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| author | alk3pInjection <webmaster@raspii.tech> | 2024-02-04 16:16:35 +0800 |
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| committer | alk3pInjection <webmaster@raspii.tech> | 2024-02-04 16:16:35 +0800 |
| commit | abdaadbcae30fe0c9a66c7516798279fdfd97750 (patch) | |
| tree | 00a54a6e25601e43876d03c1a4a12a749d4a914c /share/doc/cpp/Self_002dReferential-Macros.html | |
https://developer.arm.com/downloads/-/arm-gnu-toolchain-downloads
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diff --git a/share/doc/cpp/Self_002dReferential-Macros.html b/share/doc/cpp/Self_002dReferential-Macros.html new file mode 100644 index 0000000..e223eaf --- /dev/null +++ b/share/doc/cpp/Self_002dReferential-Macros.html @@ -0,0 +1,155 @@ +<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> +<html> +<!-- Copyright (C) 1987-2023 Free Software Foundation, Inc. + +Permission is granted to copy, distribute and/or modify this document +under the terms of the GNU Free Documentation License, Version 1.3 or +any later version published by the Free Software Foundation. A copy of +the license is included in the +section entitled "GNU Free Documentation License". + +This manual contains no Invariant Sections. The Front-Cover Texts are +(a) (see below), and the Back-Cover Texts are (b) (see below). + +(a) The FSF's Front-Cover Text is: + +A GNU Manual + +(b) The FSF's Back-Cover Text is: + +You have freedom to copy and modify this GNU Manual, like GNU + software. Copies published by the Free Software Foundation raise + funds for GNU development. --> +<!-- Created by GNU Texinfo 5.1, http://www.gnu.org/software/texinfo/ --> +<head> +<title>The C Preprocessor: Self-Referential Macros</title> + +<meta name="description" content="The C Preprocessor: Self-Referential Macros"> +<meta name="keywords" content="The C Preprocessor: Self-Referential Macros"> +<meta name="resource-type" content="document"> +<meta name="distribution" content="global"> +<meta name="Generator" content="makeinfo"> +<meta http-equiv="Content-Type" content="text/html; charset=utf-8"> +<link href="index.html#Top" rel="start" title="Top"> +<link href="Index-of-Directives.html#Index-of-Directives" rel="index" title="Index of Directives"> +<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents"> +<link href="Macro-Pitfalls.html#Macro-Pitfalls" rel="up" title="Macro Pitfalls"> +<link href="Argument-Prescan.html#Argument-Prescan" rel="next" title="Argument Prescan"> +<link href="Duplication-of-Side-Effects.html#Duplication-of-Side-Effects" rel="previous" title="Duplication of Side Effects"> +<style type="text/css"> +<!-- +a.summary-letter {text-decoration: none} +blockquote.smallquotation {font-size: smaller} +div.display {margin-left: 3.2em} +div.example {margin-left: 3.2em} +div.indentedblock {margin-left: 3.2em} +div.lisp {margin-left: 3.2em} +div.smalldisplay {margin-left: 3.2em} +div.smallexample {margin-left: 3.2em} +div.smallindentedblock {margin-left: 3.2em; font-size: smaller} +div.smalllisp {margin-left: 3.2em} +kbd {font-style:oblique} +pre.display {font-family: inherit} +pre.format {font-family: inherit} +pre.menu-comment {font-family: serif} +pre.menu-preformatted {font-family: serif} +pre.smalldisplay {font-family: inherit; font-size: smaller} +pre.smallexample {font-size: smaller} +pre.smallformat {font-family: inherit; font-size: smaller} +pre.smalllisp {font-size: smaller} +span.nocodebreak {white-space:nowrap} +span.nolinebreak {white-space:nowrap} +span.roman {font-family:serif; font-weight:normal} +span.sansserif {font-family:sans-serif; font-weight:normal} +ul.no-bullet {list-style: none} +--> +</style> + + +</head> + +<body lang="en" bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#800080" alink="#FF0000"> +<a name="Self_002dReferential-Macros"></a> +<div class="header"> +<p> +Next: <a href="Argument-Prescan.html#Argument-Prescan" accesskey="n" rel="next">Argument Prescan</a>, Previous: <a href="Duplication-of-Side-Effects.html#Duplication-of-Side-Effects" accesskey="p" rel="previous">Duplication of Side Effects</a>, Up: <a href="Macro-Pitfalls.html#Macro-Pitfalls" accesskey="u" rel="up">Macro Pitfalls</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Index-of-Directives.html#Index-of-Directives" title="Index" rel="index">Index</a>]</p> +</div> +<hr> +<a name="Self_002dReferential-Macros-1"></a> +<h4 class="subsection">3.10.5 Self-Referential Macros</h4> +<a name="index-self_002dreference"></a> + +<p>A <em>self-referential</em> macro is one whose name appears in its +definition. Recall that all macro definitions are rescanned for more +macros to replace. If the self-reference were considered a use of the +macro, it would produce an infinitely large expansion. To prevent this, +the self-reference is not considered a macro call. It is passed into +the preprocessor output unchanged. Consider an example: +</p> +<div class="smallexample"> +<pre class="smallexample">#define foo (4 + foo) +</pre></div> + +<p>where <code>foo</code> is also a variable in your program. +</p> +<p>Following the ordinary rules, each reference to <code>foo</code> will expand +into <code>(4 + foo)</code>; then this will be rescanned and will expand into +<code>(4 + (4 + foo))</code>; and so on until the computer runs out of memory. +</p> +<p>The self-reference rule cuts this process short after one step, at +<code>(4 + foo)</code>. Therefore, this macro definition has the possibly +useful effect of causing the program to add 4 to the value of <code>foo</code> +wherever <code>foo</code> is referred to. +</p> +<p>In most cases, it is a bad idea to take advantage of this feature. A +person reading the program who sees that <code>foo</code> is a variable will +not expect that it is a macro as well. The reader will come across the +identifier <code>foo</code> in the program and think its value should be that +of the variable <code>foo</code>, whereas in fact the value is four greater. +</p> +<p>One common, useful use of self-reference is to create a macro which +expands to itself. If you write +</p> +<div class="smallexample"> +<pre class="smallexample">#define EPERM EPERM +</pre></div> + +<p>then the macro <code>EPERM</code> expands to <code>EPERM</code>. Effectively, it is +left alone by the preprocessor whenever it’s used in running text. You +can tell that it’s a macro with ‘<samp>#ifdef</samp>’. You might do this if you +want to define numeric constants with an <code>enum</code>, but have +‘<samp>#ifdef</samp>’ be true for each constant. +</p> +<p>If a macro <code>x</code> expands to use a macro <code>y</code>, and the expansion of +<code>y</code> refers to the macro <code>x</code>, that is an <em>indirect +self-reference</em> of <code>x</code>. <code>x</code> is not expanded in this case +either. Thus, if we have +</p> +<div class="smallexample"> +<pre class="smallexample">#define x (4 + y) +#define y (2 * x) +</pre></div> + +<p>then <code>x</code> and <code>y</code> expand as follows: +</p> +<div class="smallexample"> +<pre class="smallexample">x → (4 + y) + → (4 + (2 * x)) + +y → (2 * x) + → (2 * (4 + y)) +</pre></div> + +<p>Each macro is expanded when it appears in the definition of the other +macro, but not when it indirectly appears in its own definition. +</p> +<hr> +<div class="header"> +<p> +Next: <a href="Argument-Prescan.html#Argument-Prescan" accesskey="n" rel="next">Argument Prescan</a>, Previous: <a href="Duplication-of-Side-Effects.html#Duplication-of-Side-Effects" accesskey="p" rel="previous">Duplication of Side Effects</a>, Up: <a href="Macro-Pitfalls.html#Macro-Pitfalls" accesskey="u" rel="up">Macro Pitfalls</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Index-of-Directives.html#Index-of-Directives" title="Index" rel="index">Index</a>]</p> +</div> + + + +</body> +</html> |