp>
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#include <boost/preprocessor/repetition.hpp>
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#ifndef MAX_IS_FUNCTION_TESTER_PARAMS
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#define MAX_IS_FUNCTION_TESTER_PARAMS 15
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#endif
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#define IS_FUNCTION_TESTER(Z, N, _) \
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template<class R BOOST_PP_COMMA_IF(N) BOOST_PP_ENUM_PARAMS(N, class A)> \
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yes_type is_function_tester(R (*)(BOOST_PP_ENUM_PARAMS(N, A))); \
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/**/
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BOOST_PP_REPEAT(BOOST_PP_INC(MAX_IS_FUNCTION_TESTER_PARAMS), IS_FUNCTION_TESTER, _)
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#undef IS_FUNCTION_TESTER
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</pre></div>
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<div>
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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.
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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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</div>
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<div style="margin-left: 0px;">
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Permission to copy, use, modify, sell and distribute this document is granted provided this copyright notice appears in all copies.
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This document is provided "as is" without express or implied warranty and with no claim as to its suitability for any purpose.
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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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