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<div><h2 class="title">
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<a name="tuple_advanced_interface"></a>Tuple library advanced features</h2></div>
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<div><p class="copyright">Copyright © 2001 Jaakko Järvi</p></div>
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<div><div class="legalnotice">
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<a name="tuple_advanced_interface.legal"></a><p>
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Distributed under the <a href="http://boost.org/LICENSE_1_0.txt" target="_top">Boost
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Software License, Version 1.0</a>.
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</p>
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<div class="toc">
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<p><b>Table of Contents</b></p>
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<dl class="toc">
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<dt><span class="section"><a href="tuple_advanced_interface.html#tuple_advanced_interface.metafunctions_for_tuple_types">Metafunctions
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for tuple types</a></span></dt>
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<dt><span class="section"><a href="tuple_advanced_interface.html#tuple_advanced_interface.cons_lists">Cons lists</a></span></dt>
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<dt><span class="section"><a href="tuple_advanced_interface.html#tuple_advanced_interface.traits_classes_for_tuple_element">Traits
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classes for tuple element types</a></span></dt>
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</dl>
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</div>
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<p>
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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>
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</p>
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<div class="section">
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<div class="titlepage"><div><div><h3 class="title">
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<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
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for tuple types</a>
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</h3></div></div></div>
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<p>
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Suppose <code class="computeroutput"><span class="identifier">T</span></code> is a tuple type,
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and <code class="computeroutput"><span class="identifier">N</span></code> is a constant integral
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expression.
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</p>
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<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>
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</pre>
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<p>
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gives the type of the <code class="computeroutput"><span class="identifier">N</span></code>-th
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element in the tuple type <code class="computeroutput"><span class="identifier">T</span></code>.
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If <code class="computeroutput"><span class="identifier">T</span></code> is <code class="computeroutput"><span class="keyword">const</span></code>,
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the resulting type is <code class="computeroutput"><span class="keyword">const</span></code>
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qualified as well. Note that the constness of <code class="computeroutput"><span class="identifier">T</span></code>
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does not affect reference type elements.
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</p>
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<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>
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</pre>
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<p>
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gives the length of the tuple type <code class="computeroutput"><span class="identifier">T</span></code>.
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</p>
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</div>
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<div class="section">
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<div class="titlepage"><div><div><h3 class="title">
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<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>
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</h3></div></div></div>
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<p>
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Tuples are internally represented as <span class="emphasis"><em>cons lists</em></span>. For
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example, the tuple
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</p>
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<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>
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</pre>
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<p>
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inherits from the type
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</p>
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<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>
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</pre>
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<p>
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The tuple template provides the typedef inherited to access the cons list
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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>
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<span class="identifier">null_type</span><span class="special">></span></code>.
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</p>
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<div class="section">
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<div class="titlepage"><div><div><h4 class="title">
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<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
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tuple</a>
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</h4></div></div></div>
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<p>
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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>.
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</p>
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</div>
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<div class="section">
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<div class="titlepage"><div><div><h4 class="title">
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<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
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and tail</a>
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</h4></div></div></div>
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<p>
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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>.
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The <code class="computeroutput"><span class="identifier">head_type</span></code> typedef gives
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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
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cons list after removing the first element. The head element is stored
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in the member variable <code class="computeroutput"><span class="identifier">head</span></code>
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and the tail list in the member variable <code class="computeroutput"><span class="identifier">tail</span></code>.
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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
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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.
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There are const and non-const versions of both functions.
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</p>
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<p>
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Note that in a one element tuple, <code class="computeroutput"><span class="identifier">tail_type</span></code>
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equals <code class="computeroutput"><span class="identifier">null_type</span></code> and the
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<code class="computeroutput"><span class="identifier">get_tail</span><span class="special">()</span></code>
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function returns an object of type <code class="computeroutput"><span class="identifier">null_type</span></code>.
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</p>
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<p>
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The empty tuple (<code class="computeroutput"><span class="identifier">null_type</span></code>)
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has no head or tail, hence the <code class="computeroutput"><span class="identifier">get_head</span></code>
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and <code class="computeroutput"><span class="identifier">get_tail</span></code> functions
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are not provided.
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</p>
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<p>
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Treating tuples as cons lists gives a convenient means to define generic
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functions to manipulate tuples. For example, the following pair of function
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templates assign <code class="computeroutput"><span class="number">0</span></code> to each
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element of a tuple (obviously, the assignments must be valid operations
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for the element types):
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</p>
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<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>
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<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>
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<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>
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</pre>
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</div>
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<div class="section">
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<div class="titlepage"><div><div><h4 class="title">
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<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
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cons lists</a>
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</h4></div></div></div>
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<p>
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A cons list can be default constructed provided that all its elements can
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be default constructed.
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</p>
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<p>
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A cons list can be constructed from its head and tail. The prototype of
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the constructor is:
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</p>
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<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>
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</pre>
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<p>
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The traits template for the head parameter selects correct parameter types
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for different kinds of element types (for reference elements the parameter
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type equals the element type, for non-reference types the parameter type
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is a reference to const non-volatile element type).
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</p>
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<p>
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For a one-element cons list the tail argument (<code class="computeroutput"><span class="identifier">null_type</span></code>)
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can be omitted.
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</p>
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</div>
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</div>
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<div class="section">
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<div class="titlepage"><div><div><h3 class="title">
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<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
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classes for tuple element types</a>
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</h3></div></div></div>
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<div class="section">
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<div class="titlepage"><div><div><h4 class="title">
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<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>
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</h4></div></div></div>
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<p>
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The template <code class="computeroutput"><span class="identifier">access_traits</span></code>
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defines three type functions. Let <code class="computeroutput"><span class="identifier">T</span></code>
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be a type of an element in a tuple:
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</p>
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<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
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<li class="listitem">
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<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
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access functions (nonmember and member <code class="computeroutput"><span class="identifier">get</span></code>
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functions, and the <code class="computeroutput"><span class="identifier">get_head</span></code>
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function).
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</li>
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<li class="listitem">
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<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>
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to the return type of the const access functions.
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</li>
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<li class="listitem">
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<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
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constructor.
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</li>
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</ul></div>
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</div>
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<div class="section">
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<div class="titlepage"><div><div><h4 class="title">
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<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>
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</h4></div></div></div>
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<p>
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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
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the type function <code class="computeroutput"><span class="identifier">make_tuple_traits</span></code>.
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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>
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implements the following type mapping:
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</p>
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<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
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<li class="listitem">
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<span class="emphasis"><em>any reference type</em></span> -> <span class="emphasis"><em>compile time
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error</em></span>
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</li>
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<li class="listitem">
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<span class="emphasis"><em>any array type</em></span> -> <span class="emphasis"><em>constant reference
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to the array type</em></span>
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</li>
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<li class="listitem">
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<code class="computeroutput"><span class="identifier">reference_wrapper</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>
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-> <code class="computeroutput"><span class="identifier">T</span><span class="special">&</span></code>
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</li>
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<li class="listitem">
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<code class="computeroutput"><span class="identifier">T</span></code> -> <code class="computeroutput"><span class="identifier">T</span></code>
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</li>
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</ul></div>
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<p>
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Objects of type <code class="computeroutput"><span class="identifier">reference_wrapper</span></code>
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are created with the <code class="computeroutput"><span class="identifier">ref</span></code>
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and <code class="computeroutput"><span class="identifier">cref</span></code> functions (see
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<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>).
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</p>
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<p>
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Reference wrappers were originally part of the tuple library, but they
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are now a general utility of boost. The <code class="computeroutput"><span class="identifier">reference_wrapper</span></code>
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template and the <code class="computeroutput"><span class="identifier">ref</span></code> and
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<code class="computeroutput"><span class="identifier">cref</span></code> functions are defined
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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
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directory; and directly in the <code class="computeroutput"><span class="identifier">boost</span></code>
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namespace.
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</p>
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</div>
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</div>
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</div>
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<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
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<td align="left"><p><small>Last revised: December 14, 2017 at 00:09:39 GMT</small></p></td>
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<td align="right"><div class="copyright-footer">Copyright © 2001 Jaakko Järvi<p>
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Distributed under the <a href="http://boost.org/LICENSE_1_0.txt" target="_top">Boost
|
|
Software License, Version 1.0</a>.
|
|
</p>
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</div></td>
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</tr></table>
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<hr>
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<div class="spirit-nav">
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