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  26 <div class="titlepage"><div><div><h2 class="title" style="clear: both">
  27 <a name="intrusive.advanced_lookups_insertions"></a><a class="link" href="advanced_lookups_insertions.html" title="Advanced lookup and insertion functions for associative containers">Advanced lookup
  28     and insertion functions for associative containers</a>
  29 </h2></div></div></div>
  30 <div class="toc"><dl class="toc">
  31 <dt><span class="section"><a href="advanced_lookups_insertions.html#intrusive.advanced_lookups_insertions.advanced_lookups">Advanced
  32       lookups</a></span></dt>
  33 <dt><span class="section"><a href="advanced_lookups_insertions.html#intrusive.advanced_lookups_insertions.advanced_insertions">Advanced
  34       insertions</a></span></dt>
  35 <dt><span class="section"><a href="advanced_lookups_insertions.html#intrusive.advanced_lookups_insertions.positional_insertions">Positional
  36       insertions</a></span></dt>
  37 </dl></div>
  38 <div class="section">
  39 <div class="titlepage"><div><div><h3 class="title">
  40 <a name="intrusive.advanced_lookups_insertions.advanced_lookups"></a><a class="link" href="advanced_lookups_insertions.html#intrusive.advanced_lookups_insertions.advanced_lookups" title="Advanced lookups">Advanced
  41       lookups</a>
  42 </h3></div></div></div>
  43 <p>
  44         <span class="bold"><strong>Boost.Intrusive</strong></span> associative containers offer
  45         the same interface as STL associative containers. However, STL and TR1 ordered
  46         and unordered simple associative containers (<code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">set</span></code>,
  47         <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">multiset</span></code>, <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">tr1</span><span class="special">::</span><span class="identifier">unordered_set</span></code> and <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">tr1</span><span class="special">::</span><span class="identifier">unordered_multiset</span></code>) have some inefficiencies
  48         caused by the interface: the user can only operate with <code class="computeroutput"><span class="identifier">value_type</span></code>
  49         objects. When using these containers we must use <code class="computeroutput"><span class="identifier">iterator</span>
  50         <span class="identifier">find</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">value_type</span>
  51         <span class="special">&amp;</span><span class="identifier">value</span><span class="special">)</span></code> to find a value. The same happens in other
  52         functions like <code class="computeroutput"><span class="identifier">equal_range</span></code>,
  53         <code class="computeroutput"><span class="identifier">lower_bound</span></code>, <code class="computeroutput"><span class="identifier">upper_bound</span></code>, etc.
  54       </p>
  55 <p>
  56         However, sometimes the object to be searched is quite expensive to construct:
  57       </p>
  58 <p>
  59 </p>
  60 <pre class="programlisting"><span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">intrusive</span><span class="special">/</span><span class="identifier">set</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
  61 <span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">intrusive</span><span class="special">/</span><span class="identifier">unordered_set</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
  62 <span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">cstring</span><span class="special">&gt;</span>
  63
  64 <span class="keyword">using</span> <span class="keyword">namespace</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">intrusive</span><span class="special">;</span>
  65
  66 <span class="comment">// Hash function for strings</span>
  67 <span class="keyword">struct</span> <span class="identifier">StrHasher</span>
  68 <span class="special">{</span>
  69    <span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span class="keyword">operator</span><span class="special">()(</span><span class="keyword">const</span> <span class="keyword">char</span> <span class="special">*</span><span class="identifier">str</span><span class="special">)</span> <span class="keyword">const</span>
  70    <span class="special">{</span>
  71       <span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span class="identifier">seed</span> <span class="special">=</span> <span class="number">0</span><span class="special">;</span>
  72       <span class="keyword">for</span><span class="special">(;</span> <span class="special">*</span><span class="identifier">str</span><span class="special">;</span> <span class="special">++</span><span class="identifier">str</span><span class="special">)</span>   <span class="identifier">boost</span><span class="special">::</span><span class="identifier">hash_combine</span><span class="special">(</span><span class="identifier">seed</span><span class="special">,</span> <span class="special">*</span><span class="identifier">str</span><span class="special">);</span>
  73       <span class="keyword">return</span> <span class="identifier">seed</span><span class="special">;</span>
  74    <span class="special">}</span>
  75 <span class="special">};</span>
  76
  77 <span class="keyword">class</span> <span class="identifier">Expensive</span> <span class="special">:</span> <span class="keyword">public</span> <span class="identifier">set_base_hook</span><span class="special">&lt;&gt;,</span> <span class="keyword">public</span> <span class="identifier">unordered_set_base_hook</span><span class="special">&lt;&gt;</span>
  78 <span class="special">{</span>
  79    <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span> <span class="identifier">key_</span><span class="special">;</span>
  80    <span class="comment">// Other members...</span>
  81
  82    <span class="keyword">public</span><span class="special">:</span>
  83    <span class="identifier">Expensive</span><span class="special">(</span><span class="keyword">const</span> <span class="keyword">char</span> <span class="special">*</span><span class="identifier">key</span><span class="special">)</span>
  84       <span class="special">:</span>  <span class="identifier">key_</span><span class="special">(</span><span class="identifier">key</span><span class="special">)</span>
  85       <span class="special">{}</span>  <span class="comment">//other expensive initializations...</span>
  86
  87    <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">get_key</span><span class="special">()</span> <span class="keyword">const</span>
  88       <span class="special">{</span>  <span class="keyword">return</span> <span class="identifier">key_</span><span class="special">;</span>   <span class="special">}</span>
  89
  90    <span class="keyword">friend</span> <span class="keyword">bool</span> <span class="keyword">operator</span> <span class="special">&lt;</span>  <span class="special">(</span><span class="keyword">const</span> <span class="identifier">Expensive</span> <span class="special">&amp;</span><span class="identifier">a</span><span class="special">,</span> <span class="keyword">const</span> <span class="identifier">Expensive</span> <span class="special">&amp;</span><span class="identifier">b</span><span class="special">)</span>
  91       <span class="special">{</span>  <span class="keyword">return</span> <span class="identifier">a</span><span class="special">.</span><span class="identifier">key_</span> <span class="special">&lt;</span> <span class="identifier">b</span><span class="special">.</span><span class="identifier">key_</span><span class="special">;</span>  <span class="special">}</span>
  92
  93    <span class="keyword">friend</span> <span class="keyword">bool</span> <span class="keyword">operator</span> <span class="special">==</span> <span class="special">(</span><span class="keyword">const</span> <span class="identifier">Expensive</span> <span class="special">&amp;</span><span class="identifier">a</span><span class="special">,</span> <span class="keyword">const</span> <span class="identifier">Expensive</span> <span class="special">&amp;</span><span class="identifier">b</span><span class="special">)</span>
  94       <span class="special">{</span>  <span class="keyword">return</span> <span class="identifier">a</span><span class="special">.</span><span class="identifier">key_</span> <span class="special">==</span> <span class="identifier">b</span><span class="special">.</span><span class="identifier">key_</span><span class="special">;</span>  <span class="special">}</span>
  95
  96    <span class="keyword">friend</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span class="identifier">hash_value</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">Expensive</span> <span class="special">&amp;</span><span class="identifier">object</span><span class="special">)</span>
  97       <span class="special">{</span>  <span class="keyword">return</span> <span class="identifier">StrHasher</span><span class="special">()(</span><span class="identifier">object</span><span class="special">.</span><span class="identifier">get_key</span><span class="special">().</span><span class="identifier">c_str</span><span class="special">());</span>  <span class="special">}</span>
  98 <span class="special">};</span>
  99
 100 <span class="comment">// A set and unordered_set that store Expensive objects</span>
 101 <span class="keyword">typedef</span> <span class="identifier">set</span><span class="special">&lt;</span><span class="identifier">Expensive</span><span class="special">&gt;</span>           <span class="identifier">Set</span><span class="special">;</span>
 102 <span class="keyword">typedef</span> <span class="identifier">unordered_set</span><span class="special">&lt;</span><span class="identifier">Expensive</span><span class="special">&gt;</span> <span class="identifier">UnorderedSet</span><span class="special">;</span>
 103
 104 <span class="comment">// Search functions</span>
 105 <span class="identifier">Expensive</span> <span class="special">*</span><span class="identifier">get_from_set</span><span class="special">(</span><span class="keyword">const</span> <span class="keyword">char</span><span class="special">*</span> <span class="identifier">key</span><span class="special">,</span> <span class="identifier">Set</span> <span class="special">&amp;</span><span class="identifier">set_object</span><span class="special">)</span>
 106 <span class="special">{</span>
 107    <span class="identifier">Set</span><span class="special">::</span><span class="identifier">iterator</span> <span class="identifier">it</span> <span class="special">=</span> <span class="identifier">set_object</span><span class="special">.</span><span class="identifier">find</span><span class="special">(</span><span class="identifier">Expensive</span><span class="special">(</span><span class="identifier">key</span><span class="special">));</span>
 108    <span class="keyword">if</span><span class="special">(</span> <span class="identifier">it</span> <span class="special">==</span> <span class="identifier">set_object</span><span class="special">.</span><span class="identifier">end</span><span class="special">()</span> <span class="special">)</span>     <span class="keyword">return</span> <span class="number">0</span><span class="special">;</span>
 109    <span class="keyword">return</span> <span class="special">&amp;*</span><span class="identifier">it</span><span class="special">;</span>
 110 <span class="special">}</span>
 111
 112 <span class="identifier">Expensive</span> <span class="special">*</span><span class="identifier">get_from_uset</span><span class="special">(</span><span class="keyword">const</span> <span class="keyword">char</span><span class="special">*</span> <span class="identifier">key</span><span class="special">,</span> <span class="identifier">UnorderedSet</span> <span class="special">&amp;</span><span class="identifier">uset_object</span><span class="special">)</span>
 113 <span class="special">{</span>
 114    <span class="identifier">UnorderedSet</span><span class="special">::</span><span class="identifier">iterator</span> <span class="identifier">it</span> <span class="special">=</span> <span class="identifier">uset_object</span><span class="special">.</span><span class="identifier">find</span><span class="special">(</span><span class="identifier">Expensive</span> <span class="special">(</span><span class="identifier">key</span><span class="special">));</span>
 115    <span class="keyword">if</span><span class="special">(</span> <span class="identifier">it</span> <span class="special">==</span> <span class="identifier">uset_object</span><span class="special">.</span><span class="identifier">end</span><span class="special">()</span> <span class="special">)</span>  <span class="keyword">return</span> <span class="number">0</span><span class="special">;</span>
 116    <span class="keyword">return</span> <span class="special">&amp;*</span><span class="identifier">it</span><span class="special">;</span>
 117 <span class="special">}</span>
 118 </pre>
 119 <p>
 120       </p>
 121 <p>
 122         <code class="computeroutput"><span class="identifier">Expensive</span></code> is an expensive
 123         object to construct. If "key" c-string is quite long <code class="computeroutput"><span class="identifier">Expensive</span></code> has to construct a <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span></code>
 124         using heap memory. Like <code class="computeroutput"><span class="identifier">Expensive</span></code>,
 125         many times the only member taking part in ordering issues is just a small
 126         part of the class. For example, with <code class="computeroutput"><span class="identifier">Expensive</span></code>,
 127         only the internal <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span></code> is needed to compare the object.
 128       </p>
 129 <p>
 130         In both containers, if we call <code class="computeroutput"><span class="identifier">get_from_set</span><span class="special">/</span><span class="identifier">get_from_unordered_set</span></code>
 131         in a loop, we might get a performance penalty, because we are forced to create
 132         a whole <code class="computeroutput"><span class="identifier">Expensive</span></code> object
 133         to be able to find an equivalent one.
 134       </p>
 135 <p>
 136         Sometimes this interface limitation is severe, because we <span class="bold"><strong>might
 137         not have enough information to construct the object</strong></span> but we might
 138         <span class="bold"><strong>have enough information to find the object</strong></span>.
 139         In this case, a name is enough to search <code class="computeroutput"><span class="identifier">Expensive</span></code>
 140         in the container but constructing an <code class="computeroutput"><span class="identifier">Expensive</span></code>
 141         might require more information that the user might not have.
 142       </p>
 143 <p>
 144         To solve this, <code class="computeroutput"><a class="link" href="../boost/intrusive/set.html" title="Class template set">set</a></code>/<code class="computeroutput"><a class="link" href="../boost/intrusive/multiset.html" title="Class template multiset">multiset</a></code> offer alternative functions,
 145         which take any type comparable with the value and a functor that should be
 146         compatible with the ordering function of the associative container. <code class="computeroutput"><a class="link" href="../boost/intrusive/unordered_set.html" title="Class template unordered_set">unordered_set</a></code>/<code class="computeroutput"><a class="link" href="../boost/intrusive/unordered_multiset.html" title="Class template unordered_multiset">unordered_multiset</a></code>
 147         offers functions that take any key type and compatible hash and equality
 148         functions. Now, let's see the optimized search function:
 149       </p>
 150 <p>
 151 </p>
 152 <pre class="programlisting"><span class="comment">// These compare Expensive and a c-string</span>
 153 <span class="keyword">struct</span> <span class="identifier">StrExpComp</span>
 154 <span class="special">{</span>
 155    <span class="keyword">bool</span> <span class="keyword">operator</span><span class="special">()(</span><span class="keyword">const</span> <span class="keyword">char</span> <span class="special">*</span><span class="identifier">str</span><span class="special">,</span> <span class="keyword">const</span> <span class="identifier">Expensive</span> <span class="special">&amp;</span><span class="identifier">c</span><span class="special">)</span> <span class="keyword">const</span>
 156    <span class="special">{</span>  <span class="keyword">return</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">strcmp</span><span class="special">(</span><span class="identifier">str</span><span class="special">,</span> <span class="identifier">c</span><span class="special">.</span><span class="identifier">get_key</span><span class="special">().</span><span class="identifier">c_str</span><span class="special">())</span> <span class="special">&lt;</span> <span class="number">0</span><span class="special">;</span>  <span class="special">}</span>
 157
 158    <span class="keyword">bool</span> <span class="keyword">operator</span><span class="special">()(</span><span class="keyword">const</span> <span class="identifier">Expensive</span> <span class="special">&amp;</span><span class="identifier">c</span><span class="special">,</span> <span class="keyword">const</span> <span class="keyword">char</span> <span class="special">*</span><span class="identifier">str</span><span class="special">)</span> <span class="keyword">const</span>
 159    <span class="special">{</span>  <span class="keyword">return</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">strcmp</span><span class="special">(</span><span class="identifier">c</span><span class="special">.</span><span class="identifier">get_key</span><span class="special">().</span><span class="identifier">c_str</span><span class="special">(),</span> <span class="identifier">str</span><span class="special">)</span> <span class="special">&lt;</span> <span class="number">0</span><span class="special">;</span>  <span class="special">}</span>
 160 <span class="special">};</span>
 161
 162 <span class="keyword">struct</span> <span class="identifier">StrExpEqual</span>
 163 <span class="special">{</span>
 164    <span class="keyword">bool</span> <span class="keyword">operator</span><span class="special">()(</span><span class="keyword">const</span> <span class="keyword">char</span> <span class="special">*</span><span class="identifier">str</span><span class="special">,</span> <span class="keyword">const</span> <span class="identifier">Expensive</span> <span class="special">&amp;</span><span class="identifier">c</span><span class="special">)</span> <span class="keyword">const</span>
 165    <span class="special">{</span>  <span class="keyword">return</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">strcmp</span><span class="special">(</span><span class="identifier">str</span><span class="special">,</span> <span class="identifier">c</span><span class="special">.</span><span class="identifier">get_key</span><span class="special">().</span><span class="identifier">c_str</span><span class="special">())</span> <span class="special">==</span> <span class="number">0</span><span class="special">;</span>  <span class="special">}</span>
 166
 167    <span class="keyword">bool</span> <span class="keyword">operator</span><span class="special">()(</span><span class="keyword">const</span> <span class="identifier">Expensive</span> <span class="special">&amp;</span><span class="identifier">c</span><span class="special">,</span> <span class="keyword">const</span> <span class="keyword">char</span> <span class="special">*</span><span class="identifier">str</span><span class="special">)</span> <span class="keyword">const</span>
 168    <span class="special">{</span>  <span class="keyword">return</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">strcmp</span><span class="special">(</span><span class="identifier">c</span><span class="special">.</span><span class="identifier">get_key</span><span class="special">().</span><span class="identifier">c_str</span><span class="special">(),</span> <span class="identifier">str</span><span class="special">)</span> <span class="special">==</span> <span class="number">0</span><span class="special">;</span>  <span class="special">}</span>
 169 <span class="special">};</span>
 170
 171 <span class="comment">// Optimized search functions</span>
 172 <span class="identifier">Expensive</span> <span class="special">*</span><span class="identifier">get_from_set_optimized</span><span class="special">(</span><span class="keyword">const</span> <span class="keyword">char</span><span class="special">*</span> <span class="identifier">key</span><span class="special">,</span> <span class="identifier">Set</span> <span class="special">&amp;</span><span class="identifier">set_object</span><span class="special">)</span>
 173 <span class="special">{</span>
 174    <span class="identifier">Set</span><span class="special">::</span><span class="identifier">iterator</span> <span class="identifier">it</span> <span class="special">=</span> <span class="identifier">set_object</span><span class="special">.</span><span class="identifier">find</span><span class="special">(</span><span class="identifier">key</span><span class="special">,</span> <span class="identifier">StrExpComp</span><span class="special">());</span>
 175    <span class="keyword">if</span><span class="special">(</span> <span class="identifier">it</span> <span class="special">==</span> <span class="identifier">set_object</span><span class="special">.</span><span class="identifier">end</span><span class="special">()</span> <span class="special">)</span>   <span class="keyword">return</span> <span class="number">0</span><span class="special">;</span>
 176    <span class="keyword">return</span> <span class="special">&amp;*</span><span class="identifier">it</span><span class="special">;</span>
 177 <span class="special">}</span>
 178
 179 <span class="identifier">Expensive</span> <span class="special">*</span><span class="identifier">get_from_uset_optimized</span><span class="special">(</span><span class="keyword">const</span> <span class="keyword">char</span><span class="special">*</span> <span class="identifier">key</span><span class="special">,</span> <span class="identifier">UnorderedSet</span> <span class="special">&amp;</span><span class="identifier">uset_object</span><span class="special">)</span>
 180 <span class="special">{</span>
 181    <span class="identifier">UnorderedSet</span><span class="special">::</span><span class="identifier">iterator</span> <span class="identifier">it</span> <span class="special">=</span> <span class="identifier">uset_object</span><span class="special">.</span><span class="identifier">find</span><span class="special">(</span><span class="identifier">key</span><span class="special">,</span> <span class="identifier">StrHasher</span><span class="special">(),</span> <span class="identifier">StrExpEqual</span><span class="special">());</span>
 182    <span class="keyword">if</span><span class="special">(</span> <span class="identifier">it</span> <span class="special">==</span> <span class="identifier">uset_object</span><span class="special">.</span><span class="identifier">end</span><span class="special">()</span> <span class="special">)</span>  <span class="keyword">return</span> <span class="number">0</span><span class="special">;</span>
 183    <span class="keyword">return</span> <span class="special">&amp;*</span><span class="identifier">it</span><span class="special">;</span>
 184 <span class="special">}</span>
 185 </pre>
 186 <p>
 187       </p>
 188 <p>
 189         This new arbitrary key overload is also available for other functions taking
 190         values as arguments:
 191       </p>
 192 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
 193 <li class="listitem">
 194             equal_range
 195           </li>
 196 <li class="listitem">
 197             lower_bound
 198           </li>
 199 <li class="listitem">
 200             upper_bound
 201           </li>
 202 <li class="listitem">
 203             count
 204           </li>
 205 <li class="listitem">
 206             find
 207           </li>
 208 <li class="listitem">
 209             erase
 210           </li>
 211 </ul></div>
 212 <p>
 213         Check <code class="computeroutput"><a class="link" href="../boost/intrusive/set.html" title="Class template set">set</a></code>, <code class="computeroutput"><a class="link" href="../boost/intrusive/multiset.html" title="Class template multiset">multiset</a></code>, <code class="computeroutput"><a class="link" href="../boost/intrusive/unordered_set.html" title="Class template unordered_set">unordered_set</a></code>,
 214         <code class="computeroutput"><a class="link" href="../boost/intrusive/unordered_multiset.html" title="Class template unordered_multiset">unordered_multiset</a></code>
 215         references to know more about those functions.
 216       </p>
 217 </div>
 218 <div class="section">
 219 <div class="titlepage"><div><div><h3 class="title">
 220 <a name="intrusive.advanced_lookups_insertions.advanced_insertions"></a><a class="link" href="advanced_lookups_insertions.html#intrusive.advanced_lookups_insertions.advanced_insertions" title="Advanced insertions">Advanced
 221       insertions</a>
 222 </h3></div></div></div>
 223 <p>
 224         A similar issue happens with insertions in simple ordered and unordered associative
 225         containers with unique keys (<code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">set</span></code> and
 226         <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">tr1</span><span class="special">::</span><span class="identifier">unordered_set</span></code>).
 227         In these containers, if a value is already present, the value to be inserted
 228         is discarded. With expensive values, if the value is already present, we
 229         can suffer efficiency problems.
 230       </p>
 231 <p>
 232         <code class="computeroutput"><a class="link" href="../boost/intrusive/set.html" title="Class template set">set</a></code> and <code class="computeroutput"><a class="link" href="../boost/intrusive/unordered_set.html" title="Class template unordered_set">unordered_set</a></code>
 233         have insertion functions to check efficiently, without constructing the value,
 234         if a value is present or not and if it's not present, a function to insert
 235         it immediately without any further lookup. For example, using the same <code class="computeroutput"><span class="identifier">Expensive</span></code> class, this function can be inefficient:
 236       </p>
 237 <p>
 238 </p>
 239 <pre class="programlisting"><span class="comment">// Insertion functions</span>
 240 <span class="keyword">bool</span> <span class="identifier">insert_to_set</span><span class="special">(</span><span class="keyword">const</span> <span class="keyword">char</span><span class="special">*</span> <span class="identifier">key</span><span class="special">,</span> <span class="identifier">Set</span> <span class="special">&amp;</span><span class="identifier">set_object</span><span class="special">)</span>
 241 <span class="special">{</span>
 242    <span class="identifier">Expensive</span> <span class="special">*</span><span class="identifier">pobject</span> <span class="special">=</span> <span class="keyword">new</span> <span class="identifier">Expensive</span><span class="special">(</span><span class="identifier">key</span><span class="special">);</span>
 243    <span class="keyword">bool</span> <span class="identifier">success</span> <span class="special">=</span> <span class="identifier">set_object</span><span class="special">.</span><span class="identifier">insert</span><span class="special">(*</span><span class="identifier">pobject</span><span class="special">).</span><span class="identifier">second</span><span class="special">;</span>
 244    <span class="keyword">if</span><span class="special">(!</span><span class="identifier">success</span><span class="special">)</span>   <span class="keyword">delete</span> <span class="identifier">pobject</span><span class="special">;</span>
 245    <span class="keyword">return</span> <span class="identifier">success</span><span class="special">;</span>
 246 <span class="special">}</span>
 247
 248 <span class="keyword">bool</span> <span class="identifier">insert_to_uset</span><span class="special">(</span><span class="keyword">const</span> <span class="keyword">char</span><span class="special">*</span> <span class="identifier">key</span><span class="special">,</span> <span class="identifier">UnorderedSet</span> <span class="special">&amp;</span><span class="identifier">uset_object</span><span class="special">)</span>
 249 <span class="special">{</span>
 250    <span class="identifier">Expensive</span> <span class="special">*</span><span class="identifier">pobject</span> <span class="special">=</span> <span class="keyword">new</span> <span class="identifier">Expensive</span><span class="special">(</span><span class="identifier">key</span><span class="special">);</span>
 251    <span class="keyword">bool</span> <span class="identifier">success</span> <span class="special">=</span> <span class="identifier">uset_object</span><span class="special">.</span><span class="identifier">insert</span><span class="special">(*</span><span class="identifier">pobject</span><span class="special">).</span><span class="identifier">second</span><span class="special">;</span>
 252    <span class="keyword">if</span><span class="special">(!</span><span class="identifier">success</span><span class="special">)</span>   <span class="keyword">delete</span> <span class="identifier">pobject</span><span class="special">;</span>
 253    <span class="keyword">return</span> <span class="identifier">success</span><span class="special">;</span>
 254 <span class="special">}</span>
 255 </pre>
 256 <p>
 257       </p>
 258 <p>
 259         If the object is already present, we are constructing an <code class="computeroutput"><span class="identifier">Expensive</span></code>
 260         that will be discarded, and this is a waste of resources. Instead of that,
 261         let's use <code class="computeroutput"><span class="identifier">insert_check</span></code> and
 262         <code class="computeroutput"><span class="identifier">insert_commit</span></code> functions:
 263       </p>
 264 <p>
 265 </p>
 266 <pre class="programlisting"><span class="comment">// Optimized insertion functions</span>
 267 <span class="keyword">bool</span> <span class="identifier">insert_to_set_optimized</span><span class="special">(</span><span class="keyword">const</span> <span class="keyword">char</span><span class="special">*</span> <span class="identifier">key</span><span class="special">,</span> <span class="identifier">Set</span> <span class="special">&amp;</span><span class="identifier">set_object</span><span class="special">)</span>
 268 <span class="special">{</span>
 269    <span class="identifier">Set</span><span class="special">::</span><span class="identifier">insert_commit_data</span> <span class="identifier">insert_data</span><span class="special">;</span>
 270    <span class="keyword">bool</span> <span class="identifier">success</span> <span class="special">=</span> <span class="identifier">set_object</span><span class="special">.</span><span class="identifier">insert_check</span><span class="special">(</span><span class="identifier">key</span><span class="special">,</span> <span class="identifier">StrExpComp</span><span class="special">(),</span> <span class="identifier">insert_data</span><span class="special">).</span><span class="identifier">second</span><span class="special">;</span>
 271    <span class="keyword">if</span><span class="special">(</span><span class="identifier">success</span><span class="special">)</span> <span class="identifier">set_object</span><span class="special">.</span><span class="identifier">insert_commit</span><span class="special">(*</span><span class="keyword">new</span> <span class="identifier">Expensive</span><span class="special">(</span><span class="identifier">key</span><span class="special">),</span> <span class="identifier">insert_data</span><span class="special">);</span>
 272    <span class="keyword">return</span> <span class="identifier">success</span><span class="special">;</span>
 273 <span class="special">}</span>
 274
 275 <span class="keyword">bool</span> <span class="identifier">insert_to_uset_optimized</span><span class="special">(</span><span class="keyword">const</span> <span class="keyword">char</span><span class="special">*</span> <span class="identifier">key</span><span class="special">,</span> <span class="identifier">UnorderedSet</span> <span class="special">&amp;</span><span class="identifier">uset_object</span><span class="special">)</span>
 276 <span class="special">{</span>
 277    <span class="identifier">UnorderedSet</span><span class="special">::</span><span class="identifier">insert_commit_data</span> <span class="identifier">insert_data</span><span class="special">;</span>
 278    <span class="keyword">bool</span> <span class="identifier">success</span> <span class="special">=</span> <span class="identifier">uset_object</span><span class="special">.</span><span class="identifier">insert_check</span>
 279       <span class="special">(</span><span class="identifier">key</span><span class="special">,</span> <span class="identifier">StrHasher</span><span class="special">(),</span> <span class="identifier">StrExpEqual</span><span class="special">(),</span> <span class="identifier">insert_data</span><span class="special">).</span><span class="identifier">second</span><span class="special">;</span>
 280    <span class="keyword">if</span><span class="special">(</span><span class="identifier">success</span><span class="special">)</span> <span class="identifier">uset_object</span><span class="special">.</span><span class="identifier">insert_commit</span><span class="special">(*</span><span class="keyword">new</span> <span class="identifier">Expensive</span><span class="special">(</span><span class="identifier">key</span><span class="special">),</span> <span class="identifier">insert_data</span><span class="special">);</span>
 281    <span class="keyword">return</span> <span class="identifier">success</span><span class="special">;</span>
 282 <span class="special">}</span>
 283 </pre>
 284 <p>
 285       </p>
 286 <p>
 287         <code class="computeroutput"><span class="identifier">insert_check</span></code> is similar to
 288         a normal <code class="computeroutput"><span class="identifier">insert</span></code> but:
 289       </p>
 290 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
 291 <li class="listitem">
 292             <code class="computeroutput"><span class="identifier">insert_check</span></code> can be used
 293             with arbitrary keys
 294           </li>
 295 <li class="listitem">
 296             if the insertion is possible (there is no equivalent value) <code class="computeroutput"><span class="identifier">insert_check</span></code> collects all the needed
 297             information in an <code class="computeroutput"><span class="identifier">insert_commit_data</span></code>
 298             structure, so that <code class="computeroutput"><span class="identifier">insert_commit</span></code>:
 299             <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: circle; ">
 300 <li class="listitem">
 301                   <span class="bold"><strong>does not execute</strong></span> further comparisons
 302                 </li>
 303 <li class="listitem">
 304                   can be executed with <span class="bold"><strong>constant-time complexity</strong></span>
 305                 </li>
 306 <li class="listitem">
 307                   has <span class="bold"><strong>no-throw guarantee</strong></span>.
 308                 </li>
 309 </ul></div>
 310           </li>
 311 </ul></div>
 312 <p>
 313         These functions must be used with care, since no other insertion or erasure
 314         must be executed between an <code class="computeroutput"><span class="identifier">insert_check</span></code>
 315         and an <code class="computeroutput"><span class="identifier">insert_commit</span></code> pair.
 316         Otherwise, the behaviour is undefined. <code class="computeroutput"><span class="identifier">insert_check</span></code>
 317         and <code class="computeroutput"><span class="identifier">insert_commit</span></code> will come
 318         in handy for developers programming efficient non-intrusive associative containers.
 319         See <code class="computeroutput"><a class="link" href="../boost/intrusive/set.html" title="Class template set">set</a></code> and <code class="computeroutput"><a class="link" href="../boost/intrusive/unordered_set.html" title="Class template unordered_set">unordered_set</a></code> reference
 320         for more information about <code class="computeroutput"><span class="identifier">insert_check</span></code>
 321         and <code class="computeroutput"><span class="identifier">insert_commit</span></code>.
 322       </p>
 323 <p>
 324         With multiple ordered and unordered associative containers (<code class="computeroutput"><a class="link" href="../boost/intrusive/multiset.html" title="Class template multiset">multiset</a></code>
 325         and <code class="computeroutput"><a class="link" href="../boost/intrusive/unordered_multiset.html" title="Class template unordered_multiset">unordered_multiset</a></code>)
 326         there is no need for these advanced insertion functions, since insertions
 327         are always successful.
 328       </p>
 329 </div>
 330 <div class="section">
 331 <div class="titlepage"><div><div><h3 class="title">
 332 <a name="intrusive.advanced_lookups_insertions.positional_insertions"></a><a class="link" href="advanced_lookups_insertions.html#intrusive.advanced_lookups_insertions.positional_insertions" title="Positional insertions">Positional
 333       insertions</a>
 334 </h3></div></div></div>
 335 <p>
 336         Some ordered associative containers offer low-level functions to bypass ordering
 337         checks and insert nodes directly in desired tree positions. These functions
 338         are provided for performance reasons when values to be inserted in the container
 339         are known to fulfill order (sets and multisets) and uniqueness (sets) invariants.
 340         A typical usage of these functions is when intrusive associative containers
 341         are used to build non-intrusive containers and the programmer wants to speed
 342         up assignments from other associative containers: if the ordering and uniqueness
 343         properties are the same, there is no need to waste time checking the position
 344         of each source value, because values are already ordered: back insertions
 345         will be much more efficient.
 346       </p>
 347 <p>
 348         <span class="bold"><strong>Note:</strong></span> These functions <span class="bold"><strong>don't
 349         check preconditions</strong></span> so they must used with care. These are functions
 350         are low-level operations <span class="bold"><strong>will break container invariants
 351         if ordering and uniqueness preconditions are not assured by the caller.</strong></span>
 352       </p>
 353 <p>
 354         Let's see an example:
 355       </p>
 356 <p>
 357 </p>
 358 <pre class="programlisting"><span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">intrusive</span><span class="special">/</span><span class="identifier">set</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
 359 <span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">vector</span><span class="special">&gt;</span>
 360 <span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">algorithm</span><span class="special">&gt;</span>
 361 <span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">cassert</span><span class="special">&gt;</span>
 362
 363 <span class="keyword">using</span> <span class="keyword">namespace</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">intrusive</span><span class="special">;</span>
 364
 365 <span class="comment">//A simple class with a set hook</span>
 366 <span class="keyword">class</span> <span class="identifier">MyClass</span> <span class="special">:</span> <span class="keyword">public</span> <span class="identifier">set_base_hook</span><span class="special">&lt;&gt;</span>
 367 <span class="special">{</span>
 368    <span class="keyword">public</span><span class="special">:</span>
 369    <span class="keyword">int</span> <span class="identifier">int_</span><span class="special">;</span>
 370
 371    <span class="identifier">MyClass</span><span class="special">(</span><span class="keyword">int</span> <span class="identifier">i</span><span class="special">)</span> <span class="special">:</span>  <span class="identifier">int_</span><span class="special">(</span><span class="identifier">i</span><span class="special">)</span> <span class="special">{}</span>
 372    <span class="keyword">friend</span> <span class="keyword">bool</span> <span class="keyword">operator</span><span class="special">&lt;</span> <span class="special">(</span><span class="keyword">const</span> <span class="identifier">MyClass</span> <span class="special">&amp;</span><span class="identifier">a</span><span class="special">,</span> <span class="keyword">const</span> <span class="identifier">MyClass</span> <span class="special">&amp;</span><span class="identifier">b</span><span class="special">)</span>
 373       <span class="special">{</span>  <span class="keyword">return</span> <span class="identifier">a</span><span class="special">.</span><span class="identifier">int_</span> <span class="special">&lt;</span> <span class="identifier">b</span><span class="special">.</span><span class="identifier">int_</span><span class="special">;</span>  <span class="special">}</span>
 374    <span class="keyword">friend</span> <span class="keyword">bool</span> <span class="keyword">operator</span><span class="special">&gt;</span> <span class="special">(</span><span class="keyword">const</span> <span class="identifier">MyClass</span> <span class="special">&amp;</span><span class="identifier">a</span><span class="special">,</span> <span class="keyword">const</span> <span class="identifier">MyClass</span> <span class="special">&amp;</span><span class="identifier">b</span><span class="special">)</span>
 375       <span class="special">{</span>  <span class="keyword">return</span> <span class="identifier">a</span><span class="special">.</span><span class="identifier">int_</span> <span class="special">&gt;</span> <span class="identifier">b</span><span class="special">.</span><span class="identifier">int_</span><span class="special">;</span>  <span class="special">}</span>
 376 <span class="special">};</span>
 377
 378 <span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span>
 379 <span class="special">{</span>
 380    <span class="comment">//Create some ORDERED elements</span>
 381    <span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special">&lt;</span><span class="identifier">MyClass</span><span class="special">&gt;</span> <span class="identifier">values</span><span class="special">;</span>
 382    <span class="keyword">for</span><span class="special">(</span><span class="keyword">int</span> <span class="identifier">i</span> <span class="special">=</span> <span class="number">0</span><span class="special">;</span> <span class="identifier">i</span> <span class="special">&lt;</span> <span class="number">100</span><span class="special">;</span> <span class="special">++</span><span class="identifier">i</span><span class="special">)</span>  <span class="identifier">values</span><span class="special">.</span><span class="identifier">push_back</span><span class="special">(</span><span class="identifier">MyClass</span><span class="special">(</span><span class="identifier">i</span><span class="special">));</span>
 383
 384    <span class="special">{</span>  <span class="comment">//Data is naturally ordered in the vector with the same criteria</span>
 385       <span class="comment">//as multiset's comparison predicate, so we can just push back</span>
 386       <span class="comment">//all elements, which is more efficient than normal insertion</span>
 387       <span class="identifier">multiset</span><span class="special">&lt;</span><span class="identifier">MyClass</span><span class="special">&gt;</span> <span class="identifier">mset</span><span class="special">;</span>
 388       <span class="keyword">for</span><span class="special">(</span><span class="keyword">int</span> <span class="identifier">i</span> <span class="special">=</span> <span class="number">0</span><span class="special">;</span> <span class="identifier">i</span> <span class="special">&lt;</span> <span class="number">100</span><span class="special">;</span> <span class="special">++</span><span class="identifier">i</span><span class="special">)</span>  <span class="identifier">mset</span><span class="special">.</span><span class="identifier">push_back</span><span class="special">(</span><span class="identifier">values</span><span class="special">[</span><span class="identifier">i</span><span class="special">]);</span>
 389
 390       <span class="comment">//Now check orderd invariant</span>
 391       <span class="identifier">multiset</span><span class="special">&lt;</span><span class="identifier">MyClass</span><span class="special">&gt;::</span><span class="identifier">const_iterator</span> <span class="identifier">next</span><span class="special">(</span><span class="identifier">mset</span><span class="special">.</span><span class="identifier">cbegin</span><span class="special">()),</span> <span class="identifier">it</span><span class="special">(</span><span class="identifier">next</span><span class="special">++);</span>
 392       <span class="keyword">for</span><span class="special">(</span><span class="keyword">int</span> <span class="identifier">i</span> <span class="special">=</span> <span class="number">0</span><span class="special">;</span> <span class="identifier">i</span> <span class="special">&lt;</span> <span class="number">99</span><span class="special">;</span> <span class="special">++</span><span class="identifier">i</span><span class="special">,</span> <span class="special">++</span><span class="identifier">it</span><span class="special">,</span> <span class="special">++</span><span class="identifier">next</span><span class="special">)</span> <span class="identifier">assert</span><span class="special">(*</span><span class="identifier">it</span> <span class="special">&lt;</span> <span class="special">*</span><span class="identifier">next</span><span class="special">);</span>
 393    <span class="special">}</span>
 394    <span class="special">{</span>  <span class="comment">//Now the correct order for the set is the reverse order</span>
 395       <span class="comment">//so let's push front all elements</span>
 396       <span class="identifier">multiset</span><span class="special">&lt;</span><span class="identifier">MyClass</span><span class="special">,</span> <span class="identifier">compare</span><span class="special">&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">greater</span><span class="special">&lt;</span><span class="identifier">MyClass</span><span class="special">&gt;</span> <span class="special">&gt;</span> <span class="special">&gt;</span> <span class="identifier">mset</span><span class="special">;</span>
 397       <span class="keyword">for</span><span class="special">(</span><span class="keyword">int</span> <span class="identifier">i</span> <span class="special">=</span> <span class="number">0</span><span class="special">;</span> <span class="identifier">i</span> <span class="special">&lt;</span> <span class="number">100</span><span class="special">;</span> <span class="special">++</span><span class="identifier">i</span><span class="special">)</span>  <span class="identifier">mset</span><span class="special">.</span><span class="identifier">push_front</span><span class="special">(</span><span class="identifier">values</span><span class="special">[</span><span class="identifier">i</span><span class="special">]);</span>
 398
 399       <span class="comment">//Now check orderd invariant</span>
 400       <span class="identifier">multiset</span><span class="special">&lt;</span><span class="identifier">MyClass</span><span class="special">,</span> <span class="identifier">compare</span><span class="special">&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">greater</span><span class="special">&lt;</span><span class="identifier">MyClass</span><span class="special">&gt;</span> <span class="special">&gt;</span> <span class="special">&gt;::</span>
 401          <span class="identifier">const_iterator</span> <span class="identifier">next</span><span class="special">(</span><span class="identifier">mset</span><span class="special">.</span><span class="identifier">cbegin</span><span class="special">()),</span> <span class="identifier">it</span><span class="special">(</span><span class="identifier">next</span><span class="special">++);</span>
 402       <span class="keyword">for</span><span class="special">(</span><span class="keyword">int</span> <span class="identifier">i</span> <span class="special">=</span> <span class="number">0</span><span class="special">;</span> <span class="identifier">i</span> <span class="special">&lt;</span> <span class="number">99</span><span class="special">;</span> <span class="special">++</span><span class="identifier">i</span><span class="special">,</span> <span class="special">++</span><span class="identifier">it</span><span class="special">,</span> <span class="special">++</span><span class="identifier">next</span><span class="special">)</span> <span class="identifier">assert</span><span class="special">(*</span><span class="identifier">it</span> <span class="special">&gt;</span> <span class="special">*</span><span class="identifier">next</span><span class="special">);</span>
 403    <span class="special">}</span>
 404    <span class="special">{</span>  <span class="comment">//Now push the first and the last and insert the rest</span>
 405       <span class="comment">//before the last position using "insert_before"</span>
 406       <span class="identifier">multiset</span><span class="special">&lt;</span><span class="identifier">MyClass</span><span class="special">&gt;</span> <span class="identifier">mset</span><span class="special">;</span>
 407       <span class="identifier">mset</span><span class="special">.</span><span class="identifier">insert_before</span><span class="special">(</span><span class="identifier">mset</span><span class="special">.</span><span class="identifier">begin</span><span class="special">(),</span> <span class="identifier">values</span><span class="special">[</span><span class="number">0</span><span class="special">]);</span>
 408       <span class="identifier">multiset</span><span class="special">&lt;</span><span class="identifier">MyClass</span><span class="special">&gt;::</span><span class="identifier">const_iterator</span> <span class="identifier">pos</span> <span class="special">=</span>
 409          <span class="identifier">mset</span><span class="special">.</span><span class="identifier">insert_before</span><span class="special">(</span><span class="identifier">mset</span><span class="special">.</span><span class="identifier">end</span><span class="special">(),</span> <span class="identifier">values</span><span class="special">[</span><span class="number">99</span><span class="special">]);</span>
 410       <span class="keyword">for</span><span class="special">(</span><span class="keyword">int</span> <span class="identifier">i</span> <span class="special">=</span> <span class="number">1</span><span class="special">;</span> <span class="identifier">i</span> <span class="special">&lt;</span> <span class="number">99</span><span class="special">;</span> <span class="special">++</span><span class="identifier">i</span><span class="special">)</span>  <span class="identifier">mset</span><span class="special">.</span><span class="identifier">insert_before</span><span class="special">(</span><span class="identifier">pos</span><span class="special">,</span> <span class="identifier">values</span><span class="special">[</span><span class="identifier">i</span><span class="special">]);</span>
 411
 412       <span class="comment">//Now check orderd invariant</span>
 413       <span class="identifier">multiset</span><span class="special">&lt;</span><span class="identifier">MyClass</span><span class="special">&gt;::</span><span class="identifier">const_iterator</span> <span class="identifier">next</span><span class="special">(</span><span class="identifier">mset</span><span class="special">.</span><span class="identifier">cbegin</span><span class="special">()),</span> <span class="identifier">it</span><span class="special">(</span><span class="identifier">next</span><span class="special">++);</span>
 414       <span class="keyword">for</span><span class="special">(</span><span class="keyword">int</span> <span class="identifier">i</span> <span class="special">=</span> <span class="number">0</span><span class="special">;</span> <span class="identifier">i</span> <span class="special">&lt;</span> <span class="number">99</span><span class="special">;</span> <span class="special">++</span><span class="identifier">i</span><span class="special">,</span> <span class="special">++</span><span class="identifier">it</span><span class="special">,</span> <span class="special">++</span><span class="identifier">next</span><span class="special">)</span> <span class="identifier">assert</span><span class="special">(*</span><span class="identifier">it</span> <span class="special">&lt;</span> <span class="special">*</span><span class="identifier">next</span><span class="special">);</span>
 415    <span class="special">}</span>
 416
 417    <span class="keyword">return</span> <span class="number">0</span><span class="special">;</span>
 418 <span class="special">}</span>
 419 </pre>
 420 <p>
 421       </p>
 422 </div>
 423 <p>
 424       For more information about advanced lookup and insertion functions see associative
 425       containers' documentation (e.g. <code class="computeroutput"><a class="link" href="../boost/intrusive/set.html" title="Class template set">set</a></code>,
 426       <code class="computeroutput"><a class="link" href="../boost/intrusive/multiset.html" title="Class template multiset">multiset</a></code>, <code class="computeroutput"><a class="link" href="../boost/intrusive/unordered_set.html" title="Class template unordered_set">unordered_set</a></code> and <code class="computeroutput"><a class="link" href="../boost/intrusive/unordered_multiset.html" title="Class template unordered_multiset">unordered_multiset</a></code> references).
 427     </p>
 428 </div>
 429 <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
 430 <td align="left"></td>
 431 <td align="right"><div class="copyright-footer">Copyright &#169; 2005 Olaf Krzikalla<br>Copyright &#169; 2006-2013 Ion Gaztanaga<p>
 432         Distributed under the Boost Software License, Version 1.0. (See accompanying
 433         file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
 434       </p>
 435 </div></td>
 436 </tr></table>
 437 <hr>
 438 <div class="spirit-nav">
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