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| <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. |
| This leads to unnecessary code repetition. |
| </div> |
| <div> |
| Consider the implementation of the <code>is_function<></code> metafunction is Boost. |
| 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. |
| Because of the special treatment of parameter lists, it is not possible to directly match a function with an arbitrary parameter list. |
| Instead, the <code>is_function_tester()</code> must be overloaded for every distinct number of parameters that is to be supported. |
| For example: |
| </div> |
| <div class="code"><pre> |
| template<class R> |
| yes_type is_function_tester(R (*)()); |
| |
| template<class R, class A0> |
| yes_type is_function_tester(R (*)(A0)); |
| |
| template<class R, class A0, class A1> |
| yes_type is_function_tester(R (*)(A0, A1)); |
| |
| template<class R, class A0, class A1, class A2> |
| 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. |
| 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. |
| Unfortunately, using external code generators has many disadvantages: |
| <ul> |
| <li>Writing the generator takes time. (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. (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. |
| 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. |
| 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 <boost/preprocessor/arithmetic/inc.hpp> |
| #include <boost/preprocessor/punctuation/comma_if.hpp> |
| #include <boost/preprocessor/repetition.hpp> |
| |
| #ifndef MAX_IS_FUNCTION_TESTER_PARAMS |
| #define MAX_IS_FUNCTION_TESTER_PARAMS 15 |
| #endif |
| |
| #define IS_FUNCTION_TESTER(Z, N, _) \ |
| template<class R BOOST_PP_COMMA_IF(N) BOOST_PP_ENUM_PARAMS(N, class A)> \ |
| 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> |
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| <i>© Copyright <a href="http://www.housemarque.com" target="_top">Housemarque Oy</a> 2002</i> |
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| Permission to copy, use, modify, sell and distribute this document is granted provided this copyright notice appears in all copies. |
| 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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| <i>© Copyright <a href="http://www.housemarque.com" target="_top">Housemarque Oy</a> 2002</i> |
| </br><i>© Copyright Paul Mensonides 2002</i> |
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| <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> |
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