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3        <title>problems.html</title>
4        <link rel="stylesheet" type="text/css" href="../styles.css">
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6<body>
7<h4>Known Problems of the C/C++ Preprocessor</h4>
8<div>
9        Preprocessor metaprogramming is subject to heated discussions.&nbsp;
10        Part of this is caused by bad experiences with dangerous techniques,
11        such as defining inline functions using macros.&nbsp;
12        As a rule of thumb, if you can find a clean and manageable way to do something
13        without the preprocessor, then you should do it that way.
14</div>
15<div>
16        Let's survey some of the widely known problems of the preprocessor in a problem/solution format.
17</div>
18<h4>Problem #1</h4>
19<div>
20        The preprocessor does not respect scope, therefore macros can accidentally and sometimes silently replace code.
21</div>
22<div>
23        <b>Solution A</b>
24        <div>
25                Use all caps identifiers for macros and only macros.&nbsp;
26                This practically eliminates the possibility that a macro might replace other kinds of code accidentally.
27        </div>
28</div>
29<div>
30        <b>Solution B</b>
31        <div>
32                Use the local macro idiom:
33        </div>
34<div class="code"><pre>
35#define MACRO ...
36// use MACRO
37#undef MACRO
38</pre></div>
39        <div>
40                This makes sure that a macro cannot accidentally replace code outside of the scope of the local macro.
41        </div>
42        <div>
43                A problem with this solution is that the #undef cannot be automated and may be forgotten.&nbsp;
44                Experienced programmers generally write the #undef either immediately before (in time)
45                or immediately after writing the macro definition.
46        </div>
47</div>
48<div>
49        <b>Solution C</b>
50        <div>
51                Use the unique macro prefix idiom.
52        </div>
53<div class="code"><pre>
54#define UMP_MACRO
55// use UMP_MACRO
56</pre></div>
57        <div>
58                This makes accidental substitution and collisions highly unlikely.&nbsp;
59                Problems with this solution include:
60        </div>
61        <ul>
62                <li>There can still be naming collisions inside a large project.</li>
63                <li>Macros still pollute the global namespace.</li>
64        </ul>
65        <i>By combining all solutions, whenever possible, the scope problem can be largely avoided.</i>
66</div>
67<h4>Problem #2</h4>
68<div>
69        Preprocessor code is difficult to read.&nbsp;
70        It requires an understanding of the basic process of how the preprocessor recursively expands macros,
71        finding macro definitions, and mentally substituting the parameters of the macro.
72</div>
73<div>
74        <b>Solution</b>
75        <div>
76                Any kind of programming requires a basic understanding of how the code is executed.&nbsp;
77                Any parameterization technique, including simple functions and templates requires finding
78                the definition and mentally substituting parameters.
79        </div>
80        <div>
81                However, it is good to know a few techniques:
82        </div>
83        <ul>
84                <li>By using as many local macros as reasonable, the bulk of the searching process can be eliminated.</li>
85                <li>Code browsers and text search tools make it easier to find the definitions.</li>
86                <li>The compiler can be used for generating the preprocessed source code in order to look for bugs.</li>
87                <li>
88                        Before turning something into a preprocessor metaprogram, first implement a small scale version
89                        of it without the preprocessor.&nbsp;
90                        The work bottom-up, replacing hand-written constructs by using the preprocessor.&nbsp;
91                        This way you can test the code incrementally.&nbsp;
92                        Experienced programmers often skip many stages, but if something proves too complex to write
93                        directly, it is always possible to fall back to incremental methods.
94                </li>
95                <li>
96                        If you insert a special symbol into the preprocessor code in places where there should be a line break,
97                        you can make code readable after preprocessing simply by using a search and replace tool.
98                </li>
99        </ul>
100        <i>An especially important thing to remember is to limit the use of the preprocessor to
101        structured, well-understood, and safe methods.&nbsp;
102        Structure helps to understand complex systems <a href="../bibliography.html#mcconnell">[McConnell]</a>.</i>
103</div>
104<h4>Problem #3</h4>
105<div>
106        "I'd like to see Cpp abolished." - <i>Bjarne Stroustrup</i> in <a href="../bibliography.html#stroustrup">[Stroustrup]</a>.
107</div>
108<div>
109        <b>Solution</b>
110        <div>
111                The C/C++ preprocessor will be here for a long time.
112        </div>
113        <i>In practice, preprocessor metaprogramming is far simpler and more portable than template metaprogramming <a href="../bibliography.html#czarnecki">[Czarnecki]</a>.</i>
114</div>
115<hr size="1">
116<div style="margin-left: 0px;">
117        <i>© Copyright <a href="http://www.housemarque.com" target="_top">Housemarque Oy</a> 2002</i>
118</div>
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120Permission to copy, use, modify, sell and distribute this document is granted provided this copyright notice appears in all copies.&nbsp;
121This 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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