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<title>Tuple library advanced features</title>
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<h1>Tuple library advanced features</h1>
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The advanced features described in this document are all under namespace <code>::boost::tuples</code>
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<h2>Metafunctions for tuple types</h2>
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Suppose <code>T</code> is a tuple type, and <code>N</code> is a constant integral expression.</p>
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<pre><code>element<N, T>::type</code></pre>
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<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.
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Note that the constness of <code>T</code> does not affect reference type
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<pre><code>length<T>::value</code></pre>
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<p>gives the length of the tuple type <code>T</code>.
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Tuples are internally represented as <i>cons lists</i>.
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For example, the tuple </p>
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<pre><code>tuple<A, B, C, D></code></pre>
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<p>inherits from the type</p>
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<pre><code>cons<A, cons<B, cons<C, cons<D, null_type> > > >
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<p>The tuple template provides the typedef <code>inherited</code> to access the cons list representation. E.g.:
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<code>tuple<A>::inherited</code> is the type <code>cons<A, null_type></code>.
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The internal representation of the empty tuple <code>tuple<></code> is <code>null_type</code>.
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<h4>Head and tail</h4>
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Both tuple template and the cons templates provide the typedefs <code>head_type</code> and <code>tail_type</code>.
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The <code>head_type</code> typedef gives the type of the first element of the tuple (or the cons list).
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<code>tail_type</code> typedef gives the remaining cons list after removing the first element.
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The head element is stored in the member variable <code>head</code> and the tail list in the member variable <code>tail</code>.
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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.
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There are const and non-const versions of both functions.
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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>.
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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.
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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):
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<pre><code>inline void set_to_zero(const null_type&) {};
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template <class H, class T>
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inline void set_to_zero(cons<H, T>& x) { x.get_head() = 0; set_to_zero(x.get_tail()); }
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<h4>Constructing cons lists</h4>
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A cons list can be default constructed provided that all its elements can be default constructed.
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A cons list can be constructed from its head and tail. The prototype of the constructor is:</p>
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<pre><code>cons(typename access_traits<head_type>::parameter_type h,
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const tail_type& t)
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<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).
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For a one-element cons list the tail argument (<code>null_type</code>) can be omitted.
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<h2>Traits classes for tuple element types</h2>
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<h4><code>access_traits</code></h4>
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The template <code>access_traits</code> defines three type functions. Let <code>T</code> be a type of an element in a tuple:</p>
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<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>
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<li><code>access_traits<T>::const_type</code> maps <code>T</code> to the return type of the const access functions.</li>
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<li><code>access_traits<T>::parameter_type</code> maps <code>T</code> to the parameter type of the tuple constructor.</li>
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<h4><code>make_tuple_traits</code></h4>
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<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>.
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The type function call <code>make_tuple_traits<T>::type</code> implements the following type mapping:</p>
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<li><i>any reference type</i> -> <i>compile time error</i>
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<li><i>any array type</i> -> <i>constant reference to the array type</i>
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<li><code>reference_wrapper<T></code> -> <code>T&</code>
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<li><code>T</code> -> <code>T</code>
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<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>.)
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<p>Reference wrappers were originally part of the tuple library, but they are now a general utility of boost.
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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.
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<p>© Copyright Jaakko Järvi 2001.</p>