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