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diff --git a/doc/html/typeof.html b/doc/html/typeof.html new file mode 100644 index 0000000000..615490c4ef --- /dev/null +++ b/doc/html/typeof.html @@ -0,0 +1,186 @@ +<html> +<head> +<meta http-equiv="Content-Type" content="text/html; charset=US-ASCII"> +<title>Chapter 31. Boost.Typeof</title> +<link rel="stylesheet" href="../../doc/src/boostbook.css" type="text/css"> +<meta name="generator" content="DocBook XSL Stylesheets V1.76.1"> +<link rel="home" href="index.html" title="The Boost C++ Libraries BoostBook Documentation Subset"> +<link rel="up" href="libraries.html" title="Part I. The Boost C++ Libraries (BoostBook Subset)"> +<link rel="prev" href="tribool/tests.html" title="Testsuite"> +<link rel="next" href="typeof/tuto.html" title="Tutorial"> +</head> +<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"> +<table cellpadding="2" width="100%"><tr> +<td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../boost.png"></td> +<td align="center"><a href="../../index.html">Home</a></td> +<td align="center"><a href="../../libs/libraries.htm">Libraries</a></td> +<td align="center"><a href="http://www.boost.org/users/people.html">People</a></td> +<td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td> +<td align="center"><a href="../../more/index.htm">More</a></td> +</tr></table> +<hr> +<div class="spirit-nav"> +<a accesskey="p" href="tribool/tests.html"><img src="../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="libraries.html"><img src="../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="index.html"><img src="../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="typeof/tuto.html"><img src="../../doc/src/images/next.png" alt="Next"></a> +</div> +<div class="chapter"> +<div class="titlepage"><div> +<div><h2 class="title"> +<a name="typeof"></a>Chapter 31. Boost.Typeof</h2></div> +<div><div class="author"><h3 class="author"> +<span class="firstname">Arkadiy</span> <span class="surname">Vertleyb</span> +</h3></div></div> +<div><div class="author"><h3 class="author"> +<span class="firstname">Peder</span> <span class="surname">Holt</span> +</h3></div></div> +<div><p class="copyright">Copyright © 2004, 2005 Arkadiy Vertleyb, Peder Holt</p></div> +<div><div class="legalnotice"> +<a name="typeof.legal"></a><p> + Distributed under the Boost Software License, Version 1.0. (See accompanying + file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top"> + http://www.boost.org/LICENSE_1_0.txt </a>) + </p> +</div></div> +</div></div> +<div class="toc"> +<p><b>Table of Contents</b></p> +<dl> +<dt><span class="section"><a href="typeof.html#typeof.moti">Motivation</a></span></dt> +<dt><span class="section"><a href="typeof/tuto.html">Tutorial</a></span></dt> +<dt><span class="section"><a href="typeof/refe.html">Reference</a></span></dt> +<dd><dl> +<dt><span class="section"><a href="typeof/refe.html#typeof.auto">AUTO, AUTO_TPL</a></span></dt> +<dt><span class="section"><a href="typeof/refe.html#typeof.compl">COMPLIANT</a></span></dt> +<dt><span class="section"><a href="typeof/refe.html#typeof.incr">INCREMENT_REGISTRATION_GROUP</a></span></dt> +<dt><span class="section"><a href="typeof/refe.html#typeof.inte">INTEGRAL</a></span></dt> +<dt><span class="section"><a href="typeof/refe.html#typeof.limit_func">LIMIT_FUNCTION_ARITY</a></span></dt> +<dt><span class="section"><a href="typeof/refe.html#typeof.messages">MESSAGES</a></span></dt> +<dt><span class="section"><a href="typeof/refe.html#typeof.limit_size">LIMIT_SIZE</a></span></dt> +<dt><span class="section"><a href="typeof/refe.html#typeof.regtype">REGISTER_TYPE</a></span></dt> +<dt><span class="section"><a href="typeof/refe.html#typeof.regtemp">REGISTER_TEMPLATE</a></span></dt> +<dt><span class="section"><a href="typeof/refe.html#typeof.temp">TEMPLATE</a></span></dt> +<dt><span class="section"><a href="typeof/refe.html#typeof.typo">TYPEOF, TYPEOF_TPL</a></span></dt> +<dt><span class="section"><a href="typeof/refe.html#typeof.typn">TYPEOF_NESTED_TYPEDEF, TYPEOF_NESTED_TYPEDEF_TPL</a></span></dt> +</dl></dd> +<dt><span class="section"><a href="typeof/other.html">Other considerations and tips</a></span></dt> +<dd><dl> +<dt><span class="section"><a href="typeof/other.html#typeof.natem">Native typeof support and emulation</a></span></dt> +<dt><span class="section"><a href="typeof/other.html#typeof.parties">The three participating parties</a></span></dt> +<dt><span class="section"><a href="typeof/other.html#typeof.features">Supported features</a></span></dt> +<dt><span class="section"><a href="typeof/other.html#typeof.what">What needs to be registered?</a></span></dt> +<dt><span class="section"><a href="typeof/other.html#typeof.limi">Limitations</a></span></dt> +</dl></dd> +<dt><span class="section"><a href="typeof/cont.html">Contributed By:</a></span></dt> +<dt><span class="section"><a href="typeof/ackn.html">Acknowledgements</a></span></dt> +</dl> +</div> +<div class="section"> +<div class="titlepage"><div><div><h2 class="title" style="clear: both"> +<a name="typeof.moti"></a>Motivation</h2></div></div></div> +<p> + Today many template libraries supply object generators to simplify object creation + by utilizing the C++ template argument deduction facility. Consider <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span></code>. + In order to instantiate this class template and create a temporary object of + this instantiation, one has to supply template parameters, as well as parameters + to the constructor: + </p> +<pre class="programlisting"><span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span><span class="special"><</span><span class="keyword">int</span><span class="special">,</span> <span class="keyword">double</span><span class="special">>(</span><span class="number">5</span><span class="special">,</span> <span class="number">3.14159</span><span class="special">);</span> +</pre> +<p> + To avoid this duplication, STL supplies the <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">make_pair</span></code> + object generator. When it is used, the types of template parameters are deduced + from supplied function arguments: + </p> +<pre class="programlisting"><span class="identifier">std</span><span class="special">::</span><span class="identifier">make_pair</span><span class="special">(</span><span class="number">5</span><span class="special">,</span> <span class="number">3.14159</span><span class="special">);</span> +</pre> +<p> + For the temporary objects it is enough. However, when a named object needs + to be allocated, the problem appears again: + </p> +<pre class="programlisting"><span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span><span class="special"><</span><span class="keyword">int</span><span class="special">,</span> <span class="keyword">double</span><span class="special">></span> <span class="identifier">p</span><span class="special">(</span><span class="number">5</span><span class="special">,</span> <span class="number">3.14159</span><span class="special">);</span> +</pre> +<p> + The object generator no longer helps: + </p> +<pre class="programlisting"><span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span><span class="special"><</span><span class="keyword">int</span><span class="special">,</span> <span class="keyword">double</span><span class="special">></span> <span class="identifier">p</span> <span class="special">=</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">make_pair</span><span class="special">(</span><span class="number">5</span><span class="special">,</span> <span class="number">3.14159</span><span class="special">);</span> +</pre> +<p> + It would be nice to deduce the type of the object (on the left) from the expression + it is initialized with (on the right), but the current C++ syntax does not + allow for this. + </p> +<p> + The above example demonstrates the essence of the problem but does not demonstrate + its scale. Many libraries, especially expression template libraries, create + objects of really complex types, and go a long way to hide this complexity + behind object generators. Consider a nit Boost.Lambda functor: + </p> +<pre class="programlisting"><span class="identifier">_1</span> <span class="special">></span> <span class="number">15</span> <span class="special">&&</span> <span class="identifier">_2</span> <span class="special"><</span> <span class="number">20</span> +</pre> +<p> + If one wanted to allocate a named copy of such an innocently looking functor, + she would have to specify something like this: + </p> +<pre class="programlisting"><span class="identifier">lambda_functor</span><span class="special"><</span> + <span class="identifier">lambda_functor_base</span><span class="special"><</span> + <span class="identifier">logical_action</span><span class="special"><</span><span class="identifier">and_action</span><span class="special">>,</span> + <span class="identifier">tuple</span><span class="special"><</span> + <span class="identifier">lambda_functor</span><span class="special"><</span> + <span class="identifier">lambda_functor_base</span><span class="special"><</span> + <span class="identifier">relational_action</span><span class="special"><</span><span class="identifier">greater_action</span><span class="special">>,</span> + <span class="identifier">tuple</span><span class="special"><</span> + <span class="identifier">lambda_functor</span><span class="special"><</span><span class="identifier">placeholder</span><span class="special"><</span><span class="number">1</span><span class="special">></span> <span class="special">>,</span> + <span class="keyword">int</span> <span class="keyword">const</span> + <span class="special">></span> + <span class="special">></span> + <span class="special">>,</span> + <span class="identifier">lambda_functor</span><span class="special"><</span> + <span class="identifier">lambda_functor_base</span><span class="special"><</span> + <span class="identifier">relational_action</span><span class="special"><</span><span class="identifier">less_action</span><span class="special">>,</span> + <span class="identifier">tuple</span><span class="special"><</span> + <span class="identifier">lambda_functor</span><span class="special"><</span><span class="identifier">placeholder</span><span class="special"><</span><span class="number">2</span><span class="special">></span> <span class="special">>,</span> + <span class="keyword">int</span> <span class="keyword">const</span> + <span class="special">></span> + <span class="special">></span> + <span class="special">></span> + <span class="special">></span> + <span class="special">></span> +<span class="special">></span> +<span class="identifier">f</span> <span class="special">=</span> <span class="identifier">_1</span> <span class="special">></span> <span class="number">15</span> <span class="special">&&</span> <span class="identifier">_2</span> <span class="special"><</span> <span class="number">20</span><span class="special">;</span> +</pre> +<p> + Not exactly elegant. To solve this problem (as well as some other problems), + the C++ standard committee is considering a few additions to the standard language, + such as <code class="computeroutput"><span class="identifier">typeof</span><span class="special">/</span><span class="identifier">decltype</span></code> and <code class="computeroutput"><span class="keyword">auto</span></code> + (see <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2004/n1607.pdf" target="_top">http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2004/n1607.pdf</a>). + </p> +<p> + The <code class="computeroutput"><span class="identifier">typeof</span></code> operator (or <code class="computeroutput"><span class="identifier">decltype</span></code>, which is a slightly different flavor + of <code class="computeroutput"><span class="identifier">typeof</span></code>) allows one to determine + the type of an expression at compile time. Using <code class="computeroutput"><span class="identifier">typeof</span></code>, + the above example can be simplified drastically: + </p> +<pre class="programlisting"><span class="identifier">typeof</span><span class="special">(</span><span class="identifier">_1</span> <span class="special">></span> <span class="number">15</span> <span class="special">&&</span> <span class="identifier">_2</span> <span class="special"><</span> <span class="number">20</span><span class="special">)</span> <span class="identifier">f</span> <span class="special">=</span> <span class="identifier">_1</span> <span class="special">></span> <span class="number">15</span> <span class="special">&&</span> <span class="identifier">_2</span> <span class="special"><</span> <span class="number">20</span><span class="special">;</span> +</pre> +<p> + Much better, but some duplication still exists. The <code class="computeroutput"><span class="keyword">auto</span></code> + type solves the rest of the problem: + </p> +<pre class="programlisting"><span class="keyword">auto</span> <span class="identifier">f</span> <span class="special">=</span> <span class="identifier">_1</span> <span class="special">></span> <span class="number">15</span> <span class="special">&&</span> <span class="identifier">_2</span> <span class="special"><</span> <span class="number">20</span><span class="special">;</span> +</pre> +<p> + The purpose of the Boost.Typeof library is to provide a library-based solution, + which could be used until the language-based facility is added to the Standard + and becomes widely available. + </p> +</div> +</div> +<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> +<td align="left"><p><small>Last revised: January 31, 2012 at 02:30:03 +0000</small></p></td> +<td align="right"><div class="copyright-footer"></div></td> +</tr></table> +<hr> +<div class="spirit-nav"> +<a accesskey="p" href="tribool/tests.html"><img src="../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="libraries.html"><img src="../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="index.html"><img src="../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="typeof/tuto.html"><img src="../../doc/src/images/next.png" alt="Next"></a> +</div> +</body> +</html> |