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diff --git a/share/doc/cpp/Operator-Precedence-Problems.html b/share/doc/cpp/Operator-Precedence-Problems.html new file mode 100644 index 0000000..7daed27 --- /dev/null +++ b/share/doc/cpp/Operator-Precedence-Problems.html @@ -0,0 +1,153 @@ +<!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. 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In addition, another pair of parentheses usually surround the +entire macro definition. Here is why it is best to write macros that +way. +</p> +<p>Suppose you define a macro as follows, +</p> +<div class="smallexample"> +<pre class="smallexample">#define ceil_div(x, y) (x + y - 1) / y +</pre></div> + +<p>whose purpose is to divide, rounding up. (One use for this operation is +to compute how many <code>int</code> objects are needed to hold a certain +number of <code>char</code> objects.) Then suppose it is used as follows: +</p> +<div class="smallexample"> +<pre class="smallexample">a = ceil_div (b & c, sizeof (int)); + → a = (b & c + sizeof (int) - 1) / sizeof (int); +</pre></div> + +<p>This does not do what is intended. The operator-precedence rules of +C make it equivalent to this: +</p> +<div class="smallexample"> +<pre class="smallexample">a = (b & (c + sizeof (int) - 1)) / sizeof (int); +</pre></div> + +<p>What we want is this: +</p> +<div class="smallexample"> +<pre class="smallexample">a = ((b & c) + sizeof (int) - 1)) / sizeof (int); +</pre></div> + +<p>Defining the macro as +</p> +<div class="smallexample"> +<pre class="smallexample">#define ceil_div(x, y) ((x) + (y) - 1) / (y) +</pre></div> + +<p>provides the desired result. +</p> +<p>Unintended grouping can result in another way. Consider <code>sizeof +ceil_div(1, 2)</code>. That has the appearance of a C expression that would +compute the size of the type of <code>ceil_div (1, 2)</code>, but in fact it +means something very different. Here is what it expands to: +</p> +<div class="smallexample"> +<pre class="smallexample">sizeof ((1) + (2) - 1) / (2) +</pre></div> + +<p>This would take the size of an integer and divide it by two. The +precedence rules have put the division outside the <code>sizeof</code> when it +was intended to be inside. +</p> +<p>Parentheses around the entire macro definition prevent such problems. +Here, then, is the recommended way to define <code>ceil_div</code>: +</p> +<div class="smallexample"> +<pre class="smallexample">#define ceil_div(x, y) (((x) + (y) - 1) / (y)) +</pre></div> + +<hr> +<div class="header"> +<p> +Next: <a href="Swallowing-the-Semicolon.html#Swallowing-the-Semicolon" accesskey="n" rel="next">Swallowing the Semicolon</a>, Previous: <a href="Misnesting.html#Misnesting" accesskey="p" rel="previous">Misnesting</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> |