mirror of
https://github.com/boostorg/preprocessor.git
synced 2025-07-19 07:22:07 +02:00
58 lines
1.8 KiB
HTML
58 lines
1.8 KiB
HTML
|
<html>
|
||
|
|
<head>
|
||
|
|
<title>sets.html</title>
|
||
|
|
<link rel="stylesheet" type="text/css" href="../styles.css">
|
||
|
|
</head>
|
||
|
|
<body>
|
||
|
|
<h4>Sets</h4>
|
||
|
|
<div>
|
||
|
|
A <i>set</i> is a group of adjacent parenthesized elements.
|
||
|
|
For example,
|
||
|
|
</div>
|
||
|
|
<div class="code">
|
||
|
|
(<i>a</i>)(<i>b</i>)(<i>c</i>)
|
||
|
|
</div>
|
||
|
|
<div>
|
||
|
|
...is a <i>set</i> of <i>3</i> elements--<i>a</i>, <i>b</i>, and <i>c</i>.
|
||
|
|
</div>
|
||
|
|
<div>
|
||
|
|
<i>Sets</i> are data structures that merge the properties of both <i>lists</i> and <i>tuples</i>
|
||
|
|
with the exception that <i>sets</i> cannot be empty.
|
||
|
|
Therefore, an "empty" <i>set</i> is considered a special case scenario that must be handled separately in C++.
|
||
|
|
</div>
|
||
|
|
<div class="code"><pre>
|
||
|
|
#define SET (x)(y)(z)
|
||
|
|
#define REVERSE(s, state, elem) (elem) state
|
||
|
|
// append to head ^
|
||
|
|
|
||
|
|
BOOST_PP_SET_FOLD_LEFT(REVERSE, BOOST_PP_EMPTY, SET)()
|
||
|
|
// #1 #2
|
||
|
|
// 1) placeholder for "empty" set
|
||
|
|
// 2) remove placeholder
|
||
|
|
|
||
|
|
#define SET_B (1)(2)(3)
|
||
|
|
#define INC(s, state, elem) state (BOOST_PP_INC(elem))
|
||
|
|
// append to tail ^
|
||
|
|
|
||
|
|
BOOST_PP_SET_FOLD_RIGHT(INC, BOOST_PP_SET_NIL, SET)
|
||
|
|
// ^
|
||
|
|
// special placeholder that will be "eaten"
|
||
|
|
// by appending to the tail
|
||
|
|
</pre></div>
|
||
|
|
<div>
|
||
|
|
<i>Sets</i> are extremely efficient.
|
||
|
|
Element access speed approaches random access--even with <i>sets</i> of up to <i>256</i> elements.
|
||
|
|
This is because element access (among other things) is implemented iteratively rather than recursively.
|
||
|
|
Therefore, elements can be accessed at extremely high indices even on preprocessors with low maximum expansion depths.
|
||
|
|
</div>
|
||
|
|
<div>
|
||
|
|
Elements of a <i>set</i> can be extracted with
|
||
|
|
<b>BOOST_PP_SET_ELEM</b>.
|
||
|
|
</div>
|
||
|
|
<h4>Primitives</h4>
|
||
|
|
<ul>
|
||
|
|
<li><a href="../ref/set_elem.html">BOOST_PP_SET_ELEM</a></li>
|
||
|
|
</ul>
|
||
|
|
</body>
|
||
|
|
</html>
|