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| <h4>Known Problems of the C/C++ Preprocessor</h4> |
| <div> |
| Preprocessor metaprogramming is subject to heated discussions. |
| Part of this is caused by bad experiences with dangerous techniques, |
| such as defining inline functions using macros. |
| As a rule of thumb, if you can find a clean and manageable way to do something |
| without the preprocessor, then you should do it that way. |
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| Let's survey some of the widely known problems of the preprocessor in a problem/solution format. |
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| <h4>Problem #1</h4> |
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| The preprocessor does not respect scope, therefore macros can accidentally and sometimes silently replace code. |
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| <b>Solution A</b> |
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| Use all caps identifiers for macros and only macros. |
| This practically eliminates the possibility that a macro might replace other kinds of code accidentally. |
| </div> |
| </div> |
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| <b>Solution B</b> |
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| Use the local macro idiom: |
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| <div class="code"><pre> |
| #define MACRO ... |
| // use MACRO |
| #undef MACRO |
| </pre></div> |
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| This makes sure that a macro cannot accidentally replace code outside of the scope of the local macro. |
| </div> |
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| A problem with this solution is that the #undef cannot be automated and may be forgotten. |
| Experienced programmers generally write the #undef either immediately before (in time) |
| or immediately after writing the macro definition. |
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| </div> |
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| <b>Solution C</b> |
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| Use the unique macro prefix idiom. |
| </div> |
| <div class="code"><pre> |
| #define UMP_MACRO |
| // use UMP_MACRO |
| </pre></div> |
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| This makes accidental substitution and collisions highly unlikely. |
| Problems with this solution include: |
| </div> |
| <ul> |
| <li>There can still be naming collisions inside a large project.</li> |
| <li>Macros still pollute the global namespace.</li> |
| </ul> |
| <i>By combining all solutions, whenever possible, the scope problem can be largely avoided.</i> |
| </div> |
| <h4>Problem #2</h4> |
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| Preprocessor code is difficult to read. |
| It requires an understanding of the basic process of how the preprocessor recursively expands macros, |
| finding macro definitions, and mentally substituting the parameters of the macro. |
| </div> |
| <div> |
| <b>Solution</b> |
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| Any kind of programming requires a basic understanding of how the code is executed. |
| Any parameterization technique, including simple functions and templates requires finding |
| the definition and mentally substituting parameters. |
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| However, it is good to know a few techniques: |
| </div> |
| <ul> |
| <li>By using as many local macros as reasonable, the bulk of the searching process can be eliminated.</li> |
| <li>Code browsers and text search tools make it easier to find the definitions.</li> |
| <li>The compiler can be used for generating the preprocessed source code in order to look for bugs.</li> |
| <li> |
| Before turning something into a preprocessor metaprogram, first implement a small scale version |
| of it without the preprocessor. |
| The work bottom-up, replacing hand-written constructs by using the preprocessor. |
| This way you can test the code incrementally. |
| Experienced programmers often skip many stages, but if something proves too complex to write |
| directly, it is always possible to fall back to incremental methods. |
| </li> |
| <li> |
| If you insert a special symbol into the preprocessor code in places where there should be a line break, |
| you can make code readable after preprocessing simply by using a search and replace tool. |
| </li> |
| </ul> |
| <i>An especially important thing to remember is to limit the use of the preprocessor to |
| structured, well-understood, and safe methods. |
| Structure helps to understand complex systems <a href="../bibliography.html#mcconnell">[McConnell]</a>.</i> |
| </div> |
| <h4>Problem #3</h4> |
| <div> |
| "I'd like to see Cpp abolished." - <i>Bjarne Stroustrup</i> in <a href="../bibliography.html#stroustrup">[Stroustrup]</a>. |
| </div> |
| <div> |
| <b>Solution</b> |
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| The C/C++ preprocessor will be here for a long time. |
| </div> |
| <i>In practice, preprocessor metaprogramming is far simpler and more portable than template metaprogramming <a href="../bibliography.html#czarnecki">[Czarnecki]</a>.</i> |
| </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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