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diff --git a/share/doc/cpp/Argument-Prescan.html b/share/doc/cpp/Argument-Prescan.html new file mode 100644 index 0000000..9e99905 --- /dev/null +++ b/share/doc/cpp/Argument-Prescan.html @@ -0,0 +1,180 @@ +<!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: Argument Prescan</title> + +<meta name="description" content="The C Preprocessor: Argument Prescan"> +<meta name="keywords" content="The C Preprocessor: Argument Prescan"> +<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="Newlines-in-Arguments.html#Newlines-in-Arguments" rel="next" title="Newlines in Arguments"> +<link href="Self_002dReferential-Macros.html#Self_002dReferential-Macros" rel="previous" title="Self-Referential Macros"> +<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="Argument-Prescan"></a> +<div class="header"> +<p> +Next: <a href="Newlines-in-Arguments.html#Newlines-in-Arguments" accesskey="n" rel="next">Newlines in Arguments</a>, Previous: <a href="Self_002dReferential-Macros.html#Self_002dReferential-Macros" accesskey="p" rel="previous">Self-Referential Macros</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="Argument-Prescan-1"></a> +<h4 class="subsection">3.10.6 Argument Prescan</h4> +<a name="index-expansion-of-arguments"></a> +<a name="index-macro-argument-expansion"></a> +<a name="index-prescan-of-macro-arguments"></a> + +<p>Macro arguments are completely macro-expanded before they are +substituted into a macro body, unless they are stringized or pasted +with other tokens. After substitution, the entire macro body, including +the substituted arguments, is scanned again for macros to be expanded. +The result is that the arguments are scanned <em>twice</em> to expand +macro calls in them. +</p> +<p>Most of the time, this has no effect. If the argument contained any +macro calls, they are expanded during the first scan. The result +therefore contains no macro calls, so the second scan does not change +it. If the argument were substituted as given, with no prescan, the +single remaining scan would find the same macro calls and produce the +same results. +</p> +<p>You might expect the double scan to change the results when a +self-referential macro is used in an argument of another macro +(see <a href="Self_002dReferential-Macros.html#Self_002dReferential-Macros">Self-Referential Macros</a>): the self-referential macro would be +expanded once in the first scan, and a second time in the second scan. +However, this is not what happens. The self-references that do not +expand in the first scan are marked so that they will not expand in the +second scan either. +</p> +<p>You might wonder, “Why mention the prescan, if it makes no difference? +And why not skip it and make the preprocessor faster?” The answer is +that the prescan does make a difference in three special cases: +</p> +<ul> +<li> Nested calls to a macro. + +<p>We say that <em>nested</em> calls to a macro occur when a macro’s argument +contains a call to that very macro. For example, if <code>f</code> is a macro +that expects one argument, <code>f (f (1))</code> is a nested pair of calls to +<code>f</code>. The desired expansion is made by expanding <code>f (1)</code> and +substituting that into the definition of <code>f</code>. The prescan causes +the expected result to happen. Without the prescan, <code>f (1)</code> itself +would be substituted as an argument, and the inner use of <code>f</code> would +appear during the main scan as an indirect self-reference and would not +be expanded. +</p> +</li><li> Macros that call other macros that stringize or concatenate. + +<p>If an argument is stringized or concatenated, the prescan does not +occur. If you <em>want</em> to expand a macro, then stringize or +concatenate its expansion, you can do that by causing one macro to call +another macro that does the stringizing or concatenation. For +instance, if you have +</p> +<div class="smallexample"> +<pre class="smallexample">#define AFTERX(x) X_ ## x +#define XAFTERX(x) AFTERX(x) +#define TABLESIZE 1024 +#define BUFSIZE TABLESIZE +</pre></div> + +<p>then <code>AFTERX(BUFSIZE)</code> expands to <code>X_BUFSIZE</code>, and +<code>XAFTERX(BUFSIZE)</code> expands to <code>X_1024</code>. (Not to +<code>X_TABLESIZE</code>. Prescan always does a complete expansion.) +</p> +</li><li> Macros used in arguments, whose expansions contain unshielded commas. + +<p>This can cause a macro expanded on the second scan to be called with the +wrong number of arguments. Here is an example: +</p> +<div class="smallexample"> +<pre class="smallexample">#define foo a,b +#define bar(x) lose(x) +#define lose(x) (1 + (x)) +</pre></div> + +<p>We would like <code>bar(foo)</code> to turn into <code>(1 + (foo))</code>, which +would then turn into <code>(1 + (a,b))</code>. Instead, <code>bar(foo)</code> +expands into <code>lose(a,b)</code>, and you get an error because <code>lose</code> +requires a single argument. In this case, the problem is easily solved +by the same parentheses that ought to be used to prevent misnesting of +arithmetic operations: +</p> +<div class="smallexample"> +<pre class="smallexample">#define foo (a,b) +</pre><pre class="smallexample">or +</pre><pre class="smallexample">#define bar(x) lose((x)) +</pre></div> + +<p>The extra pair of parentheses prevents the comma in <code>foo</code>’s +definition from being interpreted as an argument separator. +</p> +</li></ul> + +<hr> +<div class="header"> +<p> +Next: <a href="Newlines-in-Arguments.html#Newlines-in-Arguments" accesskey="n" rel="next">Newlines in Arguments</a>, Previous: <a href="Self_002dReferential-Macros.html#Self_002dReferential-Macros" accesskey="p" rel="previous">Self-Referential Macros</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> |