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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> |
| <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> |
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| <pre><code>length<T>::value</code></pre> |
| |
| <p>gives the length of the tuple type <code>T</code>. |
| </p> |
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| <h2>Cons lists</h2> |
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| <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> |
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| <h4>Empty tuple</h4> |
| <p> |
| The internal representation of the empty tuple <code>tuple<></code> is <code>null_type</code>. |
| </p> |
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| <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> |
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| <h4>Constructing cons lists</h4> |
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| <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> |
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| <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> |
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| <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> |
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| <A href="tuple_users_guide.html">Back to the user's guide</A> |
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| <p>© Copyright Jaakko Järvi 2001.</p> |
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