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33 <li><a href=
"#intro">Introduction
</a>
35 <li><a href=
"#read_write_key_extractors">Read/write key extractors
</a></li>
38 <li><a href=
"#predefined_key_extractors">Predefined key extractors
</a>
40 <li><a href=
"#identity"><code>identity
</code></a></li>
41 <li><a href=
"#member"><code>member
</code></a></li>
42 <li><a href=
"#const_mem_fun"><code>const_mem_fun
</code>
43 and
<code>mem_fun
</code></a></li>
44 <li><a href=
"#global_fun"><code>global_fun
</code></a></li>
47 <li><a href=
"#user_defined_key_extractors">User-defined key extractors
</a></li>
48 <li><a href=
"#composite_keys">Composite keys
</a>
50 <li><a href=
"#composite_keys_hash">Composite keys and hashed indices
</a></li>
53 <li><a href=
"#advanced_key_extractors">Advanced features of Boost.MultiIndex key
57 <h2><a name=
"intro">Introduction
</a></h2>
60 STL associative containers have a notion of key, albeit in a somewhat incipient
61 form. So, the keys of such containers are identified by a nested type
62 <code>key_type
</code>; for
<code>std::set
</code>s and
<code>std::multiset
</code>s,
63 <code>key_type
</code> coincides with
<code>value_type
</code>, i.e. the key is the
64 element itself.
<code>std::map
</code> and
<code>std::multimap
</code> manage
65 elements of type
<code>std::pair
<const Key,T
></code>, where the first
66 member is the key. In either case, the process of obtaining the key from a
67 given element is implicitly fixed and cannot be customized by the user.
71 Fixed key extraction mechanisms like those performed by STL associative
72 containers do not scale well in the context of Boost.MultiIndex, where
73 several indices share their
<code>value_type
</code> definition but
74 might feature completely different lookup semantics. For this reason,
75 Boost.MultiIndex formalizes the concept of a
76 <a href=
"../reference/key_extraction.html#key_extractors"><code>Key
77 Extractor
</code></a> in order to make it explicit and controllable
78 in the definition of key-based indices.
82 Intuitively speaking, a key extractor is a function object that accepts
83 a reference to an element and returns its associated key. The formal
84 concept also imposes some reasonable constraints about the stability
85 of the process, in the sense that extractors are assumed to
86 return the same key when passed the same element: this is in consonance
87 with the informal understanding that keys are actually some
"part"
88 of the element and do not depend on external data.
91 <h3><a name=
"read_write_key_extractors">Read/write key extractors
</a></h3>
94 A key extractor is called
<i>read/write
</i> if it returns a non-constant reference
95 to the key when passed a non-constant element, and it is called
<i>read-only
</i>
96 otherwise. Boost.MultiIndex requires that the key extractor be read/write
97 when using the
<code>modify_key
</code> member function of ordered and hashed
98 indices. In all other situations, read-only extractors suffice.
99 The section on
<a href=
"#advanced_key_extractors">advanced features
100 of Boost.MultiIndex key extractors
</a> details which of the predefined
101 key extractors are read/write.
104 <h2><a name=
"predefined_key_extractors">Predefined key extractors
</a></h2>
106 <h3><a name=
"identity"><code>identity
</code></a></h3>
109 The
<a href=
"../reference/key_extraction.html#identity"><code>identity
</code></a>
110 key extractor returns the entire base object as the associated key:
114 <span class=preprocessor
>#include
</span> <span class=special
><</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
>></span>
115 <span class=preprocessor
>#include
</span> <span class=special
><</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
>></span>
116 <span class=preprocessor
>#include
</span> <span class=special
><</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
>></span>
118 <span class=identifier
>multi_index_container
</span><span class=special
><</span>
119 <span class=keyword
>int
</span><span class=special
>,
</span>
120 <span class=identifier
>indexed_by
</span><span class=special
><</span>
121 <span class=identifier
>ordered_unique
</span><span class=special
><</span>
122 <span class=identifier
>identity
</span><span class=special
><</span><span class=keyword
>int
</span><span class=special
>></span> <span class=comment
>// the key is the entire element
</span>
123 <span class=special
>></span>
124 <span class=special
>></span>
125 <span class=special
>></span> <span class=identifier
>cont
</span><span class=special
>;
</span>
128 <h3><a name=
"member"><code>member
</code></a></h3>
131 <a href=
"../reference/key_extraction.html#member"><code>member
</code></a>
132 key extractors return a reference to a specified
133 data field of the base object. For instance, in the following version of our
134 familiar employee container:
138 <span class=preprocessor
>#include
</span> <span class=special
><</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
>></span>
139 <span class=preprocessor
>#include
</span> <span class=special
><</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
>></span>
140 <span class=preprocessor
>#include
</span> <span class=special
><</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
>></span>
141 <span class=preprocessor
>#include
</span> <span class=special
><</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
>></span>
143 <span class=keyword
>typedef
</span> <span class=identifier
>multi_index_container
</span><span class=special
><</span>
144 <span class=identifier
>employee
</span><span class=special
>,
</span>
145 <span class=identifier
>indexed_by
</span><span class=special
><</span>
146 <span class=identifier
>ordered_unique
</span><span class=special
><</span><span class=identifier
>identity
</span><span class=special
><</span><span class=identifier
>employee
</span><span class=special
>></span> <span class=special
>>,
</span>
147 <span class=identifier
>ordered_non_unique
</span><span class=special
><</span><span class=identifier
>member
</span><span class=special
><</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
>,
&</span><span class=identifier
>employee
</span><span class=special
>::
</span><span class=identifier
>name
</span><span class=special
>></span> <span class=special
>>,
</span>
148 <span class=identifier
>ordered_unique
</span><span class=special
><</span><span class=identifier
>member
</span><span class=special
><</span><span class=identifier
>employee
</span><span class=special
>,
</span><span class=keyword
>int
</span><span class=special
>,
&</span><span class=identifier
>employee
</span><span class=special
>::
</span><span class=identifier
>ssnumber
</span><span class=special
>></span> <span class=special
>></span>
149 <span class=special
>></span>
150 <span class=special
>></span> <span class=identifier
>employee_set
</span><span class=special
>;
</span>
154 the second and third indices use
<code>member
</code> extractors on
155 <code>employee::name
</code> and
<code>employee::ssnumber
</code>, respectively.
156 The specification of an instantiation of
<code>member
</code> is simple
157 yet a little contrived:
161 <span class=identifier
>member
</span><span class=special
><</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
>></span>
165 It might seem that the first and second parameters are superfluous,
166 since the type of the base object and of the associated data field are
167 already implicit in the pointer to member argument: unfortunately, it is
168 not possible to extract this information with current C++ mechanisms,
169 which makes the syntax of
<code>member
</code> a little too verbose.
172 <h3><a name=
"const_mem_fun"><code>const_mem_fun
</code> and
<code>mem_fun
</code></a></h3>
175 Sometimes, the key of an index is not a concrete data member of the element,
176 but rather it is a value returned by a particular member function.
177 This resembles the notion of
<i>calculated indices
</i> supported by some
178 relational databases. Boost.MultiIndex supports this
179 kind of key extraction through
180 <a href=
"../reference/key_extraction.html#const_mem_fun"><code>const_mem_fun
</code></a>.
181 Consider the following container where sorting on the third index
182 is based upon the length of the name field:
186 <span class=preprocessor
>#include
</span> <span class=special
><</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
>></span>
187 <span class=preprocessor
>#include
</span> <span class=special
><</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
>></span>
188 <span class=preprocessor
>#include
</span> <span class=special
><</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
>></span>
189 <span class=preprocessor
>#include
</span> <span class=special
><</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
>></span>
190 <span class=preprocessor
>#include
</span> <span class=special
><</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
>></span>
192 <span class=keyword
>struct
</span> <span class=identifier
>employee
</span>
193 <span class=special
>{
</span>
194 <span class=keyword
>int
</span> <span class=identifier
>id
</span><span class=special
>;
</span>
195 <span class=identifier
>std
</span><span class=special
>::
</span><span class=identifier
>string
</span> <span class=identifier
>name
</span><span class=special
>;
</span>
197 <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
>&</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>
199 <span class=keyword
>bool
</span> <span class=keyword
>operator
</span><span class=special
><(
</span><span class=keyword
>const
</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
>id
</span><span class=special
><</span><span class=identifier
>e
</span><span class=special
>.
</span><span class=identifier
>id
</span><span class=special
>;}
</span>
201 <span class=comment
>// returns the length of the name field
</span>
202 <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>
203 <span class=special
>};
</span>
205 <span class=keyword
>typedef
</span> <span class=identifier
>multi_index_container
</span><span class=special
><</span>
206 <span class=identifier
>employee
</span><span class=special
>,
</span>
207 <span class=identifier
>indexed_by
</span><span class=special
><</span>
208 <span class=comment
>// sort by employee::operator
<</span>
209 <span class=identifier
>ordered_unique
</span><span class=special
><</span><span class=identifier
>identity
</span><span class=special
><</span><span class=identifier
>employee
</span><span class=special
>></span> <span class=special
>>,
</span>
211 <span class=comment
>// sort by less
<string
> on name
</span>
212 <span class=identifier
>ordered_non_unique
</span><span class=special
><</span><span class=identifier
>member
</span><span class=special
><</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
>,
&</span><span class=identifier
>employee
</span><span class=special
>::
</span><span class=identifier
>name
</span><span class=special
>></span> <span class=special
>>,
</span>
214 <span class=comment
>// sort by less
<int
> on name_length()
</span>
215 <span class=identifier
>ordered_non_unique
</span><span class=special
><</span>
216 <span class=identifier
>const_mem_fun
</span><span class=special
><</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
>,
&</span><span class=identifier
>employee
</span><span class=special
>::
</span><span class=identifier
>name_length
</span><span class=special
>></span>
217 <span class=special
>></span>
218 <span class=special
>></span>
219 <span class=special
>></span> <span class=identifier
>employee_set
</span><span class=special
>;
</span>
223 <code>const_mem_fun
</code> usage syntax is similar to that of
224 <a href=
"#member"><code>member
</code></a>:
228 <span class=identifier
>const_mem_fun
</span><span class=special
><</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
>></span>
232 The member function referred to must be
<code>const
</code>, take no arguments and return
233 a value of the specified key type.
234 Almost always you will want to use a
<code>const
</code> member function,
235 since elements in a
<code>multi_index_container
</code> are treated as constant, much
236 as elements of an
<code>std::set
</code>. However, a
237 <a href=
"../reference/key_extraction.html#mem_fun"><code>mem_fun
</code></a>
238 counterpart is provided for use with non-constant member functions, whose
239 applicability is discussed on the paragraph on
240 <a href=
"#advanced_key_extractors">advanced features
241 of Boost.MultiIndex key extractors
</a>.
244 <p><a href=
"../examples.html#example2">Example
2</a> in the examples section
245 provides a complete program showing how to use
<code>const_mem_fun
</code>.
248 <h3><a name=
"global_fun"><code>global_fun
</code></a></h3>
251 Whereas
<code>const_mem_fun
</code> and
<code>mem_fun
</code> are based on a
252 given member function of the base type from where the key is extracted,
253 <a href=
"../reference/key_extraction.html#global_fun"><code>global_fun
</code></a>
254 takes a global function (or static member function) accepting the base
255 type as its parameter and returning the key:
259 <span class=preprocessor
>#include
</span> <span class=special
><</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
>></span>
260 <span class=preprocessor
>#include
</span> <span class=special
><</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
>></span>
261 <span class=preprocessor
>#include
</span> <span class=special
><</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
>></span>
263 <span class=keyword
>struct
</span> <span class=identifier
>rectangle
</span>
264 <span class=special
>{
</span>
265 <span class=keyword
>int
</span> <span class=identifier
>x0
</span><span class=special
>,
</span><span class=identifier
>y0
</span><span class=special
>;
</span>
266 <span class=keyword
>int
</span> <span class=identifier
>x1
</span><span class=special
>,
</span><span class=identifier
>y1
</span><span class=special
>;
</span>
267 <span class=special
>};
</span>
269 <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
>&</span> <span class=identifier
>r
</span><span class=special
>)
</span>
270 <span class=special
>{
</span>
271 <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>
272 <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>
273 <span class=special
>}
</span>
275 <span class=keyword
>typedef
</span> <span class=identifier
>multi_index_container
</span><span class=special
><</span>
276 <span class=identifier
>rectangle
</span><span class=special
>,
</span>
277 <span class=identifier
>indexed_by
</span><span class=special
><</span>
278 <span class=comment
>// sort by increasing area
</span>
279 <span class=identifier
>ordered_non_unique
</span><span class=special
><</span><span class=identifier
>global_fun
</span><span class=special
><</span><span class=keyword
>const
</span> <span class=identifier
>rectangle
</span><span class=special
>&,
</span><span class=keyword
>unsigned
</span> <span class=keyword
>long
</span><span class=special
>,
&</span><span class=identifier
>area
</span><span class=special
>></span> <span class=special
>></span>
280 <span class=special
>></span>
281 <span class=special
>></span> <span class=identifier
>rectangle_container
</span><span class=special
>;
</span>
285 The specification of
<code>global_fun
</code> obeys the following syntax:
289 <span class=identifier
>global_fun
</span><span class=special
><</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
>></span>
293 where the argument type and key type must match
<i>exactly
</i> those in the
294 signature of the function used; for instance, in the example above the argument
295 type is
<code>const rectangle
&</code>, without omitting the
"<code>const</code>"
296 and
"<code>&</code>" parts. So, although most of the time the base type will be
297 accepted by constant reference,
<code>global_fun
</code> is also prepared to take
298 functions accepting their argument by value or by non-constant reference: this
299 latter case cannot generally be used directly in the specification of
300 <code>multi_index_container
</code>s as their elements are treated as constant,
301 but the section on
<a href=
"#advanced_key_extractors">advanced features
302 of Boost.MultiIndex key extractors
</a> describes valid use cases of
303 key extraction based on such functions with a non-constant reference argument.
306 <p><a href=
"../examples.html#example2">Example
2</a> in the examples section
307 uses
<code>gobal_fun
</code>.
310 <h2><a name=
"user_defined_key_extractors">User-defined key extractors
</a></h2>
313 Although the
<a href=
"#predefined_key_extractors">predefined key extractors
</a>
314 provided by Boost.MultiIndex are intended to serve most cases,
315 the user can also provide her own key extractors in more exotic situations,
316 as long as these conform to the
317 <a href=
"../reference/key_extraction.html#key_extractors"><code>Key
318 Extractor
</code></a> concept.
322 <span class=comment
>// some record class
</span>
323 <span class=keyword
>struct
</span> <span class=identifier
>record
</span>
324 <span class=special
>{
</span>
325 <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>
326 <span class=identifier
>std
</span><span class=special
>::
</span><span class=identifier
>string
</span> <span class=identifier
>str
</span><span class=special
>;
</span>
327 <span class=special
>};
</span>
329 <span class=comment
>// extracts a record's year
</span>
330 <span class=keyword
>struct
</span> <span class=identifier
>record_year
</span>
331 <span class=special
>{
</span>
332 <span class=comment
>// result_type typedef required by Key Extractor concept
</span>
333 <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>
335 <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
>&</span> <span class=identifier
>r
</span><span class=special
>)
</span><span class=keyword
>const
</span> <span class=comment
>// operator() must be const
</span>
336 <span class=special
>{
</span>
337 <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>
338 <span class=special
>}
</span>
339 <span class=special
>};
</span>
341 <span class=comment
>// example of use of the previous key extractor
</span>
342 <span class=keyword
>typedef
</span> <span class=identifier
>multi_index_container
</span><span class=special
><</span>
343 <span class=identifier
>record
</span><span class=special
>,
</span>
344 <span class=identifier
>indexed_by
</span><span class=special
><</span>
345 <span class=identifier
>ordered_non_unique
</span><span class=special
><</span><span class=identifier
>record_year
</span><span class=special
>></span> <span class=comment
>// sorted by record's year
</span>
346 <span class=special
>></span>
347 <span class=special
>></span> <span class=identifier
>record_log
</span><span class=special
>;
</span>
351 <a href=
"../examples.html#example6">Example
6</a> in the examples section
352 applies some user-defined key extractors in a complex scenario where
353 keys are accessed via pointers.
356 <h2><a name=
"composite_keys">Composite keys
</a></h2>
359 In relational databases, composite keys depend on two or more fields of a given table.
360 The analogous concept in Boost.MultiIndex is modeled by means of
361 <a href=
"../reference/key_extraction.html#composite_key">
362 <code>composite_key
</code></a>, as shown in the example:
366 <span class=preprocessor
>#include
</span> <span class=special
><</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
>></span>
367 <span class=preprocessor
>#include
</span> <span class=special
><</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
>></span>
368 <span class=preprocessor
>#include
</span> <span class=special
><</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
>></span>
369 <span class=preprocessor
>#include
</span> <span class=special
><</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
>></span>
371 <span class=keyword
>struct
</span> <span class=identifier
>phonebook_entry
</span>
372 <span class=special
>{
</span>
373 <span class=identifier
>std
</span><span class=special
>::
</span><span class=identifier
>string
</span> <span class=identifier
>family_name
</span><span class=special
>;
</span>
374 <span class=identifier
>std
</span><span class=special
>::
</span><span class=identifier
>string
</span> <span class=identifier
>given_name
</span><span class=special
>;
</span>
375 <span class=identifier
>std
</span><span class=special
>::
</span><span class=identifier
>string
</span> <span class=identifier
>phone_number
</span><span class=special
>;
</span>
377 <span class=identifier
>phonebook_entry
</span><span class=special
>(
</span>
378 <span class=identifier
>std
</span><span class=special
>::
</span><span class=identifier
>string
</span> <span class=identifier
>family_name
</span><span class=special
>,
</span>
379 <span class=identifier
>std
</span><span class=special
>::
</span><span class=identifier
>string
</span> <span class=identifier
>given_name
</span><span class=special
>,
</span>
380 <span class=identifier
>std
</span><span class=special
>::
</span><span class=identifier
>string
</span> <span class=identifier
>phone_number
</span><span class=special
>):
</span>
381 <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>
382 <span class=special
>{}
</span>
383 <span class=special
>};
</span>
385 <span class=comment
>// define a multi_index_container with a composite key on
386 // (family_name,given_name)
</span>
387 <span class=keyword
>typedef
</span> <span class=identifier
>multi_index_container
</span><span class=special
><</span>
388 <span class=identifier
>phonebook_entry
</span><span class=special
>,
</span>
389 <span class=identifier
>indexed_by
</span><span class=special
><</span>
390 <span class=comment
>//non-unique as some subscribers might have more than one number
</span>
391 <span class=identifier
>ordered_non_unique
</span><span class=special
><</span>
392 <span class=identifier
>composite_key
</span><span class=special
><</span>
393 <span class=identifier
>phonebook_entry
</span><span class=special
>,
</span>
394 <span class=identifier
>member
</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=special
>,
&</span><span class=identifier
>phonebook_entry
</span><span class=special
>::
</span><span class=identifier
>family_name
</span><span class=special
>>,
</span>
395 <span class=identifier
>member
</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=special
>,
&</span><span class=identifier
>phonebook_entry
</span><span class=special
>::
</span><span class=identifier
>given_name
</span><span class=special
>></span>
396 <span class=special
>></span>
397 <span class=special
>>,
</span>
398 <span class=identifier
>ordered_unique
</span><span class=special
><</span> <span class=comment
>// unique as numbers belong to only one subscriber
</span>
399 <span class=identifier
>member
</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=special
>,
&</span><span class=identifier
>phonebook_entry
</span><span class=special
>::
</span><span class=identifier
>phone_number
</span><span class=special
>></span>
400 <span class=special
>></span>
401 <span class=special
>></span>
402 <span class=special
>></span> <span class=identifier
>phonebook
</span><span class=special
>;
</span>
406 <code>composite_key
</code> accepts two or more key extractors on the same
407 value (here,
<code>phonebook_entry
</code>). Lookup operations on a composite
408 key are accomplished by passing tuples with the values searched:
412 <span class=identifier
>phonebook
</span> <span class=identifier
>pb
</span><span class=special
>;
</span>
413 <span class=special
>...
</span>
414 <span class=comment
>// search for Dorothea White's number
</span>
415 <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
>"White
"</span><span class=special
>,
</span><span class=string
>"Dorothea
"</span><span class=special
>));
</span>
416 <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
>-
></span><span class=identifier
>phone_number
</span><span class=special
>;
</span>
420 Composite keys are sorted by lexicographical order, i.e. sorting is performed
421 by the first key, then the second key if the first one is equal, etc. This
422 order allows for partial searches where only the first keys are specified:
426 <span class=identifier
>phonebook
</span> <span class=identifier
>pb
</span><span class=special
>;
</span>
427 <span class=special
>...
</span>
428 <span class=comment
>// look for all Whites
</span>
429 <span class=identifier
>std
</span><span class=special
>::
</span><span class=identifier
>pair
</span><span class=special
><</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
>></span> <span class=identifier
>p
</span><span class=special
>=
</span>
430 <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
>"White
"</span><span class=special
>));
</span>
434 As a notational convenience, when only the first key is specified it is possible
435 to pass the argument directly without including it into a tuple:
439 <span class=identifier
>phonebook
</span> <span class=identifier
>pb
</span><span class=special
>;
</span>
440 <span class=special
>...
</span>
441 <span class=comment
>// look for all Whites
</span>
442 <span class=identifier
>std
</span><span class=special
>::
</span><span class=identifier
>pair
</span><span class=special
><</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
>></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
>"White
"</span><span class=special
>);
</span>
446 On the other hand, partial searches without specifying the first keys are not
451 By default, the corresponding
<code>std::less
</code> predicate is used
452 for each subkey of a composite key. Alternate comparison predicates can
453 be specified with
<a href=
"../reference/key_extraction.html#composite_key_compare">
454 <code>composite_key_compare
</code></a>:
458 <span class=comment
>// phonebook with given names in reverse order
</span>
460 <span class=keyword
>typedef
</span> <span class=identifier
>multi_index_container
</span><span class=special
><</span>
461 <span class=identifier
>phonebook_entry
</span><span class=special
>,
</span>
462 <span class=identifier
>indexed_by
</span><span class=special
><</span>
463 <span class=identifier
>ordered_non_unique
</span><span class=special
><</span>
464 <span class=identifier
>composite_key
</span><span class=special
><</span>
465 <span class=identifier
>phonebook_entry
</span><span class=special
>,
</span>
466 <span class=identifier
>member
</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=special
>,
&</span><span class=identifier
>phonebook_entry
</span><span class=special
>::
</span><span class=identifier
>family_name
</span><span class=special
>>,
</span>
467 <span class=identifier
>member
</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=special
>,
&</span><span class=identifier
>phonebook_entry
</span><span class=special
>::
</span><span class=identifier
>given_name
</span><span class=special
>></span>
468 <span class=special
>>,
</span>
469 <span class=identifier
>composite_key_compare
</span><span class=special
><</span>
470 <span class=identifier
>std
</span><span class=special
>::
</span><span class=identifier
>less
</span><span class=special
><</span><span class=identifier
>std
</span><span class=special
>::
</span><span class=identifier
>string
</span><span class=special
>>,
</span> <span class=comment
>// family names sorted as by default
</span>
471 <span class=identifier
>std
</span><span class=special
>::
</span><span class=identifier
>greater
</span><span class=special
><</span><span class=identifier
>std
</span><span class=special
>::
</span><span class=identifier
>string
</span><span class=special
>></span> <span class=comment
>// given names reversed
</span>
472 <span class=special
>></span>
473 <span class=special
>>,
</span>
474 <span class=identifier
>ordered_unique
</span><span class=special
><</span>
475 <span class=identifier
>member
</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=special
>,
&</span><span class=identifier
>phonebook_entry
</span><span class=special
>::
</span><span class=identifier
>phone_number
</span><span class=special
>></span>
476 <span class=special
>></span>
477 <span class=special
>></span>
478 <span class=special
>></span> <span class=identifier
>phonebook
</span><span class=special
>;
</span>
482 See
<a href=
"../examples.html#example7">example
7</a> in the examples section
483 for an application of
<code>composite_key
</code>.
486 <h3><a name=
"composite_keys_hash">Composite keys and hashed indices
</a></h3>
489 Composite keys can also be used with hashed indices in a straightforward manner:
493 <span class=keyword
>struct
</span> <span class=identifier
>street_entry
</span>
494 <span class=special
>{
</span>
495 <span class=comment
>// quadrant coordinates
</span>
496 <span class=keyword
>int
</span> <span class=identifier
>x
</span><span class=special
>;
</span>
497 <span class=keyword
>int
</span> <span class=identifier
>y
</span><span class=special
>;
</span>
499 <span class=identifier
>std
</span><span class=special
>::
</span><span class=identifier
>string
</span> <span class=identifier
>name
</span><span class=special
>;
</span>
501 <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
>&</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>
502 <span class=special
>};
</span>
504 <span class=keyword
>typedef
</span> <span class=identifier
>multi_index_container
</span><span class=special
><</span>
505 <span class=identifier
>street_entry
</span><span class=special
>,
</span>
506 <span class=identifier
>indexed_by
</span><span class=special
><</span>
507 <span class=identifier
>hashed_non_unique
</span><span class=special
><</span> <span class=comment
>// indexed by quadrant coordinates
</span>
508 <span class=identifier
>composite_key
</span><span class=special
><</span>
509 <span class=identifier
>street_entry
</span><span class=special
>,
</span>
510 <span class=identifier
>member
</span><span class=special
><</span><span class=identifier
>street_entry
</span><span class=special
>,
</span><span class=keyword
>int
</span><span class=special
>,
&</span><span class=identifier
>street_entry
</span><span class=special
>::
</span><span class=identifier
>x
</span><span class=special
>>,
</span>
511 <span class=identifier
>member
</span><span class=special
><</span><span class=identifier
>street_entry
</span><span class=special
>,
</span><span class=keyword
>int
</span><span class=special
>,
&</span><span class=identifier
>street_entry
</span><span class=special
>::
</span><span class=identifier
>y
</span><span class=special
>></span>
512 <span class=special
>></span>
513 <span class=special
>>,
</span>
514 <span class=identifier
>hashed_non_unique
</span><span class=special
><</span> <span class=comment
>// indexed by street name
</span>
515 <span class=identifier
>member
</span><span class=special
><</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
>,
&</span><span class=identifier
>street_entry
</span><span class=special
>::
</span><span class=identifier
>name
</span><span class=special
>></span>
516 <span class=special
>></span>
517 <span class=special
>></span>
518 <span class=special
>></span> <span class=identifier
>street_locator
</span><span class=special
>;
</span>
520 <span class=identifier
>street_locator
</span> <span class=identifier
>sl
</span><span class=special
>;
</span>
521 <span class=special
>...
</span>
522 <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>
523 <span class=special
>{
</span>
524 <span class=identifier
>std
</span><span class=special
>::
</span><span class=identifier
>pair
</span><span class=special
><</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
>></span> <span class=identifier
>p
</span><span class=special
>=
</span>
525 <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>
527 <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>
528 <span class=identifier
>std
</span><span class=special
>::
</span><span class=identifier
>cout
</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
>name
</span><span class=special
><<</span><span class=identifier
>std
</span><span class=special
>::
</span><span class=identifier
>endl
</span><span class=special
>;
</span>
529 <span class=special
>++
</span><span class=identifier
>p
</span><span class=special
>.
</span><span class=identifier
>first
</span><span class=special
>;
</span>
530 <span class=special
>}
</span>
531 <span class=special
>}
</span>
535 Note that hashing is automatically taken care of:
<code>boost::hash
</code> is
536 specialized to hash a composite key as a function of the
<code>boost::hash
</code>
537 values of its elements. Should we need to specify different hash functions for the
538 elements of a composite key, we can explicitly do so by using the
539 <a href=
"../reference/key_extraction.html#composite_key_hash"><code>composite_key_hash
</code></a>
544 <span class=keyword
>struct
</span> <span class=identifier
>tuned_int_hash
</span>
545 <span class=special
>{
</span>
546 <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>
547 <span class=special
>{
</span>
548 <span class=comment
>// specially tuned hash for this application
</span>
549 <span class=special
>}
</span>
550 <span class=special
>};
</span>
552 <span class=keyword
>typedef
</span> <span class=identifier
>multi_index_container
</span><span class=special
><</span>
553 <span class=identifier
>street_entry
</span><span class=special
>,
</span>
554 <span class=identifier
>indexed_by
</span><span class=special
><</span>
555 <span class=identifier
>hashed_non_unique
</span><span class=special
><</span> <span class=comment
>// indexed by quadrant coordinates
</span>
556 <span class=identifier
>composite_key
</span><span class=special
><</span>
557 <span class=identifier
>street_entry
</span><span class=special
>,
</span>
558 <span class=identifier
>member
</span><span class=special
><</span><span class=identifier
>street_entry
</span><span class=special
>,
</span><span class=keyword
>int
</span><span class=special
>,
&</span><span class=identifier
>street_entry
</span><span class=special
>::
</span><span class=identifier
>x
</span><span class=special
>>,
</span>
559 <span class=identifier
>member
</span><span class=special
><</span><span class=identifier
>street_entry
</span><span class=special
>,
</span><span class=keyword
>int
</span><span class=special
>,
&</span><span class=identifier
>street_entry
</span><span class=special
>::
</span><span class=identifier
>y
</span><span class=special
>></span>
560 <span class=special
>>,
</span>
561 <span class=identifier
>composite_key_hash
</span><span class=special
><</span>
562 <span class=identifier
>tuned_int_hash
</span><span class=special
>,
</span>
563 <span class=identifier
>tuned_int_hash
</span>
564 <span class=special
>></span>
565 <span class=special
>>,
</span>
566 <span class=identifier
>hashed_non_unique
</span><span class=special
><</span> <span class=comment
>// indexed by street name
</span>
567 <span class=identifier
>member
</span><span class=special
><</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
>,
&</span><span class=identifier
>street_entry
</span><span class=special
>::
</span><span class=identifier
>name
</span><span class=special
>></span>
568 <span class=special
>></span>
569 <span class=special
>></span>
570 <span class=special
>></span> <span class=identifier
>street_locator
</span><span class=special
>;
</span>
574 Also, equality of composite keys can be tuned with
575 <a href=
"../reference/key_extraction.html#composite_key_equal_to"><code>composite_key_equal_to
</code></a>,
576 though in most cases the default equality predicate (relying on
577 the
<code>std::equal_to
</code> instantiations for the element types)
578 will be the right choice.
582 Unlike with ordered indices, we cannot perform partial searches specifying
583 only the first elements of a composite key:
587 <span class=comment
>// try to locate streets in quadrants with x==
0
588 // compile-time error: hashed indices do not allow such operations
</span>
589 <span class=identifier
>std
</span><span class=special
>::
</span><span class=identifier
>pair
</span><span class=special
><</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
>></span> <span class=identifier
>p
</span><span class=special
>=
</span>
590 <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>
594 The reason for this limitation is quite logical: as the hash value of a composite
595 key depends on all of its elements, it is impossible to calculate it from
599 <h2><a name=
"advanced_key_extractors">Advanced features of Boost.MultiIndex key
603 The
<a href=
"../reference/key_extraction.html#key_extractors"><code>Key Extractor
</code></a>
604 concept allows the same object to extract keys from several different types,
605 possibly through suitably defined overloads of
<code>operator()
</code>:
609 <span class=comment
>// example of a name extractor from employee and employee *
</span>
610 <span class=keyword
>struct
</span> <span class=identifier
>name_extractor
</span>
611 <span class=special
>{
</span>
612 <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>
614 <span class=keyword
>const
</span> <span class=identifier
>result_type
</span><span class=special
>&</span> <span class=keyword
>operator
</span><span class=special
>()(
</span><span class=keyword
>const
</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
>.
</span><span class=identifier
>name
</span><span class=special
>;}
</span>
615 <span class=identifier
>result_type
</span><span class=special
>&</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
>-
></span><span class=identifier
>name
</span><span class=special
>;}
</span>
616 <span class=special
>};
</span>
618 <span class=comment
>// name_extractor can handle elements of type employee...
</span>
619 <span class=keyword
>typedef
</span> <span class=identifier
>multi_index_container
</span><span class=special
><</span>
620 <span class=identifier
>employee
</span><span class=special
>,
</span>
621 <span class=identifier
>indexed_by
</span><span class=special
><</span>
622 <span class=identifier
>ordered_unique
</span><span class=special
><</span><span class=identifier
>name_extractor
</span><span class=special
>></span>
623 <span class=special
>></span>
624 <span class=special
>></span> <span class=identifier
>employee_set
</span><span class=special
>;
</span>
626 <span class=comment
>// ...as well as elements of type employee *
</span>
627 <span class=keyword
>typedef
</span> <span class=identifier
>multi_index_container
</span><span class=special
><</span>
628 <span class=identifier
>employee
</span><span class=special
>*,
</span>
629 <span class=identifier
>indexed_by
</span><span class=special
><</span>
630 <span class=identifier
>ordered_unique
</span><span class=special
><</span><span class=identifier
>name_extractor
</span><span class=special
>></span>
631 <span class=special
>></span>
632 <span class=special
>></span> <span class=identifier
>employee_ptr_set
</span><span class=special
>;
</span>
636 This possibility is fully exploited by predefined key extractors provided
637 by Boost.MultiIndex, making it simpler to define
<code>multi_index_container
</code>s
638 where elements are pointers or references to the actual objects. The following
639 specifies a
<code>multi_index_container
</code> of pointers to employees sorted by their
644 <span class=keyword
>typedef
</span> <span class=identifier
>multi_index_container
</span><span class=special
><</span>
645 <span class=identifier
>employee
</span> <span class=special
>*,
</span>
646 <span class=identifier
>indexed_by
</span><span class=special
><</span>
647 <span class=identifier
>ordered_non_unique
</span><span class=special
><</span><span class=identifier
>member
</span><span class=special
><</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
>,
&</span><span class=identifier
>employee
</span><span class=special
>::
</span><span class=identifier
>name
</span><span class=special
>></span> <span class=special
>></span> <span class=special
>></span>
648 <span class=special
>></span> <span class=identifier
>employee_set
</span><span class=special
>;
</span>
652 Note that this is specified in exactly the same manner as a
<code>multi_index_container
</code>
653 of actual
<code>employee
</code> objects:
<code>member
</code> takes care of the
654 extra dereferencing needed to gain access to
<code>employee::name
</code>. A similar
655 functionality is provided for interoperability with reference wrappers from
656 <a href=
"../../../../doc/html/ref.html">Boost.Ref
</a>:
660 <span class=keyword
>typedef
</span> <span class=identifier
>multi_index_container
</span><span class=special
><</span>
661 <span class=identifier
>boost
</span><span class=special
>::
</span><span class=identifier
>reference_wrapper
</span><span class=special
><</span><span class=keyword
>const
</span> <span class=identifier
>employee
</span><span class=special
>>,
</span>
662 <span class=identifier
>indexed_by
</span><span class=special
><</span>
663 <span class=identifier
>ordered_non_unique
</span><span class=special
><</span><span class=identifier
>member
</span><span class=special
><</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
>,
&</span><span class=identifier
>employee
</span><span class=special
>::
</span><span class=identifier
>name
</span><span class=special
>></span> <span class=special
>></span> <span class=special
>></span>
664 <span class=special
>></span> <span class=identifier
>employee_set
</span><span class=special
>;
</span>
668 In fact, support for pointers is further extended to accept what we call
669 <i>chained pointers
</i>. Such a chained pointer is defined by induction as a raw or
670 smart pointer or iterator to the actual element, to a reference wrapper of the
671 element or
<i>to another chained pointer
</i>; that is, chained pointers are arbitrary
672 compositions of pointer-like types ultimately dereferencing
673 to the element from where the key is to be extracted. Examples of chained
674 pointers to
<code>employee
</code> are:
676 <li><code>employee *
</code>,
</li>
677 <li><code>const employee *
</code>,
</li>
678 <li><code>std::unique_ptr
<employee
></code>,
</li>
679 <li><code>std::list
<boost::reference_wrapper
<employee
> >::iterator
</code>,
</li>
680 <li><code>employee **
</code>,
</li>
681 <li><code>boost::shared_ptr
<const employee *
></code>.
</li>
683 In general, chained pointers with dereferencing distance greater than
1 are not
684 likely to be used in a normal program, but they can arise in frameworks
685 which construct
"views" as
<code>multi_index_container
</code>s from preexisting
686 <code>multi_index_container
</code>s.
690 In order to present a short summary of the different usages of Boost.MultiIndex
691 key extractors in the presence of reference wrappers and pointers, consider the
692 following final type:
696 <span class=keyword
>struct
</span> <span class=identifier
>T
</span>
697 <span class=special
>{
</span>
698 <span class=keyword
>int
</span> <span class=identifier
>i
</span><span class=special
>;
</span>
699 <span class=keyword
>const
</span> <span class=keyword
>int
</span> <span class=identifier
>j
</span><span class=special
>;
</span>
700 <span class=keyword
>int
</span> <span class=identifier
>f
</span><span class=special
>()
</span><span class=keyword
>const
</span><span class=special
>;
</span>
701 <span class=keyword
>int
</span> <span class=identifier
>g
</span><span class=special
>();
</span>
702 <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
>&);
</span>
703 <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
>&);
</span>
704 <span class=special
>};
</span>
708 The table below lists the appropriate key extractors to be used for
709 different pointer and reference wrapper types based on
<code>T
</code>, for
714 <table cellspacing=
"0">
715 <caption><b>Use cases for Boost.MultiIndex key extractors.
</b></caption>
717 <th>element type
</th>
718 <th> key
</th>
719 <th>key extractor
</th>
720 <th>applicable to
<br><code>const
</code> elements?
</th>
724 <td align=
"center" rowspan=
"6"><code>T
</code></td>
725 <td><code>i
</code></td>
726 <td><code>member
<T,int,
&T::i
></code></td>
727 <td align=
"center">yes
</td>
728 <td align=
"center">yes
</td>
731 <td><code>j
</code></td>
732 <td><code>member
<T,const int,
&T::j
></code></td>
733 <td align=
"center">yes
</td>
734 <td align=
"center">no
</td>
737 <td><code>f()
</code></td>
738 <td><code>const_mem_fun
<T,int,
&T::f
></code></td>
739 <td align=
"center">yes
</td>
740 <td align=
"center">no
</td>
743 <td><code>g()
</code></td>
744 <td><code>mem_fun
<T,int,
&T::g
></code></td>
745 <td align=
"center">no
</td>
746 <td align=
"center">no
</td>
749 <td><code>gf()
</code></td>
750 <td><code>global_fun
<const T
&,int,
&T::gf
></code></td>
751 <td align=
"center">yes
</td>
752 <td align=
"center">no
</td>
755 <td><code>gg()
</code></td>
756 <td><code>global_fun
<T
&,int,
&T::gg
></code></td>
757 <td align=
"center">no
</td>
758 <td align=
"center">no
</td>
762 <td align=
"center" rowspan=
"6"><code>reference_wrapper
<T
></code></td>
763 <td><code>i
</code></td>
764 <td><code>member
<T,int,
&T::i
></code></td>
765 <td align=
"center">yes
</td>
766 <td align=
"center">yes
</td>
769 <td><code>j
</code></td>
770 <td><code>member
<T,const int,
&T::j
></code></td>
771 <td align=
"center">yes
</td>
772 <td align=
"center">no
</td>
775 <td><code>f()
</code></td>
776 <td><code>const_mem_fun
<T,int,
&T::f
></code></td>
777 <td align=
"center">yes
</td>
778 <td align=
"center">no
</td>
781 <td><code>g()
</code></td>
782 <td><code>mem_fun
<T,int,
&T::g
></code></td>
783 <td align=
"center">yes
</td>
784 <td align=
"center">no
</td>
787 <td><code>gf()
</code></td>
788 <td><code>global_fun
<const T
&,int,
&T::gf
></code></td>
789 <td align=
"center">yes
</td>
790 <td align=
"center">no
</td>
793 <td><code>gg()
</code></td>
794 <td><code>global_fun
<T
&,int,
&T::gg
></code></td>
795 <td align=
"center">yes
</td>
796 <td align=
"center">no
</td>
800 <td align=
"center" rowspan=
"6"><code>reference_wrapper
<const T
></code></td>
801 <td><code>i
</code></td>
802 <td><code>member
<T,const int,
&T::i
></code></td>
803 <td align=
"center">yes
</td>
804 <td align=
"center">no
</td>
807 <td><code>j
</code></td>
808 <td><code>member
<T,const int,
&T::j
></code></td>
809 <td align=
"center">yes
</td>
810 <td align=
"center">no
</td>
813 <td><code>f()
</code></td>
814 <td><code>const_mem_fun
<T,int,
&T::f
></code></td>
815 <td align=
"center">yes
</td>
816 <td align=
"center">no
</td>
819 <td><code>g()
</code></td>
820 <td colspan=
"3"> </td>
823 <td><code>gf()
</code></td>
824 <td><code>global_fun
<const T
&,int,
&T::gf
></code></td>
825 <td align=
"center">yes
</td>
826 <td align=
"center">no
</td>
829 <td><code>gg()
</code></td>
830 <td colspan=
"3"> </td>
834 <td align=
"center" rowspan=
"6">chained pointer to
<code>T
</code><br>
835 or to
<code>reference_wrapper
<T
></code></td>
836 <td><code>i
</code></td>
837 <td><code>member
<T,int,
&T::i
></code></td>
838 <td align=
"center">yes
</td>
839 <td align=
"center">yes
</td>
842 <td><code>j
</code></td>
843 <td><code>member
<T,const int,
&T::j
></code></td>
844 <td align=
"center">yes
</td>
845 <td align=
"center">no
</td>
848 <td><code>f()
</code></td>
849 <td><code>const_mem_fun
<T,int,
&T::f
></code></td>
850 <td align=
"center">yes
</td>
851 <td align=
"center">no
</td>
854 <td><code>g()
</code></td>
855 <td><code>mem_fun
<T,int,
&T::g
></code></td>
856 <td align=
"center">yes
</td>
857 <td align=
"center">no
</td>
860 <td><code>gf()
</code></td>
861 <td><code>global_fun
<const T
&,int,
&T::gf
></code></td>
862 <td align=
"center">yes
</td>
863 <td align=
"center">no
</td>
866 <td><code>gg()
</code></td>
867 <td><code>global_fun
<T
&,int,
&T::gg
></code></td>
868 <td align=
"center">yes
</td>
869 <td align=
"center">no
</td>
873 <td align=
"center" rowspan=
"6">chained pointer to
<code>const T
</code><br>
874 or to
<code>reference_wrapper
<const T
></code></td>
875 <td><code>i
</code></td>
876 <td><code>member
<T,const int,
&T::i
></code></td>
877 <td align=
"center">yes
</td>
878 <td align=
"center">no
</td>
881 <td><code>j
</code></td>
882 <td><code>member
<T,const int,
&T::j
></code></td>
883 <td align=
"center">yes
</td>
884 <td align=
"center">no
</td>
887 <td><code>f()
</code></td>
888 <td><code>const_mem_fun
<T,int,
&T::f
></code></td>
889 <td align=
"center">yes
</td>
890 <td align=
"center">no
</td>
893 <td><code>g()
</code></td>
894 <td colspan=
"3"> </td>
897 <td><code>gf()
</code></td>
898 <td><code>global_fun
<const T
&,int,
&T::gf
></code></td>
899 <td align=
"center">yes
</td>
900 <td align=
"center">no
</td>
903 <td><code>gg()
</code></td>
904 <td colspan=
"3"> </td>
911 The column
"applicable to <code>const</code> elements?" states whether the
912 corresponding key extractor can be used when passed constant elements (this
913 relates to the elements specified in the first column, not the referenced
914 <code>T
</code> objects). The only negative cases are for
<code>T::g
</code> and
915 <code>T:gg
</code> when the elements are raw
<code>T
</code> objects, which make sense
916 as we are dealing with a non-constant member function (
<code>T::g
</code>)
917 and a function taking
<code>T
</code> by
918 non-constant reference: this also implies that
<code>multi_index_container
</code>s
919 of elements of
<code>T
</code> cannot be sorted by
<code>T::g
</code> or
<code>T::gg
</code>, because
920 elements contained within a
<code>multi_index_container
</code> are treated as constant.
924 The column
"read/write?" shows which combinations yield
925 <a href=
"#read_write_key_extractors">read/write key extractors
</a>.
929 Some care has to be taken to preserve
<code>const
</code>-correctness in the
930 specification of
<code>member
</code> key extractors: in some sense, the
<code>const
</code>
931 qualifier is carried along to the member part, even if that particular
932 member is not defined as
<code>const
</code>. For instance, if the elements
933 are of type
<code>const T *
</code>, sorting by
<code>T::i
</code> is
<i>not
</i>
934 specified as
<code>member
<const T,int,
&T::i
></code>, but rather as
935 <code>member
<T,const int,
&T::i
></code>.
939 For practical demonstrations of use of these key extractors, refer to
940 <a href=
"../examples.html#example2">example
2</a> and
941 <a href=
"../examples.html#example6">example
6</a> in the examples section.
946 <div class=
"prev_link"><a href=
"indices.html"><img src=
"../prev.gif" alt=
"index types" border=
"0"><br>
949 <div class=
"up_link"><a href=
"index.html"><img src=
"../up.gif" alt=
"Boost.MultiIndex tutorial" border=
"0"><br>
950 Boost.MultiIndex tutorial
952 <div class=
"next_link"><a href=
"creation.html"><img src=
"../next.gif" alt=
"container creation" border=
"0"><br>
954 </a></div><br clear=
"all" style=
"clear: all;">
958 <p>Revised March
25th
2015</p>
960 <p>© Copyright
2003-
2015 Joaqu
ín M L
ópez Mu
ñoz.
961 Distributed under the Boost Software
962 License, Version
1.0. (See accompanying file
<a href=
"../../../../LICENSE_1_0.txt">
963 LICENSE_1_0.txt
</a> or copy at
<a href=
"http://www.boost.org/LICENSE_1_0.txt">
964 http://www.boost.org/LICENSE_1_0.txt
</a>)