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     <h1>Tuple library advanced features</h1>

 The advanced features described in this document are all under namespace <code>::boost::tuples</code>

 <h2>Metafunctions for tuple types</h2>
 <p>
 Suppose <code>T</code> is a tuple type, and <code>N</code> is a constant integral expression.</p>

 <pre><code>element&lt;N, T&gt;::type</code></pre>

 <p>gives the type of the <code>N</code>th element in the tuple type <code>T</code>. If <code>T</code> is const, the resulting type is const qualified as well.
 Note that the constness of <code>T</code> does not affect reference type
 elements.
 </p>

 <pre><code>length&lt;T&gt;::value</code></pre>

 <p>gives the length of the tuple type <code>T</code>.
 </p>

 <h2>Cons lists</h2>

 <p>
 Tuples are internally represented as <i>cons lists</i>.
 For example, the tuple </p>

 <pre><code>tuple&lt;A, B, C, D&gt;</code></pre>

 <p>inherits from the type</p>
 <pre><code>cons&lt;A, cons&lt;B, cons&lt;C, cons&lt;D, null_type&gt; &gt; &gt; &gt;
 </code></pre>

 <p>The tuple template provides the typedef <code>inherited</code> to access the cons list representation. E.g.:
 <code>tuple&lt;A&gt;::inherited</code> is the type <code>cons&lt;A, null_type&gt;</code>.
 </p>

 <h4>Empty tuple</h4>
 <p>
 The internal representation of the empty tuple <code>tuple&lt;&gt;</code> is <code>null_type</code>.
 </p>

 <h4>Head and tail</h4>
 <p>
 Both tuple template and the cons templates provide the typedefs <code>head_type</code> and <code>tail_type</code>.
 The <code>head_type</code> typedef gives the type of the first element of the tuple (or the cons list).
 The
 <code>tail_type</code> typedef gives the remaining cons list after removing the first element.
 The head element is stored in the member variable <code>head</code> and the tail list in the member variable <code>tail</code>.
 Cons lists provide the member function <code>get_head()</code> for getting a reference to the head of a cons list, and <code>get_tail()</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>tail_type</code> equals <code>null_type</code> and the <code>get_tail()</code> function returns an object of type <code>null_type</code>.
 </p>
 <p>
 The empty tuple (<code>null_type</code>) has no head or tail, hence the <code>get_head</code> and <code>get_tail</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 0 to each element of a tuple (obviously, the assignments must be valid operations for the element types):

 <pre><code>inline void set_to_zero(const null_type&amp;) {};

 template &lt;class H, class T&gt;
 inline void set_to_zero(cons&lt;H, T&gt;&amp; x) { x.get_head() = 0; set_to_zero(x.get_tail()); }
 </code></pre>
 <p>

 <h4>Constructing cons lists</h4>

 <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><code>cons(typename access_traits&lt;head_type&gt;::parameter_type h,
      const tail_type&amp; t)
 </code></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>null_type</code>) can be omitted.
 </p>


 <h2>Traits classes for tuple element types</h2>

 <h4><code>access_traits</code></h4>
 <p>
 The template <code>access_traits</code> defines three type functions. Let <code>T</code> be a type of an element in a tuple:</p>
 <ol>
 <li><code>access_traits&lt;T&gt;::non_const_type</code> maps <code>T</code> to the return type of the non-const access functions (nonmember and member <code>get</code> functions, and the <code>get_head</code> function).</li>
 <li><code>access_traits&lt;T&gt;::const_type</code> maps <code>T</code> to the return type of the const access functions.</li>
 <li><code>access_traits&lt;T&gt;::parameter_type</code> maps <code>T</code> to the parameter type of the tuple constructor.</li>
 </ol>
 <h4><code>make_tuple_traits</code></h4>

 <p>The element types of the tuples that are created with the <code>make_tuple</code> functions are computed with the type function <code>make_tuple_traits</code>.
 The type function call <code>make_tuple_traits&lt;T&gt;::type</code> implements the following type mapping:</p>
 <ul>
 <li><i>any reference type</i> -&gt; <i>compile time error</i>
 </li>
 <li><i>any array type</i> -&gt; <i>constant reference to the array type</i>
 </li>
 <li><code>reference_wrapper&lt;T&gt;</code> -&gt; <code>T&amp;</code>
 </li>
 <li><code>T</code> -&gt; <code>T</code>
 </li>
 </ul>

 <p>Objects of type <code>reference_wrapper</code> are created with the <code>ref</code> and <code>cref</code> functions (see <A href="tuple_users_guide.html#make_tuple">The <code>make_tuple</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>reference_wrapper</code> template and the <code>ref</code> and <code>cref</code> functions are defined in a separate file <code>ref.hpp</code> in the main boost include directory; and directly in the <code>boost</code> namespace.
 </p>

 <A href="tuple_users_guide.html">Back to the user's guide</A>
 <hr>

 <p>&copy; Copyright Jaakko J&auml;rvi 2001.</p>
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 </html>
	<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
	<html>
	<head>
	<title>Tuple library advanced features</title>
	</head>

	<body bgcolor="#FFFFFF" text="#000000">

	<IMG SRC="../../../boost.png"
	ALT="C++ Boost" width="277" height="86">

	<h1>Tuple library advanced features</h1>

	The advanced features described in this document are all under namespace <code>::boost::tuples</code>

	<h2>Metafunctions for tuple types</h2>
	<p>
	Suppose <code>T</code> is a tuple type, and <code>N</code> is a constant integral expression.</p>

	<pre><code>element<N, T>::type</code></pre>

	<p>gives the type of the <code>N</code>th element in the tuple type <code>T</code>. If <code>T</code> is const, the resulting type is const qualified as well.
	Note that the constness of <code>T</code> does not affect reference type
	elements.
	</p>

	<pre><code>length<T>::value</code></pre>

	<p>gives the length of the tuple type <code>T</code>.
	</p>

	<h2>Cons lists</h2>

	<p>
	Tuples are internally represented as <i>cons lists</i>.
	For example, the tuple </p>

	<pre><code>tuple<A, B, C, D></code></pre>

	<p>inherits from the type</p>
	<pre><code>cons<A, cons<B, cons<C, cons<D, null_type> > > >
	</code></pre>

	<p>The tuple template provides the typedef <code>inherited</code> to access the cons list representation. E.g.:
	<code>tuple<A>::inherited</code> is the type <code>cons<A, null_type></code>.
	</p>

	<h4>Empty tuple</h4>
	<p>
	The internal representation of the empty tuple <code>tuple<></code> is <code>null_type</code>.
	</p>

	<h4>Head and tail</h4>
	<p>
	Both tuple template and the cons templates provide the typedefs <code>head_type</code> and <code>tail_type</code>.
	The <code>head_type</code> typedef gives the type of the first element of the tuple (or the cons list).
	The
	<code>tail_type</code> typedef gives the remaining cons list after removing the first element.
	The head element is stored in the member variable <code>head</code> and the tail list in the member variable <code>tail</code>.
	Cons lists provide the member function <code>get_head()</code> for getting a reference to the head of a cons list, and <code>get_tail()</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>tail_type</code> equals <code>null_type</code> and the <code>get_tail()</code> function returns an object of type <code>null_type</code>.
	</p>
	<p>
	The empty tuple (<code>null_type</code>) has no head or tail, hence the <code>get_head</code> and <code>get_tail</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 0 to each element of a tuple (obviously, the assignments must be valid operations for the element types):

	<pre><code>inline void set_to_zero(const null_type&) {};

	template <class H, class T>
	inline void set_to_zero(cons<H, T>& x) { x.get_head() = 0; set_to_zero(x.get_tail()); }
	</code></pre>
	<p>

	<h4>Constructing cons lists</h4>

	<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><code>cons(typename access_traits<head_type>::parameter_type h,
	const tail_type& t)
	</code></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>null_type</code>) can be omitted.
	</p>


	<h2>Traits classes for tuple element types</h2>

	<h4><code>access_traits</code></h4>
	<p>
	The template <code>access_traits</code> defines three type functions. Let <code>T</code> be a type of an element in a tuple:</p>
	<ol>
	<li><code>access_traits<T>::non_const_type</code> maps <code>T</code> to the return type of the non-const access functions (nonmember and member <code>get</code> functions, and the <code>get_head</code> function).</li>
	<li><code>access_traits<T>::const_type</code> maps <code>T</code> to the return type of the const access functions.</li>
	<li><code>access_traits<T>::parameter_type</code> maps <code>T</code> to the parameter type of the tuple constructor.</li>
	</ol>
	<h4><code>make_tuple_traits</code></h4>

	<p>The element types of the tuples that are created with the <code>make_tuple</code> functions are computed with the type function <code>make_tuple_traits</code>.
	The type function call <code>make_tuple_traits<T>::type</code> implements the following type mapping:</p>
	<ul>
	<li><i>any reference type</i> -> <i>compile time error</i>
	</li>
	<li><i>any array type</i> -> <i>constant reference to the array type</i>
	</li>
	<li><code>reference_wrapper<T></code> -> <code>T&</code>
	</li>
	<li><code>T</code> -> <code>T</code>
	</li>
	</ul>

	<p>Objects of type <code>reference_wrapper</code> are created with the <code>ref</code> and <code>cref</code> functions (see <A href="tuple_users_guide.html#make_tuple">The <code>make_tuple</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>reference_wrapper</code> template and the <code>ref</code> and <code>cref</code> functions are defined in a separate file <code>ref.hpp</code> in the main boost include directory; and directly in the <code>boost</code> namespace.
	</p>

	<A href="tuple_users_guide.html">Back to the user's guide</A>
	<hr>

	<p>© Copyright Jaakko Järvi 2001.</p>
	</body>
	</html>