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<h1><img src="../../../../boost.png" alt="boost.png (6897 bytes)" align=
"middle" width="277" height="86">Boost.MultiIndex Tutorial: Key extraction</h1>

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<h2>Contents</h2>

<ul>
  <li><a href="#intro">Introduction</a>
    <ul>
      <li><a href="#read_write_key_extractors">Read/write key extractors</a></li>
    </ul>
  </li>
  <li><a href="#predefined_key_extractors">Predefined key extractors</a>
    <ul>
      <li><a href="#identity"><code>identity</code></a></li>
      <li><a href="#member"><code>member</code></a></li>
      <li><a href="#const_mem_fun"><code>const_mem_fun</code> and <code>mem_fun</code></a>
        <ul>
          <li><a href="#x_mem_fun">Variants for other types of member functions</a></li>
        </ul>
      </li>
      <li><a href="#global_fun"><code>global_fun</code></a></li>
    </ul>
  </li>
  <li><a href="#user_defined_key_extractors">User-defined key extractors</a></li>
  <li><a href="#composite_keys">Composite keys</a>
    <ul>
      <li><a href="#composite_keys_hash">Composite keys and hashed indices</a></li>
    </ul>
  </li>
  <li><a href="#key">C++17 terse key specification syntax</a></li>
  <li><a href="#advanced_key_extractors">Advanced features of Boost.MultiIndex key
    extractors</a></li>
</ul>

<h2><a name="intro">Introduction</a></h2>

<p>
STL associative containers have a notion of key, albeit in a somewhat incipient
form. So, the keys of such containers are identified by a nested type
<code>key_type</code>; for <code>std::set</code>s and <code>std::multiset</code>s,
<code>key_type</code> coincides with <code>value_type</code>, i.e. the key is the
element itself. <code>std::map</code> and <code>std::multimap</code> manage
elements of type <code>std::pair&lt;const Key,T&gt;</code>, where the first
member is the key. In either case, the process of obtaining the key from a
given element is implicitly fixed and cannot be customized by the user.
</p>

<p>
Fixed key extraction mechanisms like those performed by STL associative
containers do not scale well in the context of Boost.MultiIndex, where
several indices share their <code>value_type</code> definition but
might feature completely different lookup semantics. For this reason,
Boost.MultiIndex formalizes the concept of a 
<a href="../reference/key_extraction.html#key_extractors"><code>Key
Extractor</code></a> in order to make it explicit and controllable
in the definition of key-based indices.
</p>

<p>
Intuitively speaking, a key extractor is a function object that accepts
a reference to an element and returns its associated key. The formal
concept also imposes some reasonable constraints about the stability
of the process, in the sense that extractors are assumed to
return the same key when passed the same element: this is in consonance
with the informal understanding that keys are actually some "part"
of the element and do not depend on external data.
</p>

<h3><a name="read_write_key_extractors">Read/write key extractors</a></h3>

<p>
A key extractor is called <i>read/write</i> if it returns a non-constant reference
to the key when passed a non-constant element, and it is called <i>read-only</i>
otherwise. Boost.MultiIndex requires that the key extractor be read/write
when using the <code>modify_key</code> member function of ordered and hashed
indices. In all other situations, read-only extractors suffice.
The section on <a href="#advanced_key_extractors">advanced features
of Boost.MultiIndex key extractors</a> details which of the predefined
key extractors are read/write.
</p>

<h2><a name="predefined_key_extractors">Predefined key extractors</a></h2>

<h3><a name="identity"><code>identity</code></a></h3>

<p>
The <a href="../reference/key_extraction.html#identity"><code>identity</code></a>
key extractor returns the entire base object as the associated key:
</p>

<blockquote><pre>
<span class=preprocessor>#include</span> <span class=special>&lt;</span><span class=identifier>boost</span><span class=special>/</span><span class=identifier>multi_index_container</span><span class=special>.</span><span class=identifier>hpp</span><span class=special>&gt;</span>
<span class=preprocessor>#include</span> <span class=special>&lt;</span><span class=identifier>boost</span><span class=special>/</span><span class=identifier>multi_index</span><span class=special>/</span><span class=identifier>ordered_index</span><span class=special>.</span><span class=identifier>hpp</span><span class=special>&gt;</span>
<span class=preprocessor>#include</span> <span class=special>&lt;</span><span class=identifier>boost</span><span class=special>/</span><span class=identifier>multi_index</span><span class=special>/</span><span class=identifier>identity</span><span class=special>.</span><span class=identifier>hpp</span><span class=special>&gt;</span>

<span class=identifier>multi_index_container</span><span class=special>&lt;</span>
  <span class=keyword>int</span><span class=special>,</span>
  <span class=identifier>indexed_by</span><span class=special>&lt;</span>
    <span class=identifier>ordered_unique</span><span class=special>&lt;</span>
      <span class=identifier>identity</span><span class=special>&lt;</span><span class=keyword>int</span><span class=special>&gt;</span> <span class=comment>// the key is the entire element</span>
    <span class=special>&gt;</span>
  <span class=special>&gt;</span>
<span class=special>&gt;</span> <span class=identifier>cont</span><span class=special>;</span>
</pre></blockquote>

<h3><a name="member"><code>member</code></a></h3>

<p>
<a href="../reference/key_extraction.html#member"><code>member</code></a>
key extractors return a reference to a specified
data field of the base object. For instance, in the following version of our
familiar employee container:
</p>

<blockquote><pre>
<span class=preprocessor>#include</span> <span class=special>&lt;</span><span class=identifier>boost</span><span class=special>/</span><span class=identifier>multi_index_container</span><span class=special>.</span><span class=identifier>hpp</span><span class=special>&gt;</span>
<span class=preprocessor>#include</span> <span class=special>&lt;</span><span class=identifier>boost</span><span class=special>/</span><span class=identifier>multi_index</span><span class=special>/</span><span class=identifier>ordered_index</span><span class=special>.</span><span class=identifier>hpp</span><span class=special>&gt;</span>
<span class=preprocessor>#include</span> <span class=special>&lt;</span><span class=identifier>boost</span><span class=special>/</span><span class=identifier>multi_index</span><span class=special>/</span><span class=identifier>identity</span><span class=special>.</span><span class=identifier>hpp</span><span class=special>&gt;</span>
<span class=preprocessor>#include</span> <span class=special>&lt;</span><span class=identifier>boost</span><span class=special>/</span><span class=identifier>multi_index</span><span class=special>/</span><span class=identifier>member</span><span class=special>.</span><span class=identifier>hpp</span><span class=special>&gt;</span>

<span class=keyword>typedef</span> <span class=identifier>multi_index_container</span><span class=special>&lt;</span>
  <span class=identifier>employee</span><span class=special>,</span>
  <span class=identifier>indexed_by</span><span class=special>&lt;</span>
    <span class=identifier>ordered_unique</span><span class=special>&lt;</span><span class=identifier>identity</span><span class=special>&lt;</span><span class=identifier>employee</span><span class=special>&gt;</span> <span class=special>&gt;,</span>
    <span class=identifier>ordered_non_unique</span><span class=special>&lt;</span><span class=identifier>member</span><span class=special>&lt;</span><span class=identifier>employee</span><span class=special>,</span><span class=identifier>std</span><span class=special>::</span><span class=identifier>string</span><span class=special>,&amp;</span><span class=identifier>employee</span><span class=special>::</span><span class=identifier>name</span><span class=special>&gt;</span> <span class=special>&gt;,</span>
    <span class=identifier>ordered_unique</span><span class=special>&lt;</span><span class=identifier>member</span><span class=special>&lt;</span><span class=identifier>employee</span><span class=special>,</span><span class=keyword>int</span><span class=special>,&amp;</span><span class=identifier>employee</span><span class=special>::</span><span class=identifier>ssnumber</span><span class=special>&gt;</span> <span class=special>&gt;</span>
  <span class=special>&gt;</span>
<span class=special>&gt;</span> <span class=identifier>employee_set</span><span class=special>;</span>
</pre></blockquote>

<p>
the second and third indices use <code>member</code> extractors on
<code>employee::name</code> and <code>employee::ssnumber</code>, respectively.
The specification of an instantiation of <code>member</code> is simple
yet a little contrived:
</p>

<blockquote><pre>
<span class=identifier>member</span><span class=special>&lt;</span><span class=identifier><i>(base type)</i></span><span class=special>,</span><span class=identifier><i>(key type)</i></span><span class=special>,</span><span class=identifier><i>(pointer to member)</i></span><span class=special>&gt;</span>
</pre></blockquote>

<p>
It might seem that the first and second parameters are superfluous,
since the type of the base object and of the associated data field are
already implicit in the pointer to member argument: unfortunately, it is
not possible to extract this information with current C++ mechanisms,
which makes the syntax of <code>member</code> a little too verbose.
</p>

<h3><a name="const_mem_fun"><code>const_mem_fun</code> and <code>mem_fun</code></a></h3>

<p>
Sometimes, the key of an index is not a concrete data member of the element,
but rather it is a value returned by a particular member function.
This resembles the notion of <i>calculated indices</i> supported by some
relational databases. Boost.MultiIndex supports this
kind of key extraction through
<a href="../reference/key_extraction.html#const_mem_fun"><code>const_mem_fun</code></a>.
Consider the following container where sorting on the third index
is based upon the length of the name field:
</p>

<blockquote><pre>
<span class=preprocessor>#include</span> <span class=special>&lt;</span><span class=identifier>boost</span><span class=special>/</span><span class=identifier>multi_index_container</span><span class=special>.</span><span class=identifier>hpp</span><span class=special>&gt;</span>
<span class=preprocessor>#include</span> <span class=special>&lt;</span><span class=identifier>boost</span><span class=special>/</span><span class=identifier>multi_index</span><span class=special>/</span><span class=identifier>ordered_index</span><span class=special>.</span><span class=identifier>hpp</span><span class=special>&gt;</span>
<span class=preprocessor>#include</span> <span class=special>&lt;</span><span class=identifier>boost</span><span class=special>/</span><span class=identifier>multi_index</span><span class=special>/</span><span class=identifier>identity</span><span class=special>.</span><span class=identifier>hpp</span><span class=special>&gt;</span>
<span class=preprocessor>#include</span> <span class=special>&lt;</span><span class=identifier>boost</span><span class=special>/</span><span class=identifier>multi_index</span><span class=special>/</span><span class=identifier>member</span><span class=special>.</span><span class=identifier>hpp</span><span class=special>&gt;</span>
<span class=preprocessor>#include</span> <span class=special>&lt;</span><span class=identifier>boost</span><span class=special>/</span><span class=identifier>multi_index</span><span class=special>/</span><span class=identifier>mem_fun</span><span class=special>.</span><span class=identifier>hpp</span><span class=special>&gt;</span>

<span class=keyword>struct</span> <span class=identifier>employee</span>
<span class=special>{</span>
  <span class=keyword>int</span>         <span class=identifier>id</span><span class=special>;</span>
  <span class=identifier>std</span><span class=special>::</span><span class=identifier>string</span> <span class=identifier>name</span><span class=special>;</span>

  <span class=identifier>employee</span><span class=special>(</span><span class=keyword>int</span> <span class=identifier>id</span><span class=special>,</span><span class=keyword>const</span> <span class=identifier>std</span><span class=special>::</span><span class=identifier>string</span><span class=special>&amp;</span> <span class=identifier>name</span><span class=special>):</span><span class=identifier>id</span><span class=special>(</span><span class=identifier>id</span><span class=special>),</span><span class=identifier>name</span><span class=special>(</span><span class=identifier>name</span><span class=special>){}</span>

  <span class=keyword>bool</span> <span class=keyword>operator</span><span class=special>&lt;(</span><span class=keyword>const</span> <span class=identifier>employee</span><span class=special>&amp;</span> <span class=identifier>e</span><span class=special>)</span><span class=keyword>const</span><span class=special>{</span><span class=keyword>return</span> <span class=identifier>id</span><span class=special>&lt;</span><span class=identifier>e</span><span class=special>.</span><span class=identifier>id</span><span class=special>;}</span>

  <span class=comment>// returns the length of the name field</span>
  <span class=identifier>std</span><span class=special>::</span><span class=identifier>size_t</span> <span class=identifier>name_length</span><span class=special>()</span><span class=keyword>const</span><span class=special>{</span><span class=keyword>return</span> <span class=identifier>name</span><span class=special>.</span><span class=identifier>size</span><span class=special>();}</span>
<span class=special>};</span>

<span class=keyword>typedef</span> <span class=identifier>multi_index_container</span><span class=special>&lt;</span>
  <span class=identifier>employee</span><span class=special>,</span>
  <span class=identifier>indexed_by</span><span class=special>&lt;</span>
    <span class=comment>// sort by employee::operator&lt;</span>
    <span class=identifier>ordered_unique</span><span class=special>&lt;</span><span class=identifier>identity</span><span class=special>&lt;</span><span class=identifier>employee</span><span class=special>&gt;</span> <span class=special>&gt;,</span>
    
    <span class=comment>// sort by less&lt;string&gt; on name</span>
    <span class=identifier>ordered_non_unique</span><span class=special>&lt;</span><span class=identifier>member</span><span class=special>&lt;</span><span class=identifier>employee</span><span class=special>,</span><span class=identifier>std</span><span class=special>::</span><span class=identifier>string</span><span class=special>,&amp;</span><span class=identifier>employee</span><span class=special>::</span><span class=identifier>name</span><span class=special>&gt;</span> <span class=special>&gt;,</span>
    
    <span class=comment>// sort by less&lt;int&gt; on name_length()</span>
    <span class=identifier>ordered_non_unique</span><span class=special>&lt;</span>
      <span class=identifier>const_mem_fun</span><span class=special>&lt;</span><span class=identifier>employee</span><span class=special>,</span><span class=identifier>std</span><span class=special>::</span><span class=identifier>size_t</span><span class=special>,&amp;</span><span class=identifier>employee</span><span class=special>::</span><span class=identifier>name_length</span><span class=special>&gt;</span>
    <span class=special>&gt;</span>
  <span class=special>&gt;</span>
<span class=special>&gt;</span> <span class=identifier>employee_set</span><span class=special>;</span>
</pre></blockquote>

<p>
<code>const_mem_fun</code> usage syntax is similar to that of
<a href="#member"><code>member</code></a>:
</p>

<blockquote><pre>
<span class=identifier>const_mem_fun</span><span class=special>&lt;</span><span class=identifier><i>(base type)</i></span><span class=special>,</span><span class=identifier><i>(key type)</i></span><span class=special>,</span><span class=identifier><i>(pointer to member function)</i></span><span class=special>&gt;</span>
</pre></blockquote>

<p>
The member function referred to must be <code>const</code>, take no arguments and return
a value of the specified key type.
Almost always you will want to use a <code>const</code> member function,
since elements in a <code>multi_index_container</code> are treated as constant, much
as elements of an <code>std::set</code>. However, a
<a href="../reference/key_extraction.html#mem_fun"><code>mem_fun</code></a>
counterpart is provided for use with non-constant member functions, whose
applicability is discussed on the paragraph on
<a href="#advanced_key_extractors">advanced features
of Boost.MultiIndex key extractors</a>.
</p>

<p><a href="../examples.html#example2">Example 2</a> in the examples section
provides a complete program showing how to use <code>const_mem_fun</code>.
<p>

<h4><a name="x_mem_fun">Variants for other types of member functions</a></h4>

<p>
Consider the following, non-compiling code:
</p>

<blockquote><pre>
<span class=keyword>struct</span> <span class=identifier>employee</span>
<span class=special>{</span>
  <span class=special>...</span>
  <span class=identifier>std</span><span class=special>::</span><span class=identifier>size_t</span> <span class=identifier>salary</span><span class=special>()</span><span class=keyword>const</span><span class=special><b>&amp;</b>;</span> <span class=comment>// note the &amp;</span>
<span class=special>};</span>

<span class=keyword>typedef</span> <span class=identifier>multi_index_container</span><span class=special>&lt;</span>
  <span class=identifier>employee</span><span class=special>,</span>
  <span class=identifier>indexed_by</span><span class=special>&lt;</span>
    <span class=special>...</span>
    <span class=identifier>ordered_non_unique</span><span class=special>&lt;</span>
      <span class=comment>// compiler error: can't convert &amp;employee::salary to
      // std::size_t (employee::*)() const</span>
      <span class=identifier><b>const_mem_fun</b></span><span class=special>&lt;</span><span class=identifier>employee</span><span class=special>,</span><span class=identifier>std</span><span class=special>::</span><span class=identifier>size_t</span><span class=special>,&amp;</span><span class=identifier>employee</span><span class=special>::</span><span class=identifier>salary</span><span class=special>&gt;</span>
    <span class=special>&gt;</span>
  <span class=special>&gt;</span>
<span class=special>&gt;</span> <span class=identifier>employee_set</span><span class=special>;</span>
</pre></blockquote>

<p>
The problem here is that the type of <code>&amp;employee::salary</code>, which is
<a href="https://en.cppreference.com/w/cpp/language/member_functions#const-.2C_volatile-.2C_and_ref-qualified_member_functions">ref-qualified</a>,
does not match exactly what <code>const_mem_fun</code> expects. Fortunately, Boost.MultiIndex
provides a variant to fit:
</p>

<blockquote><pre>
<span class=keyword>typedef</span> <span class=identifier>multi_index_container</span><span class=special>&lt;</span>
  <span class=identifier>employee</span><span class=special>,</span>
  <span class=identifier>indexed_by</span><span class=special>&lt;</span>
    <span class=special>...</span>
    <span class=identifier>ordered_non_unique</span><span class=special>&lt;</span>
      <span class=identifier><b>cref_mem_fun</b></span><span class=special>&lt;</span><span class=identifier>employee</span><span class=special>,</span><span class=identifier>std</span><span class=special>::</span><span class=identifier>size_t</span><span class=special>,&amp;</span><span class=identifier>employee</span><span class=special>::</span><span class=identifier>salary</span><span class=special>&gt;</span>
    <span class=special>&gt;</span>
  <span class=special>&gt;</span>
<span class=special>&gt;</span> <span class=identifier>employee_set</span><span class=special>;</span>
</pre></blockquote>

<p>
This is the list of all available variants of <code>const_mem_fun</code> and
<code>mem_fun</code>:
</p>

<p align="center">
<table cellspacing="0">
  <caption><b>Variants of <code>const_mem_fun</code> and <code>mem_fun</code>.</b></caption>
<tr>
  <th>&nbsp;Member function example&nbsp;</th>
  <th>Suitable extractor</th>
  <th>Behaves as</th>
</tr>
<tr>
  <td><code>int f()const volatile</code></td>
  <td><a href="../reference/key_extraction.html#cv_mem_fun"><code>cv_mem_fun</code></a><code>&lt;int,X,&amp;X::f&gt;</code></td>
  <td align="center" rowspan="3">&nbsp;<code>const_mem_fun</code>&nbsp;</td>
</tr>
<tr>
  <td><code>int f()const&amp;</code></td>
  <td><a href="../reference/key_extraction.html#cv_mem_fun"><code>cref_mem_fun</code></a><code>&lt;int,X,&amp;X::f&gt;</code></td>
</tr>
<tr>
  <td><code>int f()const volatile&amp;</code></td>
  <td><a href="../reference/key_extraction.html#cv_mem_fun"><code>cvref_mem_fun</code></a><code>&lt;int,X,&amp;X::f&gt;</code></td>
</tr>
<tr>
  <td><code>int f()volatile</code></td>
  <td><a href="../reference/key_extraction.html#volatile_mem_fun"><code>volatile_mem_fun</code></a><code>&lt;int,X,&amp;X::f&gt;</code></td>
  <td align="center" rowspan="3"><code>mem_fun</code></td>
</tr>
<tr>
  <td><code>int f()&amp;</code></td>
  <td><a href="../reference/key_extraction.html#volatile_mem_fun"><code>ref_mem_fun</code></a><code>&lt;int,X,&amp;X::f&gt;</code></td>
</tr>
<tr>
  <td><code>int f()volatile&amp;</code></td>
  <td><a href="../reference/key_extraction.html#volatile_mem_fun"><code>vref_mem_fun</code></a><code>&lt;int,X,&amp;X::f&gt;</code></td>
</tr>
</table>
</p>

<h3><a name="global_fun"><code>global_fun</code></a></h3>

<p>
Whereas <code>const_mem_fun</code> and its variants are based on a
given member function of the base type from where the key is extracted,
<a href="../reference/key_extraction.html#global_fun"><code>global_fun</code></a>
takes a global function (or static member function) accepting the base
type as its parameter and returning the key:
</p>

<blockquote><pre>
<span class=preprocessor>#include</span> <span class=special>&lt;</span><span class=identifier>boost</span><span class=special>/</span><span class=identifier>multi_index_container</span><span class=special>.</span><span class=identifier>hpp</span><span class=special>&gt;</span>
<span class=preprocessor>#include</span> <span class=special>&lt;</span><span class=identifier>boost</span><span class=special>/</span><span class=identifier>multi_index</span><span class=special>/</span><span class=identifier>ordered_index</span><span class=special>.</span><span class=identifier>hpp</span><span class=special>&gt;</span>
<span class=preprocessor>#include</span> <span class=special>&lt;</span><span class=identifier>boost</span><span class=special>/</span><span class=identifier>multi_index</span><span class=special>/</span><span class=identifier>global_fun</span><span class=special>.</span><span class=identifier>hpp</span><span class=special>&gt;</span>

<span class=keyword>struct</span> <span class=identifier>rectangle</span>
<span class=special>{</span>
  <span class=keyword>int</span> <span class=identifier>x0</span><span class=special>,</span><span class=identifier>y0</span><span class=special>;</span>
  <span class=keyword>int</span> <span class=identifier>x1</span><span class=special>,</span><span class=identifier>y1</span><span class=special>;</span>
<span class=special>};</span>

<span class=keyword>unsigned</span> <span class=keyword>long</span> <span class=identifier>area</span><span class=special>(</span><span class=keyword>const</span> <span class=identifier>rectangle</span><span class=special>&amp;</span> <span class=identifier>r</span><span class=special>)</span>
<span class=special>{</span>
  <span class=keyword>return</span> <span class=special>(</span><span class=keyword>unsigned</span> <span class=keyword>long</span><span class=special>)(</span><span class=identifier>r</span><span class=special>.</span><span class=identifier>x1</span><span class=special>-</span><span class=identifier>r</span><span class=special>.</span><span class=identifier>x0</span><span class=special>)*(</span><span class=identifier>r</span><span class=special>.</span><span class=identifier>x1</span><span class=special>-</span><span class=identifier>r</span><span class=special>.</span><span class=identifier>x0</span><span class=special>)+</span>
         <span class=special>(</span><span class=keyword>unsigned</span> <span class=keyword>long</span><span class=special>)(</span><span class=identifier>r</span><span class=special>.</span><span class=identifier>y1</span><span class=special>-</span><span class=identifier>r</span><span class=special>.</span><span class=identifier>y0</span><span class=special>)*(</span><span class=identifier>r</span><span class=special>.</span><span class=identifier>y1</span><span class=special>-</span><span class=identifier>r</span><span class=special>.</span><span class=identifier>y0</span><span class=special>);</span>
<span class=special>}</span>

<span class=keyword>typedef</span> <span class=identifier>multi_index_container</span><span class=special>&lt;</span>
  <span class=identifier>rectangle</span><span class=special>,</span>
  <span class=identifier>indexed_by</span><span class=special>&lt;</span>
    <span class=comment>// sort by increasing area</span>
    <span class=identifier>ordered_non_unique</span><span class=special>&lt;</span><span class=identifier>global_fun</span><span class=special>&lt;</span><span class=keyword>const</span> <span class=identifier>rectangle</span><span class=special>&amp;,</span><span class=keyword>unsigned</span> <span class=keyword>long</span><span class=special>,&amp;</span><span class=identifier>area</span><span class=special>&gt;</span> <span class=special>&gt;</span>
  <span class=special>&gt;</span>
<span class=special>&gt;</span> <span class=identifier>rectangle_container</span><span class=special>;</span>
</pre></blockquote>

<p>
The specification of <code>global_fun</code> obeys the following syntax:
</p>

<blockquote><pre>
<span class=identifier>global_fun</span><span class=special>&lt;</span><span class=identifier><i>(argument type)</i></span><span class=special>,</span><span class=identifier><i>(key type)</i></span><span class=special>,</span><span class=identifier><i>(pointer to function)</i></span><span class=special>&gt;</span>
</pre></blockquote>

<p>
where the argument type and key type must match <i>exactly</i> those in the
signature of the function used; for instance, in the example above the argument
type is <code>const rectangle&amp;</code>, without omitting the "<code>const</code>"
and "<code>&amp;</code>" parts. So, although most of the time the base type will be
accepted by constant reference, <code>global_fun</code> is also prepared to take
functions accepting their argument by value or by non-constant reference: this
latter case cannot generally be used directly in the specification of
<code>multi_index_container</code>s as their elements are treated as constant,
but the section on <a href="#advanced_key_extractors">advanced features
of Boost.MultiIndex key extractors</a> describes valid use cases of
key extraction based on such functions with a non-constant reference argument.
</p>

<p><a href="../examples.html#example2">Example 2</a> in the examples section
uses <code>gobal_fun</code>.
<p>

<h2><a name="user_defined_key_extractors">User-defined key extractors</a></h2>

<p>
Although the <a href="#predefined_key_extractors">predefined key extractors</a>
provided by Boost.MultiIndex are intended to serve most cases,
the user can also provide her own key extractors in more exotic situations,
as long as these conform to the
<a href="../reference/key_extraction.html#key_extractors"><code>Key
Extractor</code></a> concept.
</p>

<blockquote><pre>
<span class=comment>// some record class</span>
<span class=keyword>struct</span> <span class=identifier>record</span>
<span class=special>{</span>
  <span class=identifier>boost</span><span class=special>::</span><span class=identifier>gregorian</span><span class=special>::</span><span class=identifier>date</span> <span class=identifier>d</span><span class=special>;</span>
  <span class=identifier>std</span><span class=special>::</span><span class=identifier>string</span>            <span class=identifier>str</span><span class=special>;</span>    
<span class=special>};</span>

<span class=comment>// extracts a record's year</span>
<span class=keyword>struct</span> <span class=identifier>record_year</span>
<span class=special>{</span>
  <span class=comment>// result_type typedef required by Key Extractor concept</span>
  <span class=keyword>typedef</span> <span class=identifier>boost</span><span class=special>::</span><span class=identifier>gregorian</span><span class=special>::</span><span class=identifier>greg_year</span> <span class=identifier>result_type</span><span class=special>;</span> 
  
  <span class=identifier>result_type</span> <span class=keyword>operator</span><span class=special>()(</span><span class=keyword>const</span> <span class=identifier>record</span><span class=special>&amp;</span> <span class=identifier>r</span><span class=special>)</span><span class=keyword>const</span> <span class=comment>// operator() must be const</span>
  <span class=special>{</span>
    <span class=keyword>return</span> <span class=identifier>r</span><span class=special>.</span><span class=identifier>d</span><span class=special>.</span><span class=identifier>year</span><span class=special>();</span>
  <span class=special>}</span>
<span class=special>};</span>

<span class=comment>// example of use of the previous key extractor</span>
<span class=keyword>typedef</span> <span class=identifier>multi_index_container</span><span class=special>&lt;</span>
  <span class=identifier>record</span><span class=special>,</span>
  <span class=identifier>indexed_by</span><span class=special>&lt;</span>
    <span class=identifier>ordered_non_unique</span><span class=special>&lt;</span><span class=identifier>record_year</span><span class=special>&gt;</span> <span class=comment>// sorted by record's year</span>
  <span class=special>&gt;</span>
<span class=special>&gt;</span> <span class=identifier>record_log</span><span class=special>;</span>
</pre></blockquote>

<p>
<a href="../examples.html#example6">Example 6</a> in the examples section
applies some user-defined key extractors in a complex scenario where
keys are accessed via pointers.
</p>

<h2><a name="composite_keys">Composite keys</a></h2>

<p>
In relational databases, composite keys depend on two or more fields of a given table.
The analogous concept in Boost.MultiIndex is modeled by means of
<a href="../reference/key_extraction.html#composite_key">
<code>composite_key</code></a>, as shown in the example:
</p>

<blockquote><pre>
<span class=preprocessor>#include</span> <span class=special>&lt;</span><span class=identifier>boost</span><span class=special>/</span><span class=identifier>multi_index_container</span><span class=special>.</span><span class=identifier>hpp</span><span class=special>&gt;</span>
<span class=preprocessor>#include</span> <span class=special>&lt;</span><span class=identifier>boost</span><span class=special>/</span><span class=identifier>multi_index</span><span class=special>/</span><span class=identifier>ordered_index</span><span class=special>.</span><span class=identifier>hpp</span><span class=special>&gt;</span>
<span class=preprocessor>#include</span> <span class=special>&lt;</span><span class=identifier>boost</span><span class=special>/</span><span class=identifier>multi_index</span><span class=special>/</span><span class=identifier>member</span><span class=special>.</span><span class=identifier>hpp</span><span class=special>&gt;</span>
<span class=preprocessor>#include</span> <span class=special>&lt;</span><span class=identifier>boost</span><span class=special>/</span><span class=identifier>multi_index</span><span class=special>/</span><span class=identifier>composite_key</span><span class=special>.</span><span class=identifier>hpp</span><span class=special>&gt;</span>

<span class=keyword>struct</span> <span class=identifier>phonebook_entry</span>
<span class=special>{</span>
  <span class=identifier>std</span><span class=special>::</span><span class=identifier>string</span> <span class=identifier>family_name</span><span class=special>;</span>
  <span class=identifier>std</span><span class=special>::</span><span class=identifier>string</span> <span class=identifier>given_name</span><span class=special>;</span>
  <span class=identifier>std</span><span class=special>::</span><span class=identifier>string</span> <span class=identifier>phone_number</span><span class=special>;</span>

  <span class=identifier>phonebook_entry</span><span class=special>(</span>
    <span class=identifier>std</span><span class=special>::</span><span class=identifier>string</span> <span class=identifier>family_name</span><span class=special>,</span>
    <span class=identifier>std</span><span class=special>::</span><span class=identifier>string</span> <span class=identifier>given_name</span><span class=special>,</span>
    <span class=identifier>std</span><span class=special>::</span><span class=identifier>string</span> <span class=identifier>phone_number</span><span class=special>):</span>
    <span class=identifier>family_name</span><span class=special>(</span><span class=identifier>family_name</span><span class=special>),</span><span class=identifier>given_name</span><span class=special>(</span><span class=identifier>given_name</span><span class=special>),</span><span class=identifier>phone_number</span><span class=special>(</span><span class=identifier>phone_number</span><span class=special>)</span>
  <span class=special>{}</span>
<span class=special>};</span>

<span class=comment>// define a multi_index_container with a composite key on
// (family_name,given_name)</span>
<span class=keyword>typedef</span> <span class=identifier>multi_index_container</span><span class=special>&lt;</span>
  <span class=identifier>phonebook_entry</span><span class=special>,</span>
  <span class=identifier>indexed_by</span><span class=special>&lt;</span>
    <span class=comment>// non-unique as some subscribers might have more than one number</span>
    <span class=identifier>ordered_non_unique</span><span class=special>&lt;</span> 
      <span class=identifier>composite_key</span><span class=special>&lt;</span>
        <span class=identifier>phonebook_entry</span><span class=special>,</span>
        <span class=identifier>member</span><span class=special>&lt;</span><span class=identifier>phonebook_entry</span><span class=special>,</span><span class=identifier>std</span><span class=special>::</span><span class=identifier>string</span><span class=special>,&amp;</span><span class=identifier>phonebook_entry</span><span class=special>::</span><span class=identifier>family_name</span><span class=special>&gt;,</span>
        <span class=identifier>member</span><span class=special>&lt;</span><span class=identifier>phonebook_entry</span><span class=special>,</span><span class=identifier>std</span><span class=special>::</span><span class=identifier>string</span><span class=special>,&amp;</span><span class=identifier>phonebook_entry</span><span class=special>::</span><span class=identifier>given_name</span><span class=special>&gt;</span>
      <span class=special>&gt;</span>
    <span class=special>&gt;,</span>
    <span class=identifier>ordered_unique</span><span class=special>&lt;</span> <span class=comment>// unique as numbers belong to only one subscriber</span>
      <span class=identifier>member</span><span class=special>&lt;</span><span class=identifier>phonebook_entry</span><span class=special>,</span><span class=identifier>std</span><span class=special>::</span><span class=identifier>string</span><span class=special>,&amp;</span><span class=identifier>phonebook_entry</span><span class=special>::</span><span class=identifier>phone_number</span><span class=special>&gt;</span>
    <span class=special>&gt;</span>
  <span class=special>&gt;</span>
<span class=special>&gt;</span> <span class=identifier>phonebook</span><span class=special>;</span>
</pre></blockquote>

<p>
<code>composite_key</code> accepts two or more key extractors on the same
value (here, <code>phonebook_entry</code>). Lookup operations on a composite
key are accomplished by passing tuples with the values searched:
</p>

<blockquote><pre>
<span class=identifier>phonebook</span> <span class=identifier>pb</span><span class=special>;</span>
<span class=special>...</span>
<span class=comment>// search for Dorothea White's number</span>
<span class=identifier>phonebook</span><span class=special>::</span><span class=identifier>iterator</span> <span class=identifier>it</span><span class=special>=</span><span class=identifier>pb</span><span class=special>.</span><span class=identifier>find</span><span class=special>(</span><span class=identifier>std</span><span class=special>::</span><span class=identifier>make_tuple</span><span class=special>(</span><span class=string>&quot;White&quot;</span><span class=special>,</span><span class=string>&quot;Dorothea&quot;</span><span class=special>));</span>
<span class=identifier>std</span><span class=special>::</span><span class=identifier>string</span> <span class=identifier>number</span><span class=special>=</span><span class=identifier>it</span><span class=special>-&gt;</span><span class=identifier>phone_number</span><span class=special>;</span>
</pre></blockquote>

<p>
Composite keys are sorted by lexicographical order, i.e. sorting is performed
by the first key, then the second key if the first one is equal, etc. This
order allows for partial searches where only the first keys are specified:
</p>

<blockquote><pre>
<span class=identifier>phonebook</span> <span class=identifier>pb</span><span class=special>;</span>
<span class=special>...</span>
<span class=comment>// look for all Whites</span>
<span class=identifier>std</span><span class=special>::</span><span class=identifier>pair</span><span class=special>&lt;</span><span class=identifier>phonebook</span><span class=special>::</span><span class=identifier>iterator</span><span class=special>,</span><span class=identifier>phonebook</span><span class=special>::</span><span class=identifier>iterator</span><span class=special>&gt;</span> <span class=identifier>p</span><span class=special>=</span>
  <span class=identifier>pb</span><span class=special>.</span><span class=identifier>equal_range</span><span class=special>(</span><span class=identifier>std</span><span class=special>::</span><span class=identifier>make_tuple</span><span class=special>(</span><span class=string>&quot;White&quot;</span><span class=special>));</span>
</pre></blockquote>

<p>
As a notational convenience, when only the first key is specified it is possible
to pass the argument directly without including it into a tuple:
</p>

<blockquote><pre>
<span class=identifier>phonebook</span> <span class=identifier>pb</span><span class=special>;</span>
<span class=special>...</span>
<span class=comment>// look for all Whites</span>
<span class=identifier>std</span><span class=special>::</span><span class=identifier>pair</span><span class=special>&lt;</span><span class=identifier>phonebook</span><span class=special>::</span><span class=identifier>iterator</span><span class=special>,</span><span class=identifier>phonebook</span><span class=special>::</span><span class=identifier>iterator</span><span class=special>&gt;</span> <span class=identifier>p</span><span class=special>=</span><span class=identifier>pb</span><span class=special>.</span><span class=identifier>equal_range</span><span class=special>(</span><span class=string>&quot;White&quot;</span><span class=special>);</span>
</pre></blockquote>

<p>
On the other hand, partial searches without specifying the first keys are not
allowed.
</p>

<p>
By default, the corresponding <code>std::less</code> predicate is used
for each subkey of a composite key. Alternate comparison predicates can
be specified with <a href="../reference/key_extraction.html#composite_key_compare">
<code>composite_key_compare</code></a>:
</p>

<blockquote><pre>
<span class=comment>// phonebook with given names in reverse order</span>

<span class=keyword>typedef</span> <span class=identifier>multi_index_container</span><span class=special>&lt;</span>
  <span class=identifier>phonebook_entry</span><span class=special>,</span>
  <span class=identifier>indexed_by</span><span class=special>&lt;</span>
    <span class=identifier>ordered_non_unique</span><span class=special>&lt;</span>
      <span class=identifier>composite_key</span><span class=special>&lt;</span>
        <span class=identifier>phonebook_entry</span><span class=special>,</span>
        <span class=identifier>member</span><span class=special>&lt;</span><span class=identifier>phonebook_entry</span><span class=special>,</span><span class=identifier>std</span><span class=special>::</span><span class=identifier>string</span><span class=special>,&amp;</span><span class=identifier>phonebook_entry</span><span class=special>::</span><span class=identifier>family_name</span><span class=special>&gt;,</span>
        <span class=identifier>member</span><span class=special>&lt;</span><span class=identifier>phonebook_entry</span><span class=special>,</span><span class=identifier>std</span><span class=special>::</span><span class=identifier>string</span><span class=special>,&amp;</span><span class=identifier>phonebook_entry</span><span class=special>::</span><span class=identifier>given_name</span><span class=special>&gt;</span>
      <span class=special>&gt;,</span>
      <span class=identifier>composite_key_compare</span><span class=special>&lt;</span>
        <span class=identifier>std</span><span class=special>::</span><span class=identifier>less</span><span class=special>&lt;</span><span class=identifier>std</span><span class=special>::</span><span class=identifier>string</span><span class=special>&gt;,</span>   <span class=comment>// family names sorted as by default</span>
        <span class=identifier>std</span><span class=special>::</span><span class=identifier>greater</span><span class=special>&lt;</span><span class=identifier>std</span><span class=special>::</span><span class=identifier>string</span><span class=special>&gt;</span> <span class=comment>// given names reversed</span>
      <span class=special>&gt;</span>
    <span class=special>&gt;,</span>
    <span class=identifier>ordered_unique</span><span class=special>&lt;</span>
      <span class=identifier>member</span><span class=special>&lt;</span><span class=identifier>phonebook_entry</span><span class=special>,</span><span class=identifier>std</span><span class=special>::</span><span class=identifier>string</span><span class=special>,&amp;</span><span class=identifier>phonebook_entry</span><span class=special>::</span><span class=identifier>phone_number</span><span class=special>&gt;</span>
    <span class=special>&gt;</span>
  <span class=special>&gt;</span>
<span class=special>&gt;</span> <span class=identifier>phonebook</span><span class=special>;</span>
</pre></blockquote>

<p>
See <a href="../examples.html#example7">example 7</a> in the examples section
for an application of <code>composite_key</code>.
</p>

<h3><a name="composite_keys_hash">Composite keys and hashed indices</a></h3>

<p>
Composite keys can also be used with hashed indices in a straightforward manner:
</p>

<blockquote><pre>
<span class=keyword>struct</span> <span class=identifier>street_entry</span>
<span class=special>{</span>
  <span class=comment>// quadrant coordinates</span>
  <span class=keyword>int</span> <span class=identifier>x</span><span class=special>;</span>
  <span class=keyword>int</span> <span class=identifier>y</span><span class=special>;</span>

  <span class=identifier>std</span><span class=special>::</span><span class=identifier>string</span> <span class=identifier>name</span><span class=special>;</span>

  <span class=identifier>street_entry</span><span class=special>(</span><span class=keyword>int</span> <span class=identifier>x</span><span class=special>,</span><span class=keyword>int</span> <span class=identifier>y</span><span class=special>,</span><span class=keyword>const</span> <span class=identifier>std</span><span class=special>::</span><span class=identifier>string</span><span class=special>&amp;</span> <span class=identifier>name</span><span class=special>):</span><span class=identifier>x</span><span class=special>(</span><span class=identifier>x</span><span class=special>),</span><span class=identifier>y</span><span class=special>(</span><span class=identifier>y</span><span class=special>),</span><span class=identifier>name</span><span class=special>(</span><span class=identifier>name</span><span class=special>){}</span>
<span class=special>};</span>

<span class=keyword>typedef</span> <span class=identifier>multi_index_container</span><span class=special>&lt;</span>
  <span class=identifier>street_entry</span><span class=special>,</span>
  <span class=identifier>indexed_by</span><span class=special>&lt;</span>
    <span class=identifier>hashed_non_unique</span><span class=special>&lt;</span> <span class=comment>// indexed by quadrant coordinates</span>
      <span class=identifier>composite_key</span><span class=special>&lt;</span>
        <span class=identifier>street_entry</span><span class=special>,</span>
        <span class=identifier>member</span><span class=special>&lt;</span><span class=identifier>street_entry</span><span class=special>,</span><span class=keyword>int</span><span class=special>,&amp;</span><span class=identifier>street_entry</span><span class=special>::</span><span class=identifier>x</span><span class=special>&gt;,</span>
        <span class=identifier>member</span><span class=special>&lt;</span><span class=identifier>street_entry</span><span class=special>,</span><span class=keyword>int</span><span class=special>,&amp;</span><span class=identifier>street_entry</span><span class=special>::</span><span class=identifier>y</span><span class=special>&gt;</span>
      <span class=special>&gt;</span>
    <span class=special>&gt;,</span>
    <span class=identifier>hashed_non_unique</span><span class=special>&lt;</span> <span class=comment>// indexed by street name</span>
      <span class=identifier>member</span><span class=special>&lt;</span><span class=identifier>street_entry</span><span class=special>,</span><span class=identifier>std</span><span class=special>::</span><span class=identifier>string</span><span class=special>,&amp;</span><span class=identifier>street_entry</span><span class=special>::</span><span class=identifier>name</span><span class=special>&gt;</span>
    <span class=special>&gt;</span>
  <span class=special>&gt;</span>
<span class=special>&gt;</span> <span class=identifier>street_locator</span><span class=special>;</span>

<span class=identifier>street_locator</span> <span class=identifier>sl</span><span class=special>;</span>
<span class=special>...</span>
<span class=keyword>void</span> <span class=identifier>streets_in_quadrant</span><span class=special>(</span><span class=keyword>int</span> <span class=identifier>x</span><span class=special>,</span><span class=keyword>int</span> <span class=identifier>y</span><span class=special>)</span>
<span class=special>{</span>
  <span class=identifier>std</span><span class=special>::</span><span class=identifier>pair</span><span class=special>&lt;</span><span class=identifier>street_locator</span><span class=special>::</span><span class=identifier>iterator</span><span class=special>,</span><span class=identifier>street_locator</span><span class=special>::</span><span class=identifier>iterator</span><span class=special>&gt;</span> <span class=identifier>p</span><span class=special>=</span>
    <span class=identifier>sl</span><span class=special>.</span><span class=identifier>equal_range</span><span class=special>(</span><span class=identifier>std</span><span class=special>::</span><span class=identifier>make_tuple</span><span class=special>(</span><span class=identifier>x</span><span class=special>,</span><span class=identifier>y</span><span class=special>));</span>

  <span class=keyword>while</span><span class=special>(</span><span class=identifier>p</span><span class=special>.</span><span class=identifier>first</span><span class=special>!=</span><span class=identifier>p</span><span class=special>.</span><span class=identifier>second</span><span class=special>){</span>
    <span class=identifier>std</span><span class=special>::</span><span class=identifier>cout</span><span class=special>&lt;&lt;</span><span class=identifier>p</span><span class=special>.</span><span class=identifier>first</span><span class=special>-&gt;</span><span class=identifier>name</span><span class=special>&lt;&lt;</span><span class=identifier>std</span><span class=special>::</span><span class=identifier>endl</span><span class=special>;</span>
    <span class=special>++</span><span class=identifier>p</span><span class=special>.</span><span class=identifier>first</span><span class=special>;</span>
  <span class=special>}</span>
<span class=special>}</span>
</pre></blockquote>

<p>
Note that hashing is automatically taken care of: <code>boost::hash</code> is
specialized to hash a composite key as a function of the <code>boost::hash</code>
values of its elements. Should we need to specify different hash functions for the
elements of a composite key, we can explicitly do so by using the
<a href="../reference/key_extraction.html#composite_key_hash"><code>composite_key_hash</code></a>
utility:
</p>

<blockquote><pre>
<span class=keyword>struct</span> <span class=identifier>tuned_int_hash</span>
<span class=special>{</span>
  <span class=keyword>int</span> <span class=keyword>operator</span><span class=special>()(</span><span class=keyword>int</span> <span class=identifier>x</span><span class=special>)</span><span class=keyword>const</span>
  <span class=special>{</span>
    <span class=comment>// specially tuned hash for this application</span>
  <span class=special>}</span>
<span class=special>};</span>

<span class=keyword>typedef</span> <span class=identifier>multi_index_container</span><span class=special>&lt;</span>
  <span class=identifier>street_entry</span><span class=special>,</span>
  <span class=identifier>indexed_by</span><span class=special>&lt;</span>
    <span class=identifier>hashed_non_unique</span><span class=special>&lt;</span> <span class=comment>// indexed by quadrant coordinates</span>
      <span class=identifier>composite_key</span><span class=special>&lt;</span>
        <span class=identifier>street_entry</span><span class=special>,</span>
        <span class=identifier>member</span><span class=special>&lt;</span><span class=identifier>street_entry</span><span class=special>,</span><span class=keyword>int</span><span class=special>,&amp;</span><span class=identifier>street_entry</span><span class=special>::</span><span class=identifier>x</span><span class=special>&gt;,</span>
        <span class=identifier>member</span><span class=special>&lt;</span><span class=identifier>street_entry</span><span class=special>,</span><span class=keyword>int</span><span class=special>,&amp;</span><span class=identifier>street_entry</span><span class=special>::</span><span class=identifier>y</span><span class=special>&gt;</span>
      <span class=special>&gt;,</span>
      <span class=identifier>composite_key_hash</span><span class=special>&lt;</span>
        <span class=identifier>tuned_int_hash</span><span class=special>,</span>
        <span class=identifier>tuned_int_hash</span>
      <span class=special>&gt;</span>
    <span class=special>&gt;,</span>
    <span class=identifier>hashed_non_unique</span><span class=special>&lt;</span> <span class=comment>// indexed by street name</span>
      <span class=identifier>member</span><span class=special>&lt;</span><span class=identifier>street_entry</span><span class=special>,</span><span class=identifier>std</span><span class=special>::</span><span class=identifier>string</span><span class=special>,&amp;</span><span class=identifier>street_entry</span><span class=special>::</span><span class=identifier>name</span><span class=special>&gt;</span>
    <span class=special>&gt;</span>
  <span class=special>&gt;</span>
<span class=special>&gt;</span> <span class=identifier>street_locator</span><span class=special>;</span>
</pre></blockquote>

<p>
Also, equality of composite keys can be tuned with
<a href="../reference/key_extraction.html#composite_key_equal_to"><code>composite_key_equal_to</code></a>,
though in most cases the default equality predicate (relying on
the <code>std::equal_to</code> instantiations for the element types)
will be the right choice.
</p>

<p>
Unlike with ordered indices, we cannot perform partial searches specifying
only the first elements of a composite key:
</p>

<blockquote><pre>
<span class=comment>// try to locate streets in quadrants with x==0
// compile-time error: hashed indices do not allow such operations</span>
<span class=identifier>std</span><span class=special>::</span><span class=identifier>pair</span><span class=special>&lt;</span><span class=identifier>street_locator</span><span class=special>::</span><span class=identifier>iterator</span><span class=special>,</span><span class=identifier>street_locator</span><span class=special>::</span><span class=identifier>iterator</span><span class=special>&gt;</span> <span class=identifier>p</span><span class=special>=</span>
  <span class=identifier>sl</span><span class=special>.</span><span class=identifier>equal_range</span><span class=special>(</span><span class=identifier>std</span><span class=special>::</span><span class=identifier>make_tuple</span><span class=special>(</span><span class=number>0</span><span class=special>));</span>
</pre></blockquote>

<p>
The reason for this limitation is quite logical: as the hash value of a composite
key depends on all of its elements, it is impossible to calculate it from
partial information.
</p>

<h2><a name="key">C++17 terse key specification syntax</a></h2>

<p>
C++17 introduces the declaration of <code>auto</code> template parameters, which can be
taken advantage of to eliminate some redundancy in the specification of Boost.MultiIndex
predefined key extractors. For instance, instead of the classical:
</p>

<blockquote><pre>
<span class=keyword>typedef</span> <span class=identifier>multi_index_container</span><span class=special>&lt;</span>
  <span class=identifier>employee</span><span class=special>,</span>
  <span class=identifier>indexed_by</span><span class=special>&lt;</span>
    <span class=identifier>ordered_unique</span><span class=special>&lt;</span><span class=identifier>identity</span><span class=special>&lt;</span><span class=identifier>employee</span><span class=special>&gt;</span> <span class=special>&gt;,</span>
    <span class=identifier>ordered_non_unique</span><span class=special>&lt;</span><span class=identifier>member</span><span class=special>&lt;</span><span class=identifier>employee</span><span class=special>,</span><span class=identifier>std</span><span class=special>::</span><span class=identifier>string</span><span class=special>,&amp;</span><span class=identifier>employee</span><span class=special>::</span><span class=identifier>name</span><span class=special>&gt;</span> <span class=special>&gt;,</span>
    <span class=identifier>ordered_non_unique</span><span class=special>&lt;</span>
      <span class=identifier>const_mem_fun</span><span class=special>&lt;</span><span class=identifier>employee</span><span class=special>,</span><span class=identifier>std</span><span class=special>::</span><span class=identifier>size_t</span><span class=special>,&amp;</span><span class=identifier>employee</span><span class=special>::</span><span class=identifier>name_length</span><span class=special>&gt;</span>
    <span class=special>&gt;</span>
  <span class=special>&gt;</span>
<span class=special>&gt;</span> <span class=identifier>employee_set</span><span class=special>;</span>
</pre></blockquote>

one can now write:

<blockquote><pre>
<span class=preprocessor>#include</span> <span class=special>&lt;</span><span class=identifier>boost</span><span class=special>/</span><span class=identifier>multi_index</span><span class=special>/</span><span class=identifier>key</span><span class=special>.</span><span class=identifier>hpp</span><span class=special>&gt;</span>
<span class=special>...</span>
<span class=keyword>typedef</span> <span class=identifier>multi_index_container</span><span class=special>&lt;</span>
  <span class=identifier>employee</span><span class=special>,</span>
  <span class=identifier>indexed_by</span><span class=special>&lt;</span>
    <span class=identifier>ordered_unique</span><span class=special>&lt;</span><span class=identifier>identity</span><span class=special>&lt;</span><span class=identifier>employee</span><span class=special>&gt;&gt;,</span>
    <span class=identifier>ordered_non_unique</span><span class=special>&lt;</span><span class=identifier><b>key</b></span><span class=special><b>&lt;&amp;</b></span><span class=identifier><b>employee</b></span><span class=special><b>::</b></span><span class=identifier><b>name</b></span><span class=special><b>&gt;</b>&gt;,</span>
    <span class=identifier>ordered_non_unique</span><span class=special>&lt;</span><span class=identifier><b>key</b></span><span class=special><b>&lt;&amp;</b></span><span class=identifier><b>employee</b></span><span class=special><b>::</b></span><span class=identifier><b>name_length</b></span><span class=special><b>&gt;</b>&gt;</span>
  <span class=special>&gt;</span>
<span class=special>&gt;</span> <span class=identifier>employee_set</span><span class=special>;</span>
</pre></blockquote>

<p>
which results in the exact same defined type, as
<a href="../reference/key_extraction.html#key"><code>key</code></a> constructs are mere
aliases for the old syntax. <code>key</code> can be used to shorten the specification of
<a href="../reference/key_extraction.html#member"><code>member</code></a>,
<a href="../reference/key_extraction.html#const_mem_fun"><code>const_mem_fun</code></a>
(and <a href="../reference/key_extraction.html#cv_mem_fun">variants</a>),
<a href="../reference/key_extraction.html#mem_fun"><code>mem_fun</code></a>
(and <a href="../reference/key_extraction.html#volatile_mem_fun">variants</a>),
<a href="../reference/key_extraction.html#global_fun"><code>global_fun</code></a>
and, with additional terseness benefits,
<a href="../reference/key_extraction.html#composite_key"><code>composite_key</code></a>:
</p>

<blockquote><pre>
<span class=keyword>typedef</span> <span class=identifier>multi_index_container</span><span class=special>&lt;</span>
  <span class=identifier>phonebook_entry</span><span class=special>,</span>
  <span class=identifier>indexed_by</span><span class=special>&lt;</span>
    <span class=comment>// composite key on family name and given name</span>
    <span class=identifier>ordered_non_unique</span><span class=special>&lt;
      </span><span class=identifier><b>key</b></span><span class=special><b>&lt;&amp;</b></span><span class=identifier><b>phonebook_entry</b></span><span class=special><b>::</b></span><span class=identifier><b>family_name</b></span><span class=special><b>,&amp;</b></span><span class=identifier><b>phonebook_entry</b></span><span class=special><b>::</b></span><span class=identifier><b>given_name</b></span><span class=special><b>&gt;</b></span>
    <span class=special>&gt;,</span>
    <span class=comment>// unique index on phone number</span>
    <span class=identifier>ordered_unique</span><span class=special>&lt;</span><span class=identifier>key</span><span class=special>&lt;&amp;</span><span class=identifier>phonebook_entry</span><span class=special>::</span><span class=identifier>phone_number</span><span class=special>&gt;&gt;</span>
  <span class=special>&gt;</span>
<span class=special>&gt;</span> <span class=identifier>phonebook</span><span class=special>;</span>
</pre></blockquote>

In this example, the first usage of <code>key</code> substitutes for the
decidedly more cumbersome:

<blockquote><pre>
<span class=identifier>composite_key</span><span class=special>&lt;</span>
  <span class=identifier>phonebook_entry</span><span class=special>,</span>
  <span class=identifier>member</span><span class=special>&lt;</span><span class=identifier>phonebook_entry</span><span class=special>,</span><span class=identifier>std</span><span class=special>::</span><span class=identifier>string</span><span class=special>,&amp;</span><span class=identifier>phonebook_entry</span><span class=special>::</span><span class=identifier>family_name</span><span class=special>&gt;,</span>
  <span class=identifier>member</span><span class=special>&lt;</span><span class=identifier>phonebook_entry</span><span class=special>,</span><span class=identifier>std</span><span class=special>::</span><span class=identifier>string</span><span class=special>,&amp;</span><span class=identifier>phonebook_entry</span><span class=special>::</span><span class=identifier>given_name</span><span class=special>&gt;</span>
<span class=special>&gt;</span>
</pre></blockquote>

Note that we did not even have to specify the first <code>phonebook_entry</code>
argument: the internal machinery of <code>key</code> automatically deduces it for
us. Check the <a href="../reference/key_extraction.html#key_synopsis">reference</a>
for technical details.

<h2><a name="advanced_key_extractors">Advanced features of Boost.MultiIndex key
extractors</a></h2>

<p>
The <a href="../reference/key_extraction.html#key_extractors"><code>Key Extractor</code></a>
concept allows the same object to extract keys from several different types,
possibly through suitably defined overloads of <code>operator()</code>:
</p>

<blockquote><pre>
<span class=comment>// example of a name extractor from employee and employee *</span>
<span class=keyword>struct</span> <span class=identifier>name_extractor</span>
<span class=special>{</span>
  <span class=keyword>typedef</span> <span class=identifier>std</span><span class=special>::</span><span class=identifier>string</span> <span class=identifier>result_type</span><span class=special>;</span>

  <span class=keyword>const</span> <span class=identifier>result_type</span><span class=special>&amp;</span> <span class=keyword>operator</span><span class=special>()(</span><span class=keyword>const</span> <span class=identifier>employee</span><span class=special>&amp;</span> <span class=identifier>e</span><span class=special>)</span><span class=keyword>const</span><span class=special>{</span><span class=keyword>return</span> <span class=identifier>e</span><span class=special>.</span><span class=identifier>name</span><span class=special>;}</span>
  <span class=identifier>result_type</span><span class=special>&amp;</span>       <span class=keyword>operator</span><span class=special>()(</span><span class=identifier>employee</span><span class=special>*</span> <span class=identifier>e</span><span class=special>)</span><span class=keyword>const</span><span class=special>{</span><span class=keyword>return</span> <span class=identifier>e</span><span class=special>-&gt;</span><span class=identifier>name</span><span class=special>;}</span>
<span class=special>};</span>

<span class=comment>// name_extractor can handle elements of type employee...</span>
<span class=keyword>typedef</span> <span class=identifier>multi_index_container</span><span class=special>&lt;</span>
  <span class=identifier>employee</span><span class=special>,</span>
  <span class=identifier>indexed_by</span><span class=special>&lt;</span>
    <span class=identifier>ordered_unique</span><span class=special>&lt;</span><span class=identifier>name_extractor</span><span class=special>&gt;</span>
  <span class=special>&gt;</span>
<span class=special>&gt;</span> <span class=identifier>employee_set</span><span class=special>;</span>

<span class=comment>// ...as well as elements of type employee *</span>
<span class=keyword>typedef</span> <span class=identifier>multi_index_container</span><span class=special>&lt;</span>
  <span class=identifier>employee</span><span class=special>*,</span>
  <span class=identifier>indexed_by</span><span class=special>&lt;</span>
    <span class=identifier>ordered_unique</span><span class=special>&lt;</span><span class=identifier>name_extractor</span><span class=special>&gt;</span>
  <span class=special>&gt;</span>
<span class=special>&gt;</span> <span class=identifier>employee_ptr_set</span><span class=special>;</span>
</pre></blockquote>

<p>
This possibility is fully exploited by predefined key extractors provided
by Boost.MultiIndex, making it simpler to define <code>multi_index_container</code>s
where elements are pointers or references to the actual objects. The following
specifies a <code>multi_index_container</code> of pointers to employees sorted by their
names.
</p>

<blockquote><pre>
<span class=keyword>typedef</span> <span class=identifier>multi_index_container</span><span class=special>&lt;</span>
  <span class=identifier>employee</span> <span class=special>*,</span>
  <span class=identifier>indexed_by</span><span class=special>&lt;</span>
    <span class=identifier>ordered_non_unique</span><span class=special>&lt;</span><span class=identifier>member</span><span class=special>&lt;</span><span class=identifier>employee</span><span class=special>,</span><span class=identifier>std</span><span class=special>::</span><span class=identifier>string</span><span class=special>,&amp;</span><span class=identifier>employee</span><span class=special>::</span><span class=identifier>name</span><span class=special>&gt;</span> <span class=special>&gt;</span> <span class=special>&gt;</span>
<span class=special>&gt;</span> <span class=identifier>employee_set</span><span class=special>;</span>
</pre></blockquote>

<p>
Note that this is specified in exactly the same manner as a <code>multi_index_container</code>
of actual <code>employee</code> objects: <code>member</code> takes care of the
extra dereferencing needed to gain access to <code>employee::name</code>. A similar
functionality is provided for interoperability with reference wrappers from
<a href="../../../../doc/html/ref.html">Boost.Ref</a>:
</p>

<blockquote><pre>
<span class=keyword>typedef</span> <span class=identifier>multi_index_container</span><span class=special>&lt;</span>
  <span class=identifier>boost</span><span class=special>::</span><span class=identifier>reference_wrapper</span><span class=special>&lt;</span><span class=keyword>const</span> <span class=identifier>employee</span><span class=special>&gt;,</span>
  <span class=identifier>indexed_by</span><span class=special>&lt;</span>
    <span class=identifier>ordered_non_unique</span><span class=special>&lt;</span><span class=identifier>member</span><span class=special>&lt;</span><span class=identifier>employee</span><span class=special>,</span><span class=identifier>std</span><span class=special>::</span><span class=identifier>string</span><span class=special>,&amp;</span><span class=identifier>employee</span><span class=special>::</span><span class=identifier>name</span><span class=special>&gt;</span> <span class=special>&gt;</span> <span class=special>&gt;</span>
<span class=special>&gt;</span> <span class=identifier>employee_set</span><span class=special>;</span>
</pre></blockquote>

<p>
In fact, support for pointers is further extended to accept what we call
<i>chained pointers</i>. Such a chained pointer is defined by induction as a raw or
smart pointer or iterator to the actual element, to a reference wrapper of the
element or <i>to another chained pointer</i>; that is, chained pointers are arbitrary
compositions of pointer-like types ultimately dereferencing
to the element from where the key is to be extracted. Examples of chained
pointers to <code>employee</code> are:
<ul>
  <li><code>employee *</code>,</li>
  <li><code>const employee *</code>,</li>
  <li><code>std::unique_ptr&lt;employee></code>,</li>
  <li><code>std::list&lt;boost::reference_wrapper&lt;employee> >::iterator</code>,</li>
  <li><code>employee **</code>,</li>
  <li><code>boost::shared_ptr&lt;const employee *></code>.</li>
</ul>
In general, chained pointers with dereferencing distance greater than 1 are not
likely to be used in a normal program, but they can arise in frameworks
which construct "views" as <code>multi_index_container</code>s from preexisting
<code>multi_index_container</code>s.
</p>

<p>
In order to present a short summary of the different usages of Boost.MultiIndex
key extractors in the presence of reference wrappers and pointers, consider the
following final type:
</p>

<blockquote><pre>
<span class=keyword>struct</span> <span class=identifier>T</span>
<span class=special>{</span>
  <span class=keyword>int</span>        <span class=identifier>i</span><span class=special>;</span>
  <span class=keyword>const</span> <span class=keyword>int</span>  <span class=identifier>j</span><span class=special>;</span>
  <span class=keyword>int</span>        <span class=identifier>f</span><span class=special>()</span><span class=keyword>const</span><span class=special>;</span>
  <span class=keyword>int</span>        <span class=identifier>g</span><span class=special>();</span>
  <span class=keyword>static</span> <span class=keyword>int</span> <span class=identifier>gf</span><span class=special>(</span><span class=keyword>const</span> <span class=identifier>T</span><span class=special>&amp;);</span>
  <span class=keyword>static</span> <span class=keyword>int</span> <span class=identifier>gg</span><span class=special>(</span><span class=identifier>T</span><span class=special>&amp;);</span>
<span class=special>};</span>
</pre></blockquote>

<p>
The table below lists the appropriate key extractors to be used for
different pointer and reference wrapper types based on <code>T</code>, for
each of its members.
</p>

<p align="center">
<table cellspacing="0">
  <caption><b>Use cases for Boost.MultiIndex key extractors.</b></caption>
<tr>
  <th>element type</th>
  <th>&nbsp;key&nbsp;</th>
  <th>key extractor</th>
  <th>applicable to<br><code>const</code> elements?</th>
  <th>read/write?</th>
</tr>
<tr>
  <td align="center" rowspan="6"><code>T</code></td>
  <td><code>i</code></td>
  <td><code>member&lt;T,int,&amp;T::i></code></td>
  <td align="center">yes</td>
  <td align="center">yes</td>
</tr>
<tr>
  <td><code>j</code></td>
  <td><code>member&lt;T,const int,&amp;T::j></code></td>
  <td align="center">yes</td>
  <td align="center">no</td>
</tr>
<tr>
  <td><code>f()</code></td>
  <td><code>const_mem_fun&lt;T,int,&amp;T::f></code></td>
  <td align="center">yes</td>
  <td align="center">no</td>
</tr>
<tr>
  <td><code>g()</code></td>
  <td><code>mem_fun&lt;T,int,&amp;T::g></code></td>
  <td align="center">no</td>
  <td align="center">no</td>
</tr>
<tr>
  <td><code>gf()</code></td>
  <td><code>global_fun&lt;const T&amp;,int,&amp;T::gf></code></td>
  <td align="center">yes</td>
  <td align="center">no</td>
</tr>
<tr>
  <td><code>gg()</code></td>
  <td><code>global_fun&lt;T&amp;,int,&amp;T::gg></code></td>
  <td align="center">no</td>
  <td align="center">no</td>
</tr>

<tr class="odd_tr">
  <td align="center" rowspan="6"><code>reference_wrapper&lt;T></code></td>
  <td><code>i</code></td>
  <td><code>member&lt;T,int,&amp;T::i></code></td>
  <td align="center">yes</td>
  <td align="center">yes</td>
</tr>
<tr class="odd_tr">
  <td><code>j</code></td>
  <td><code>member&lt;T,const int,&amp;T::j></code></td>
  <td align="center">yes</td>
  <td align="center">no</td>
</tr>
<tr class="odd_tr">
  <td><code>f()</code></td>
  <td><code>const_mem_fun&lt;T,int,&amp;T::f></code></td>
  <td align="center">yes</td>
  <td align="center">no</td>
</tr>
<tr class="odd_tr">
  <td><code>g()</code></td>
  <td><code>mem_fun&lt;T,int,&amp;T::g></code></td>
  <td align="center">yes</td>
  <td align="center">no</td>
</tr>
<tr class="odd_tr">
  <td><code>gf()</code></td>
  <td><code>global_fun&lt;const T&amp;,int,&amp;T::gf></code></td>
  <td align="center">yes</td>
  <td align="center">no</td>
</tr>
<tr class="odd_tr">
  <td><code>gg()</code></td>
  <td><code>global_fun&lt;T&amp;,int,&amp;T::gg></code></td>
  <td align="center">yes</td>
  <td align="center">no</td>
</tr>

<tr>
  <td align="center" rowspan="6"><code>reference_wrapper&lt;const T></code></td>
  <td><code>i</code></td>
  <td><code>member&lt;T,const int,&amp;T::i></code></td>
  <td align="center">yes</td>
  <td align="center">no</td>
</tr>
<tr>
  <td><code>j</code></td>
  <td><code>member&lt;T,const int,&amp;T::j></code></td>
  <td align="center">yes</td>
  <td align="center">no</td>
</tr>
<tr>
  <td><code>f()</code></td>
  <td><code>const_mem_fun&lt;T,int,&amp;T::f></code></td>
  <td align="center">yes</td>
  <td align="center">no</td>
</tr>
<tr>
  <td><code>g()</code></td>
  <td colspan="3">&nbsp;</td>
</tr>
<tr>
  <td><code>gf()</code></td>
  <td><code>global_fun&lt;const T&amp;,int,&amp;T::gf></code></td>
  <td align="center">yes</td>
  <td align="center">no</td>
</tr>
<tr>
  <td><code>gg()</code></td>
  <td colspan="3">&nbsp;</td>
</tr>

<tr class="odd_tr">
  <td align="center" rowspan="6">chained pointer to <code>T</code><br>
    or to <code>reference_wrapper&lt;T></code></td>
  <td><code>i</code></td>
  <td><code>member&lt;T,int,&amp;T::i></code></td>
  <td align="center">yes</td>
  <td align="center">yes</td>
</tr>
<tr class="odd_tr">
  <td><code>j</code></td>
  <td><code>member&lt;T,const int,&amp;T::j></code></td>
  <td align="center">yes</td>
  <td align="center">no</td>
</tr>
<tr class="odd_tr">
  <td><code>f()</code></td>
  <td><code>const_mem_fun&lt;T,int,&amp;T::f></code></td>
  <td align="center">yes</td>
  <td align="center">no</td>
</tr>
<tr class="odd_tr">
  <td><code>g()</code></td>
  <td><code>mem_fun&lt;T,int,&amp;T::g></code></td>
  <td align="center">yes</td>
  <td align="center">no</td>
</tr>
<tr class="odd_tr">
  <td><code>gf()</code></td>
  <td><code>global_fun&lt;const T&amp;,int,&amp;T::gf></code></td>
  <td align="center">yes</td>
  <td align="center">no</td>
</tr>
<tr class="odd_tr">
  <td><code>gg()</code></td>
  <td><code>global_fun&lt;T&amp;,int,&amp;T::gg></code></td>
  <td align="center">yes</td>
  <td align="center">no</td>
</tr>

<tr>
  <td align="center" rowspan="6">chained pointer to <code>const T</code><br>
    or to <code>reference_wrapper&lt;const T></code></td>
  <td><code>i</code></td>
  <td><code>member&lt;T,const int,&amp;T::i></code></td>
  <td align="center">yes</td>
  <td align="center">no</td>
</tr>
<tr>
  <td><code>j</code></td>
  <td><code>member&lt;T,const int,&amp;T::j></code></td>
  <td align="center">yes</td>
  <td align="center">no</td>
</tr>
<tr>
  <td><code>f()</code></td>
  <td><code>const_mem_fun&lt;T,int,&amp;T::f></code></td>
  <td align="center">yes</td>
  <td align="center">no</td>
</tr>
<tr>
  <td><code>g()</code></td>
  <td colspan="3">&nbsp;</td>
</tr>
<tr>
  <td><code>gf()</code></td>
  <td><code>global_fun&lt;const T&amp;,int,&amp;T::gf></code></td>
  <td align="center">yes</td>
  <td align="center">no</td>
</tr>
<tr>
  <td><code>gg()</code></td>
  <td colspan="3">&nbsp;</td>
</tr>

</table>
</p>

<p>
The column "applicable to <code>const</code> elements?" states whether the
corresponding key extractor can be used when passed constant elements (this
relates to the elements specified in the first column, not the referenced
<code>T</code> objects). The only negative cases are for <code>T::g</code> and
<code>T:gg</code> when the elements are raw <code>T</code> objects, which make sense
as we are dealing with a non-constant member function (<code>T::g</code>)
and a function taking <code>T</code> by
non-constant reference: this also implies that <code>multi_index_container</code>s
of elements of <code>T</code> cannot be sorted by <code>T::g</code> or <code>T::gg</code>, because
elements contained within a <code>multi_index_container</code> are treated as constant.
</p>

<p>
The column "read/write?" shows which combinations yield
<a href="#read_write_key_extractors">read/write key extractors</a>.
</p>

<p>
Some care has to be taken to preserve <code>const</code>-correctness in the
specification of <code>member</code> key extractors: in some sense, the <code>const</code>
qualifier is carried along to the member part, even if that particular
member is not defined as <code>const</code>. For instance, if the elements
are of type <code>const T *</code>, sorting by <code>T::i</code> is <i>not</i>
specified as <code>member&lt;const T,int,&amp;T::i></code>, but rather as
<code>member&lt;T,const int,&amp;T::i></code>.
</p>

<p>
For practical demonstrations of use of these key extractors, refer to
<a href="../examples.html#example2">example 2</a> and
<a href="../examples.html#example6">example 6</a> in the examples section.
</p>

<hr>

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<br>

<p>Revised June 11th 2019</p>

<p>&copy; Copyright 2003-2019 Joaqu&iacute;n M L&oacute;pez Mu&ntilde;oz.
Distributed under the Boost Software 
License, Version 1.0. (See accompanying file <a href="../../../../LICENSE_1_0.txt">
LICENSE_1_0.txt</a> or copy at <a href="http://www.boost.org/LICENSE_1_0.txt">
http://www.boost.org/LICENSE_1_0.txt</a>)
</p>

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