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<html> <head> <meta http-equiv="Content-Type" content="text/html; charset=UTF-8"> <title>Tuple library advanced features</title> <link rel="stylesheet" href="../../../../doc/src/boostbook.css" type="text/css"> <meta name="generator" content="DocBook XSL Stylesheets V1.79.1"> <link rel="home" href="tuple_users_guide.html" title="Chapter 1. Boost.Tuple"> <link rel="up" href="tuple_users_guide.html" title="Chapter 1. Boost.Tuple"> <link rel="prev" href="tuple_users_guide.html" title="Chapter 1. Boost.Tuple"> <link rel="next" href="design_decisions_rationale.html" title="Design decisions rationale"> </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="tuple_users_guide.html"><img src="../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="tuple_users_guide.html"><img src="../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="tuple_users_guide.html"><img src="../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="design_decisions_rationale.html"><img src="../../../../doc/src/images/next.png" alt="Next"></a> </div> <div class="article"> <div class="titlepage"> <div> <div><h2 class="title"> <a name="tuple_advanced_interface"></a>Tuple library advanced features</h2></div> <div><p class="copyright">Copyright © 2001 Jaakko Järvi</p></div> <div><div class="legalnotice"> <a name="tuple_advanced_interface.legal"></a><p> Distributed under the <a href="http://boost.org/LICENSE_1_0.txt" target="_top">Boost Software License, Version 1.0</a>. </p> </div></div> </div> <hr> </div> <div class="toc"> <p><b>Table of Contents</b></p> <dl class="toc"> <dt><span class="section"><a href="tuple_advanced_interface.html#tuple_advanced_interface.metafunctions_for_tuple_types">Metafunctions for tuple types</a></span></dt> <dt><span class="section"><a href="tuple_advanced_interface.html#tuple_advanced_interface.cons_lists">Cons lists</a></span></dt> <dt><span class="section"><a href="tuple_advanced_interface.html#tuple_advanced_interface.traits_classes_for_tuple_element">Traits classes for tuple element types</a></span></dt> </dl> </div> <p> The advanced features described in this document are all under namespace <code class="computeroutput"><span class="special">::</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">tuples</span></code> </p> <div class="section"> <div class="titlepage"><div><div><h3 class="title"> <a name="tuple_advanced_interface.metafunctions_for_tuple_types"></a><a class="link" href="tuple_advanced_interface.html#tuple_advanced_interface.metafunctions_for_tuple_types" title="Metafunctions for tuple types">Metafunctions for tuple types</a> </h3></div></div></div> <p> Suppose <code class="computeroutput"><span class="identifier">T</span></code> is a tuple type, and <code class="computeroutput"><span class="identifier">N</span></code> is a constant integral expression. </p> <pre class="programlisting"><span class="identifier">element</span><span class="special"><</span><span class="identifier">N</span><span class="special">,</span> <span class="identifier">T</span><span class="special">>::</span><span class="identifier">type</span> </pre> <p> gives the type of the <code class="computeroutput"><span class="identifier">N</span></code>-th element in the tuple type <code class="computeroutput"><span class="identifier">T</span></code>. If <code class="computeroutput"><span class="identifier">T</span></code> is <code class="computeroutput"><span class="keyword">const</span></code>, the resulting type is <code class="computeroutput"><span class="keyword">const</span></code> qualified as well. Note that the constness of <code class="computeroutput"><span class="identifier">T</span></code> does not affect reference type elements. </p> <pre class="programlisting"><span class="identifier">length</span><span class="special"><</span><span class="identifier">T</span><span class="special">>::</span><span class="identifier">value</span> </pre> <p> gives the length of the tuple type <code class="computeroutput"><span class="identifier">T</span></code>. </p> </div> <div class="section"> <div class="titlepage"><div><div><h3 class="title"> <a name="tuple_advanced_interface.cons_lists"></a><a class="link" href="tuple_advanced_interface.html#tuple_advanced_interface.cons_lists" title="Cons lists">Cons lists</a> </h3></div></div></div> <p> Tuples are internally represented as <span class="emphasis"><em>cons lists</em></span>. For example, the tuple </p> <pre class="programlisting"><span class="identifier">tuple</span><span class="special"><</span><span class="identifier">A</span><span class="special">,</span> <span class="identifier">B</span><span class="special">,</span> <span class="identifier">C</span><span class="special">,</span> <span class="identifier">D</span><span class="special">></span> </pre> <p> inherits from the type </p> <pre class="programlisting"><span class="identifier">cons</span><span class="special"><</span><span class="identifier">A</span><span class="special">,</span> <span class="identifier">cons</span><span class="special"><</span><span class="identifier">B</span><span class="special">,</span> <span class="identifier">cons</span><span class="special"><</span><span class="identifier">C</span><span class="special">,</span> <span class="identifier">cons</span><span class="special"><</span><span class="identifier">D</span><span class="special">,</span> <span class="identifier">null_type</span><span class="special">></span> <span class="special">></span> <span class="special">></span> <span class="special">></span> </pre> <p> The tuple template provides the typedef inherited to access the cons list representation. E.g.: <code class="computeroutput"><span class="identifier">tuple</span><span class="special"><</span><span class="identifier">A</span><span class="special">>::</span><span class="identifier">inherited</span></code> is the type <code class="computeroutput"><span class="identifier">cons</span><span class="special"><</span><span class="identifier">A</span><span class="special">,</span> <span class="identifier">null_type</span><span class="special">></span></code>. </p> <div class="section"> <div class="titlepage"><div><div><h4 class="title"> <a name="tuple_advanced_interface.cons_lists.empty_tuple"></a><a class="link" href="tuple_advanced_interface.html#tuple_advanced_interface.cons_lists.empty_tuple" title="Empty tuple">Empty tuple</a> </h4></div></div></div> <p> The internal representation of the empty tuple <code class="computeroutput"><span class="identifier">tuple</span><span class="special"><></span></code> is <code class="computeroutput"><span class="identifier">null_type</span></code>. </p> </div> <div class="section"> <div class="titlepage"><div><div><h4 class="title"> <a name="tuple_advanced_interface.cons_lists.head_and_tail"></a><a class="link" href="tuple_advanced_interface.html#tuple_advanced_interface.cons_lists.head_and_tail" title="Head and tail">Head and tail</a> </h4></div></div></div> <p> Both tuple template and the cons templates provide the typedefs <code class="computeroutput"><span class="identifier">head_type</span></code> and <code class="computeroutput"><span class="identifier">tail_type</span></code>. The <code class="computeroutput"><span class="identifier">head_type</span></code> typedef gives the type of the first element of the tuple (or the cons list). The <code class="computeroutput"><span class="identifier">tail_type</span></code> typedef gives the remaining cons list after removing the first element. The head element is stored in the member variable <code class="computeroutput"><span class="identifier">head</span></code> and the tail list in the member variable <code class="computeroutput"><span class="identifier">tail</span></code>. Cons lists provide the member function <code class="computeroutput"><span class="identifier">get_head</span><span class="special">()</span></code> for getting a reference to the head of a cons list, and <code class="computeroutput"><span class="identifier">get_tail</span><span class="special">()</span></code> for getting a reference to the tail. There are const and non-const versions of both functions. </p> <p> Note that in a one element tuple, <code class="computeroutput"><span class="identifier">tail_type</span></code> equals <code class="computeroutput"><span class="identifier">null_type</span></code> and the <code class="computeroutput"><span class="identifier">get_tail</span><span class="special">()</span></code> function returns an object of type <code class="computeroutput"><span class="identifier">null_type</span></code>. </p> <p> The empty tuple (<code class="computeroutput"><span class="identifier">null_type</span></code>) has no head or tail, hence the <code class="computeroutput"><span class="identifier">get_head</span></code> and <code class="computeroutput"><span class="identifier">get_tail</span></code> functions are not provided. </p> <p> Treating tuples as cons lists gives a convenient means to define generic functions to manipulate tuples. For example, the following pair of function templates assign <code class="computeroutput"><span class="number">0</span></code> to each element of a tuple (obviously, the assignments must be valid operations for the element types): </p> <pre class="programlisting"><span class="keyword">inline</span> <span class="keyword">void</span> <span class="identifier">set_to_zero</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">null_type</span><span class="special">&)</span> <span class="special">{};</span> <span class="keyword">template</span> <span class="special"><</span><span class="keyword">class</span> <span class="identifier">H</span><span class="special">,</span> <span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> <span class="keyword">inline</span> <span class="keyword">void</span> <span class="identifier">set_to_zero</span><span class="special">(</span><span class="identifier">cons</span><span class="special"><</span><span class="identifier">H</span><span class="special">,</span> <span class="identifier">T</span><span class="special">>&</span> <span class="identifier">x</span><span class="special">)</span> <span class="special">{</span> <span class="identifier">x</span><span class="special">.</span><span class="identifier">get_head</span><span class="special">()</span> <span class="special">=</span> <span class="number">0</span><span class="special">;</span> <span class="identifier">set_to_zero</span><span class="special">(</span><span class="identifier">x</span><span class="special">.</span><span class="identifier">get_tail</span><span class="special">());</span> <span class="special">}</span> </pre> </div> <div class="section"> <div class="titlepage"><div><div><h4 class="title"> <a name="tuple_advanced_interface.cons_lists.constructing_cons_lists"></a><a class="link" href="tuple_advanced_interface.html#tuple_advanced_interface.cons_lists.constructing_cons_lists" title="Constructing cons lists">Constructing cons lists</a> </h4></div></div></div> <p> A cons list can be default constructed provided that all its elements can be default constructed. </p> <p> A cons list can be constructed from its head and tail. The prototype of the constructor is: </p> <pre class="programlisting"><span class="identifier">cons</span><span class="special">(</span><span class="keyword">typename</span> <span class="identifier">access_traits</span><span class="special"><</span><span class="identifier">head_type</span><span class="special">>::</span><span class="identifier">parameter_type</span> <span class="identifier">h</span><span class="special">,</span> <span class="keyword">const</span> <span class="identifier">tail_type</span><span class="special">&</span> <span class="identifier">t</span><span class="special">)</span> </pre> <p> The traits template for the head parameter selects correct parameter types for different kinds of element types (for reference elements the parameter type equals the element type, for non-reference types the parameter type is a reference to const non-volatile element type). </p> <p> For a one-element cons list the tail argument (<code class="computeroutput"><span class="identifier">null_type</span></code>) can be omitted. </p> </div> </div> <div class="section"> <div class="titlepage"><div><div><h3 class="title"> <a name="tuple_advanced_interface.traits_classes_for_tuple_element"></a><a class="link" href="tuple_advanced_interface.html#tuple_advanced_interface.traits_classes_for_tuple_element" title="Traits classes for tuple element types">Traits classes for tuple element types</a> </h3></div></div></div> <div class="section"> <div class="titlepage"><div><div><h4 class="title"> <a name="tuple_advanced_interface.traits_classes_for_tuple_element.access_traits"></a><a class="link" href="tuple_advanced_interface.html#tuple_advanced_interface.traits_classes_for_tuple_element.access_traits" title="access_traits">access_traits</a> </h4></div></div></div> <p> The template <code class="computeroutput"><span class="identifier">access_traits</span></code> defines three type functions. Let <code class="computeroutput"><span class="identifier">T</span></code> be a type of an element in a tuple: </p> <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "> <li class="listitem"> <code class="computeroutput"><span class="identifier">access_traits</span><span class="special"><</span><span class="identifier">T</span><span class="special">>::</span><span class="identifier">non_const_type</span></code> maps <code class="computeroutput"><span class="identifier">T</span></code> to the return type of the no n-const access functions (nonmember and member <code class="computeroutput"><span class="identifier">get</span></code> functions, and the <code class="computeroutput"><span class="identifier">get_head</span></code> function). </li> <li class="listitem"> <code class="computeroutput"><span class="identifier">access_traits</span><span class="special"><</span><span class="identifier">T</span><span class="special">>::</span><span class="identifier">const_type</span></code> maps <code class="computeroutput"><span class="identifier">T</span></code> to the return type of the const access functions. </li> <li class="listitem"> <code class="computeroutput"><span class="identifier">access_traits</span><span class="special"><</span><span class="identifier">T</span><span class="special">>::</span><span class="identifier">parameter_type</span></code> maps <code class="computeroutput"><span class="identifier">T</span></code> to the parameter type of the tuple constructor. </li> </ul></div> </div> <div class="section"> <div class="titlepage"><div><div><h4 class="title"> <a name="tuple_advanced_interface.traits_classes_for_tuple_element.make_tuple_traits"></a><a class="link" href="tuple_advanced_interface.html#tuple_advanced_interface.traits_classes_for_tuple_element.make_tuple_traits" title="make_tuple_traits">make_tuple_traits</a> </h4></div></div></div> <p> The element types of the tuples that are created with the <code class="computeroutput"><span class="identifier">make_tuple</span></code> functions are computed with the type function <code class="computeroutput"><span class="identifier">make_tuple_traits</span></code>. The type function call <code class="computeroutput"><span class="identifier">make_tuple_traits</span><span class="special"><</span><span class="identifier">T</span><span class="special">>::</span><span class="identifier">type</span></code> implements the following type mapping: </p> <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "> <li class="listitem"> <span class="emphasis"><em>any reference type</em></span> -> <span class="emphasis"><em>compile time error</em></span> </li> <li class="listitem"> <span class="emphasis"><em>any array type</em></span> -> <span class="emphasis"><em>constant reference to the array type</em></span> </li> <li class="listitem"> <code class="computeroutput"><span class="identifier">reference_wrapper</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> -> <code class="computeroutput"><span class="identifier">T</span><span class="special">&</span></code> </li> <li class="listitem"> <code class="computeroutput"><span class="identifier">T</span></code> -> <code class="computeroutput"><span class="identifier">T</span></code> </li> </ul></div> <p> Objects of type <code class="computeroutput"><span class="identifier">reference_wrapper</span></code> are created with the <code class="computeroutput"><span class="identifier">ref</span></code> and <code class="computeroutput"><span class="identifier">cref</span></code> functions (see <a class="link" href="tuple_users_guide.html#tuple.constructing_tuples.make_tuple" title="The make_tuple function">The <code class="computeroutput"><span class="identifier">make_tuple</span></code> function</a>). </p> <p> Reference wrappers were originally part of the tuple library, but they are now a general utility of boost. The <code class="computeroutput"><span class="identifier">reference_wrapper</span></code> template and the <code class="computeroutput"><span class="identifier">ref</span></code> and <code class="computeroutput"><span class="identifier">cref</span></code> functions are defined in a separate file <a href="../../../../libs/core/doc/html/core/ref.html" target="_top"><code class="computeroutput"><span class="identifier">ref</span><span class="special">.</span><span class="identifier">hpp</span></code></a> in the main boost include directory; and directly in the <code class="computeroutput"><span class="identifier">boost</span></code> namespace. </p> </div> </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: August 11, 2020 at 14:59:08 GMT</small></p></td> <td align="right"><div class="copyright-footer">Copyright © 2001 Jaakko Järvi<p> Distributed under the <a href="http://boost.org/LICENSE_1_0.txt" target="_top">Boost Software License, Version 1.0</a>. </p> </div></td> </tr></table> <hr> <div class="spirit-nav"> <a accesskey="p" href="tuple_users_guide.html"><img src="../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="tuple_users_guide.html"><img src="../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="tuple_users_guide.html"><img src="../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="design_decisions_rationale.html"><img src="../../../../doc/src/images/next.png" alt="Next"></a> </div> </body> </html>