source: downloads/boost_1_34_1/libs/any/doc/any.xml @ 69

<?xml version="1.0" encoding="utf-8"?>
<!DOCTYPE library PUBLIC "-//Boost//DTD BoostBook XML V1.0//EN"
  "http://www.boost.org/tools/boostbook/dtd/boostbook.dtd">
<library name="Any" dirname="any" xmlns:xi="http://www.w3.org/2001/XInclude" 
         id="any" last-revision="$Date: 2005/11/04 09:34:36 $">
  <libraryinfo>
    <author>
      <firstname>Kevlin</firstname>
      <surname>Henney</surname>
    </author>

    <copyright>
      <year>2001</year>
      <holder>Kevlin Henney</holder>
    </copyright>

    <librarypurpose>
      Safe, generic container for single values of different value types
    </librarypurpose> 
    <librarycategory name="category:data-structures"/>
  </libraryinfo>

  <title>Boost.Any</title>

  <section>
    <title>Introduction</title>

    <para>There are times when a generic (in the sense of
    <emphasis>general</emphasis> as opposed to
    <emphasis>template-based programming</emphasis>) type is needed:
    variables that are truly variable, accommodating values of many
    other more specific types rather than C++'s normal strict and
    static types. We can distinguish three basic kinds of generic
    type:</para>

    <itemizedlist>
      <listitem>
        <para>Converting types that can hold one of a number of
        possible value types, e.g. <code>int</code> and
        <code>string</code>, and freely convert between them, for
        instance interpreting <code>5</code> as <code>"5"</code> or
        vice-versa.  Such types are common in scripting and other
        interpreted
        languages. 
        <code><functionname>boost::lexical_cast</functionname></code>
        supports such conversion functionality.</para>
      </listitem>
      <listitem>
        <para>
        Discriminated types that contain values of different types but
        do not attempt conversion between them, i.e. <code>5</code> is
        held strictly as an <code>int</code> and is not implicitly
        convertible either to <code>"5"</code> or to
        <code>5.0</code>. Their indifference to interpretation but
        awareness of type effectively makes them safe, generic
        containers of single values, with no scope for surprises from
        ambiguous conversions.</para>
      </listitem>
      <listitem>
        <para>
        Indiscriminate types that can refer to anything but are
        oblivious to the actual underlying type, entrusting all forms
        of access and interpretation to the programmer. This niche is
        dominated by <code>void *</code>, which offers plenty of scope
        for surprising, undefined behavior.</para>
      </listitem>
    </itemizedlist>

    <para>The <code><classname>boost::any</classname></code> class
    (based on the class of the same name described in <ulink
    url="http://www.two-sdg.demon.co.uk/curbralan/papers/ValuedConversions.pdf">"Valued
    Conversions"</ulink> by Kevlin Henney, <emphasis>C++
    Report</emphasis> 12(7), July/August 2000) is a variant value type
    based on the second category. It supports copying of any value
    type and safe checked extraction of that value strictly against
    its type. A similar design, offering more appropriate operators,
    can be used for a generalized function adaptor,
    <code>any_function</code>, a generalized iterator adaptor,
    <code>any_iterator</code>, and other object types that need
    uniform runtime treatment but support only compile-time template
    parameter conformance.</para>
  </section>

  <section>
    <title>Examples</title>

    <using-namespace name="boost"/>
    <using-class name="boost::any"/>

    <para>The following code demonstrates the syntax for using
    implicit conversions to and copying of any objects:</para>

<programlisting name="any.example.first">
#include &lt;list&gt;
#include &lt;boost/any.hpp&gt;

using <functionname>boost::any_cast</functionname>;
typedef std::list&lt;<classname>boost::any</classname>&gt; many;

void append_int(many &amp; values, int value)
{
    <classname>boost::any</classname> to_append = value;
    values.push_back(to_append);
}

void append_string(many &amp; values, const std::string &amp; value)
{
    values.push_back(value);
}

void append_char_ptr(many &amp; values, const char * value)
{
    values.push_back(value);
}

void append_any(many &amp; values, const <classname>boost::any</classname> &amp; value)
{
    values.push_back(value);
}

void append_nothing(many &amp; values)
{
    values.push_back(<classname>boost::any</classname>());
}
</programlisting>

    <para>The following predicates follow on from the previous
    definitions and demonstrate the use of queries on any
    objects:</para>

<programlisting name="any.example.second">
bool is_empty(const <classname>boost::any</classname> &amp; operand)
{
    return operand.<methodname>empty</methodname>();
}

bool is_int(const <classname>boost::any</classname> &amp; operand)
{
    return operand.<methodname>type</methodname>() == typeid(int);
}

bool is_char_ptr(const <classname>boost::any</classname> &amp; operand)
{
    try
    {
        <functionname>any_cast</functionname>&lt;const char *&gt;(operand);
        return true;
    }
    catch(const <classname>boost::bad_any_cast</classname> &amp;)
    {
        return false;
    }
}

bool is_string(const <classname>boost::any</classname> &amp; operand)
{
    return <functionname>any_cast</functionname>&lt;std::string&gt;(&amp;operand);
}

void count_all(many &amp; values, std::ostream &amp; out)
{
    out &lt;&lt; "#empty == "
        &lt;&lt; std::count_if(values.begin(), values.end(), is_empty) &lt;&lt; std::endl;
    out &lt;&lt; "#int == "
        &lt;&lt; std::count_if(values.begin(), values.end(), is_int) &lt;&lt; std::endl;
    out &lt;&lt; "#const char * == "
        &lt;&lt; std::count_if(values.begin(), values.end(), is_char_ptr) &lt;&lt; std::endl;
    out &lt;&lt; "#string == "
        &lt;&lt; std::count_if(values.begin(), values.end(), is_string) &lt;&lt; std::endl;
}
</programlisting>

    <para>The following type, patterned after the OMG's Property Service, defines name-value pairs for arbitrary value types:</para>

<programlisting>
struct property
{
    property();
    property(const std::string &amp;, const <classname>boost::any</classname> &amp;);

    std::string name;
    <classname>boost::any</classname> value;
};

typedef std::list&lt;property&gt; properties;
</programlisting>

    <para>The following base class demonstrates one approach to
    runtime polymorphism based callbacks that also require arbitrary
    argument types. The absence of virtual member templates requires
    that different solutions have different trade-offs in terms of
    efficiency, safety, and generality. Using a checked variant type
    offers one approach:</para>

<programlisting>
class consumer
{
public:
    virtual void notify(const <classname>any</classname> &amp;) = 0;
    ...
};
</programlisting>
  </section>

  <library-reference>
    <section id="any.ValueType">
      <title><emphasis>ValueType</emphasis> requirements</title>

      <para>Values are strongly informational objects for which
      identity is not significant, i.e. the focus is principally on
      their state content and any behavior organized around
      that. Another distinguishing feature of values is their
      granularity: normally fine-grained objects representing simple
      concepts in the system such as quantities.</para>

      <para>As the emphasis of a value lies in its state not its
      identity, values can be copied and typically assigned one to
      another, requiring the explicit or implicit definition of a
      public copy constructor and public assignment operator. Values
      typically live within other scopes, i.e. within objects or
      blocks, rather than on the heap. Values are therefore normally
      passed around and manipulated directly as variables or through
      references, but not as pointers that emphasize identity and
      indirection.</para>

      <para>The specific requirements on value types to be used in an
      <code><classname alt="boost::any">any</classname></code>
      are:</para>

      <itemizedlist spacing="compact">
        <listitem><simpara>A <emphasis>ValueType</emphasis> is
          <emphasis>CopyConstructible</emphasis> [20.1.3].</simpara>
        </listitem>
        
        <listitem><simpara>A <emphasis>ValueType</emphasis> is
        optionally <emphasis>Assignable</emphasis> [23.1]. The strong
        exception-safety guarantee is required for all forms of
        assignment.</simpara>
        </listitem>
        
        <listitem><simpara>The destructor for a
        <emphasis>ValueType</emphasis> upholds the no-throw
        exception-safety guarantee.</simpara>
        </listitem>
      </itemizedlist>
    </section>

    <header name="boost/any.hpp">
      <namespace name="boost">
        <class name="bad_any_cast">
          <inherit access="public">
            <classname>std::bad_cast</classname>
          </inherit>
          <purpose>The exception thrown in the event of a failed
          <code><functionname>any_cast</functionname></code> of an
          <code><classname>any</classname></code> value.</purpose>

          <method name="what" specifiers="virtual" cv="const">
            <type>const char *</type>
          </method>
        </class>

        <class name="any">
          <purpose>A class whose instances can hold instances of any
          type that satisfies <link
          linkend="any.ValueType">ValueType</link>
          requirements.</purpose>

          <constructor>
            <postconditions><simpara><code>this-&gt;<methodname>empty</methodname>()</code></simpara></postconditions>
          </constructor>

          <constructor>
            <parameter name="other">
              <paramtype>const <classname>any</classname> &amp;</paramtype>
            </parameter>

            <effects><simpara> Copy constructor that copies content of
            <code>other</code> into new instance, so that any content
            is equivalent in both type and value to the content of
            <code>other</code>, or empty if <code>other</code> is
            empty. </simpara></effects>

            <throws><simpara>May fail with a
            <code><classname>std::bad_alloc</classname></code>
            exception or any exceptions arising from the copy
            constructor of the contained type.</simpara></throws>
          </constructor>

          <constructor>
            <template>
              <template-type-parameter name="ValueType"/>
            </template>

            <parameter name="value">
              <paramtype>const ValueType &amp;</paramtype>
            </parameter>

            <effects><simpara>Makes a copy of <code>value</code>, so
            that the initial content of the new instance is equivalent
            in both type and value to
            <code>value</code>.</simpara></effects>

            <throws><simpara><code><classname>std::bad_alloc</classname></code>
            or any exceptions arising from the copy constructor of the
            contained type.</simpara></throws>
          </constructor>

          <destructor>
            <effects><simpara>Releases any and all resources used in
            management of instance.</simpara></effects>
            <throws><simpara>Nothing.</simpara></throws>
          </destructor>

          <copy-assignment>
            <type><classname>any</classname> &amp;</type>

            <parameter name="rhs">
              <paramtype>const <classname>any</classname> &amp;</paramtype>
            </parameter>

            <effects><simpara>Copies content of <code>rhs</code> into
            current instance, discarding previous content, so that the
            new content is equivalent in both type and value to the
            content of <code>rhs</code>, or empty if
            <code>rhs.<methodname>empty</methodname>()</code>.</simpara></effects>

            <throws><simpara><code><classname>std::bad_alloc</classname></code>
            or any exceptions arising from the copy constructor of the
            contained type. Assignment satisfies the strong guarantee
            of exception safety.</simpara></throws>
          </copy-assignment>

          <copy-assignment>
             <template>
              <template-type-parameter name="ValueType"/>
            </template>

            <type><classname>any</classname> &amp;</type>

            <parameter name="rhs">
              <paramtype>const ValueType &amp;</paramtype>
            </parameter>

            <effects><simpara>Makes a copy of <code>rhs</code>,
            discarding previous content, so that the new content of is
            equivalent in both type and value to
            <code>rhs</code>.</simpara></effects>

            <throws><simpara><code><classname>std::bad_alloc</classname></code>
            or any exceptions arising from the copy constructor of the
            contained type. Assignment satisfies the strong guarantee
            of exception safety.</simpara></throws>
          </copy-assignment>

          <method-group name="modifiers">
            <method name="swap">
              <type><classname>any</classname> &amp;</type>

              <parameter name="rhs">
                <paramtype><classname>any</classname> &amp;</paramtype>
              </parameter>

              <effects><simpara>Exchange of the contents of
              <code>*this</code> and
              <code>rhs</code>.</simpara></effects>

              <returns><simpara><code>*this</code></simpara></returns>

              <throws><simpara>Nothing.</simpara></throws>
            </method>
          </method-group>

          <method-group name="queries">
            <method name="empty" cv="const">
              <type>bool</type>

              <returns><simpara><code>true</code> if instance is
              empty, otherwise <code>false</code>.</simpara></returns>
              
              <throws><simpara>Will not throw.</simpara></throws>
            </method>

            <method name="type" cv="const">
              <type>const <classname>std::type_info</classname> &amp;</type>
              
              <returns><simpara>the <code>typeid</code> of the
              contained value if instance is non-empty, otherwise
              <code>typeid(void)</code>.</simpara></returns>

              <notes><simpara>Useful for querying against types known
              either at compile time or only at
              runtime.</simpara></notes>
            </method>
          </method-group>
        </class>

        <overloaded-function name="any_cast">
          <signature>
            <template>
              <template-type-parameter name="T"/>
            </template>
            
            <type>T</type>
            
            <parameter name="operand">
              <paramtype><classname>any</classname> &amp;</paramtype>
            </parameter>
          </signature>
          
          <signature>
            <template>
              <template-type-parameter name="T"/>
            </template>
            
            <type>T</type>
            
            <parameter name="operand">
              <paramtype>const <classname>any</classname> &amp;</paramtype>
            </parameter>
          </signature>
          
          <signature>
            <template>
              <template-type-parameter name="ValueType"/>
            </template>
            
            <type>const ValueType *</type>
            
            <parameter name="operand">
              <paramtype>const <classname>any</classname> *</paramtype>
            </parameter>
          </signature>
          
          <signature>
            <template>
              <template-type-parameter name="ValueType"/>
            </template>
            
            <type>ValueType *</type>
            
            <parameter name="operand">
              <paramtype><classname>any</classname> *</paramtype>
            </parameter>
          </signature>
          
          <purpose><simpara>Custom keyword cast for extracting a value
          of a given type from an
          <code><classname>any</classname></code>.</simpara></purpose>

          <returns><simpara> If passed a pointer, it returns a
          similarly qualified pointer to the value content if
          successful, otherwise null is returned.
          If T is ValueType, it returns a copy of the held value, otherwise, if T is a reference 
          to (possibly const qualified) ValueType, it returns a reference to the held 
          value.</simpara></returns>

          <throws><simpara>Overloads taking an
          <code><classname>any</classname></code> pointer do not
          throw; overloads taking an
          <code><classname>any</classname></code> value or reference
          throws <code><classname>bad_any_cast</classname></code> if
          unsuccessful.</simpara></throws>

        </overloaded-function>
      </namespace>
    </header>
  </library-reference>

  <section>
    <title>Acknowledgements</title>

    <para>Doug Gregor ported the documentation to the BoostBook format.</para>
  </section>
</library>