2001-11-25 18:32:11 +00:00
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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
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<HTML><HEAD><TITLE>Boost PREPROCESSOR library</TITLE>
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<h1>Boost PREPROCESSOR library: Widely known problems with the C preprocessor</h1>
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<p>
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Preprocessor metaprogramming is subject to heated discussions. Part of this is caused by
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bad experiences with dangerous techniques, such as defining inline functions using macros. As a rule
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of thumb, if you can find a clean and
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manageable way to do something without using the preprocessor, then
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you should do it that way.</p>
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<p>Let's survey some of the widely known problems with the preprocessor in a problem/solution
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format.</p>
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<HR>
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<p><B>PROBLEM:</B> Preprocessor does not
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respect scope, therefore macros can accidentally and sometimes silently replace
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code.</p>
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<p><B>SOLUTION A:</B> Use all caps identifiers
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for macros and only macros. This practically eliminates the possibility that a
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macro might replace other kind of code accidentally.</p>
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<p><B>SOLUTION B:</B> Use the Local Macro
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idiom:</p>
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<blockquote><pre>#define MACRO ...
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// Use MACRO
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#undef MACRO
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</pre></blockquote>
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<p>This makes sure that a macro can not accidentally
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replace code outside of the scope of the local macro.</p>
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<P>A problem with this solution is that the #undef can not be automated and may be
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forgotten. Experienced programmers generally write the #undef either immediately
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before (in time) or immediately after writing the macro definition.</P>
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<P><B>SOLUTION C:</B> Use the Unique Macro Prefix idiom:</P>
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<blockquote><pre>#define UMP_MACRO
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// Use UMP_MACRO
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</pre></blockquote>
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<P>This makes accidental substitution and collisions highly
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unlikely. Problems with this solution:</P>
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<UL>
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<LI>
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There can still be naming collisions inside a large project.
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<LI>
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Macros still pollute the global namespace. </LI></UL>
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<P><EM><B>By combining all solutions, whenever
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possible, the scope problem can be largely avoided.</B></EM></P>
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<HR>
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<P><B>PROBLEM:</B> Preprocessor code is difficult to read.
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It requires understanding the basic process of how
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the preprocessor recursively expands macros, finding the macro definition and mentally
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substituting the parameters of the macro. </P>
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<P><B>SOLUTION:</B> Any kind of programming requires basic understanding
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of how the code is executed. Any parameterization technique, including simple
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functions and templates requires finding the definition and mentally substituting
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parameters. </P>
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<P>However, it is good to know a few techniques:</P>
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<UL>
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<LI>
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By using as many Local Macros as reasonable, the bulk of the searching
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process can be eliminated.
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<LI>
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Code browsers and text search tools make it easier to find the
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definitions.
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<LI>
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The compiler can be used for generating the preprocessed source code in
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order to look for bugs.
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<LI>
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Before turning something into a preprocessor metaprogram, first
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implement a small scale version of it without preprocessor. Then work
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bottom->up replacing hand written constructs by using preprocessor. This
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way you can test the code incrementally. Experienced programmers often skip
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many stages, but if something proves too complex to write directly, it is
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always possible to fall back to incremental methods.
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<LI>
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If you insert a special symbol into the preprocessor code in places where
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there should be a line break, you can make code readable after preprocessing
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simply by using a search and replace tool. </LI></UL>
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<P><B><EM> An especially important thing to remember is to limit the use of preprocessor
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to the structured, well understood and safe methods. Structure helps to understand
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complex systems <A href="references.htm#[4]">[4]</A>.</EM></B></P>
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<HR>
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<P><B>PROBLEM:</B> "I'd
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like to see Cpp abolished." - Bjarne Stroustrup in <A href="references.htm#[1]">[1]</A></P>
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<P><B>SOLUTION:</B> The C preprocessor will be here for a
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long time.</P>
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<P><EM><B>In practice, preprocessor metaprogramming is far simpler and more portable
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than template metaprogramming <A href="references.htm#[2]">[2]</A>.</B></EM></P>
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<hr>
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2001-12-03 21:20:27 +00:00
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<P><EFBFBD> Copyright Housemarque Oy 2001</P>
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2001-11-25 18:32:11 +00:00
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<p>Permission to copy, use, modify, sell and distribute this document is granted
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provided this copyright notice appears in all copies. This document is provided
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"as is" without express or implied warranty, and with no claim as to its suitability
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for any purpose. </p>
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<p>Updated: <!--webbot bot="Timestamp" s-type="EDITED" s-format="%d %b %Y" startspan --><!--webbot bot="Timestamp" endspan i-checksum="15246" --></p>
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<p></p>
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