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13 <h1><img src=
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14 "middle" width=
"277" height=
"86">Header
<cite><<a href=
15 "../../boost/operators.hpp">boost/operators.hpp
</a>></cite></h1>
17 <p>The header
<cite><<a href=
18 "../../boost/operators.hpp">boost/operators.hpp
</a>></cite> supplies
19 several sets of class templates (in namespace
<code>boost
</code>). These
20 templates define operators at namespace scope in terms of a minimal
21 number of fundamental operators provided by the class.
</p>
23 <h2><a name=
"contents">Contents
</a></h2>
26 <li><a href=
"#contents">Contents
</a></li>
29 <a href=
"#rationale">Rationale
</a>
32 <li><a href=
"#semantics">Summary of Template Semantics
</a></li>
34 <li><a href=
"#concepts_note">Use of
<i>concepts
</i></a></li>
39 <a href=
"#usage">Usage
</a>
43 <a href=
"#two_arg">Two-Argument Template Forms
</a>
46 <li><a href=
"#two_arg_gen">General Considerations
</a></li>
48 <li><a href=
"#mixed_arithmetics">Mixed arithmetics
</a></li>
52 <li><a href=
"#chaining">Base Class Chaining and Object
55 <li><a href=
"#explicit_instantiation">Separate, Explicit
56 Instantiation
</a></li>
58 <li><a href=
"#portability">Requirement Portability
</a></li>
62 <li><a href=
"#example">Example
</a></li>
65 <a href=
"#arithmetic">Arithmetic operators
</a>
69 <a href=
"#smpl_oprs">Simple Arithmetic Operators
</a>
72 <li><a href=
"#ordering">Ordering Note
</a></li>
74 <li><a href=
"#symmetry">Symmetry Note
</a></li>
78 <li><a href=
"#grpd_oprs">Grouped Arithmetic Operators
</a></li>
80 <li><a href=
"#ex_oprs">Example Templates
</a></li>
82 <li><a href=
"#a_demo">Arithmetic Operators Demonstration and Test
88 <a href=
"#deref">Dereference Operators and Iterator Helpers
</a>
91 <li><a href=
"#dereference">Dereference operators
</a></li>
93 <li><a href=
"#grpd_iter_oprs">Grouped Iterator Operators
</a></li>
96 <a href=
"#iterator">Iterator Helpers
</a>
99 <li><a href=
"#iterator_helpers_notes">Iterator Helper
104 <li><a href=
"#i_demo">Iterator Demonstration and Test
109 <li><a href=
"#contributors">Contributors
</a></li>
111 <li><a href=
"#old_lib_note">Note for Users of Older Versions
</a></li>
114 <h2><a name=
"rationale">Rationale
</a></h2>
116 <p>Overloaded operators for class types typically occur in groups. If you
117 can write
<code>x
+
y
</code>, you probably also want to be able
118 to write
<code>x += y
</code>. If you can write
<code>x
< y,
</code> you
119 also want
<code>x
> y, x
>= y,
</code> and
<code>x
<= y
</code>.
120 Moreover, unless your class has really surprising behavior, some of these
121 related operators can be defined in terms of others (e.g.
<code>x
>= y
122 <=
> !(x
< y)
</code>). Replicating this boilerplate for multiple
123 classes is both tedious and error-prone. The
<cite><a href=
124 "../../boost/operators.hpp">boost/operators.hpp
</a></cite> templates help
125 by generating operators for you at namespace scope based on other
126 operators you've defined in your class.
</p>
128 <p>If, for example, you declare a class like this:
</p>
133 : boost::operators
<MyInt
>
135 bool operator
<(const MyInt
& x) const;
136 bool operator==(const MyInt
& x) const;
137 MyInt
& operator+=(const MyInt
& x);
138 MyInt
& operator-=(const MyInt
& x);
139 MyInt
& operator*=(const MyInt
& x);
140 MyInt
& operator/=(const MyInt
& x);
141 MyInt
& operator%=(const MyInt
& x);
142 MyInt
& operator|=(const MyInt
& x);
143 MyInt
& operator
&=(const MyInt
& x);
144 MyInt
& operator^=(const MyInt
& x);
145 MyInt
& operator++();
146 MyInt
& operator--();
151 <p>then the
<code><a href=
"#operators1">operators
<></a></code>
152 template adds more than a dozen additional operators, such as
153 <code>operator
></code>,
<code><=
</code>,
<code>>=
</code>, and
154 (binary)
<code>+
</code>.
<a href=
"#two_arg">Two-argument forms
</a> of the
155 templates are also provided to allow interaction with other types.
</p>
157 <h3>Summary of Template
<a name=
"semantics">Semantics
</a></h3>
160 <li>Each operator template completes the concept(s) it describes by
161 defining overloaded operators for its target class.
</li>
163 <li>The name of an operator class template indicates the
<a href=
164 "#concepts_note">concept
</a> that its target class will model.
</li>
166 <li>Usually, the target class uses an instantation of the operator
167 class template as a base class. Some operator templates support an
<a
168 href=
"#explicit_instantiation">alternate method
</a>.
</li>
170 <li>The concept can be compound,
<i>i.e.
</i> it may represent a common
171 combination of other, simpler concepts.
</li>
173 <li>Most operator templates require their target class to support
174 operations related to the operators supplied by the template. In
175 accordance with widely accepted
<a href=
176 "http://www.gotw.ca/gotw/004.htm">coding style recommendations
</a>, the
177 target class is often required to supply the assignment counterpart
178 operator of the concept's
"main operator." For example, the
179 <code>addable
</code> template requires
<code>operator+=(T
180 const
&)
</code> and in turn supplies
<code>operator+(T const
&, T
181 const
&)
</code>.
</li>
184 <h3>Use of
<i><a name=
"concepts_note">concepts
</a></i></h3>
186 <p>The discussed concepts are not necessarily the standard library's
187 concepts (CopyConstructible,
<i>etc.
</i>), although some of them could
188 be; they are what we call
<i>concepts with a small 'c'
</i>. In
189 particular, they are different from the former ones in that they
<em>do
190 not
</em> describe precise semantics of the operators they require to be
191 defined, except the requirements that (a) the semantics of the operators
192 grouped in one concept should be consistent (
<i>e.g.
</i> effects of
193 evaluating of
<code>a += b
</code> and
194 <code>a
=
a
+
b
</code> expressions should be the
195 same), and (b) that the return types of the operators should follow
196 semantics of return types of corresponding operators for built-in types
197 (
<i>e.g.
</i> <code>operator
<</code> should return a type convertible
198 to
<code>bool
</code>, and
<code>T::operator-=
</code> should return type
199 convertible to
<code>T
</code>). Such
"loose" requirements make operators
200 library applicable to broader set of target classes from different
201 domains,
<i>i.e.
</i> eventually more useful.
</p>
203 <h2><a name=
"usage">Usage
</a></h2>
205 <h3><a name=
"two_arg">Two-Argument
</a> Template Forms
</h3>
207 <h4><a name=
"two_arg_gen">General Considerations
</a></h4>
209 <p>The arguments to a binary operator commonly have identical types, but
210 it is not unusual to want to define operators which combine different
211 types. For
<a href=
"#example">example
</a>, one might want to multiply a
212 mathematical vector by a scalar. The two-argument template forms of the
213 arithmetic operator templates are supplied for this purpose. When
214 applying the two-argument form of a template, the desired return type of
215 the operators typically determines which of the two types in question
216 should be derived from the operator template. For example, if the result
217 of
<code>T
+
U
</code> is of type
<code>T
</code>, then
218 <code>T
</code> (not
<code>U
</code>) should be derived from
<code><a href=
219 "#addable2">addable
<T, U
></a></code>. The comparison templates
220 (
<code><a href=
"#less_than_comparable2">less_than_comparable
<T,
221 U
></a></code>,
<code><a href=
222 "#equality_comparable2">equality_comparable
<T, U
></a></code>,
223 <code><a href=
"#equivalent2">equivalent
<T, U
></a></code>, and
224 <code><a href=
"#partially_ordered2">partially_ordered
<T,
225 U
></a></code>) are exceptions to this guideline, since the return type
226 of the operators they define is
<code>bool
</code>.
</p>
228 <p>On compilers which do not support partial specialization, the
229 two-argument forms must be specified by using the names shown below with
230 the trailing
<code>'
2'
</code>. The single-argument forms with the
231 trailing
<code>'
1'
</code> are provided for symmetry and to enable certain
232 applications of the
<a href=
"#chaining">base class chaining
</a>
235 <h4><a name=
"mixed_arithmetics">Mixed Arithmetics
</a></h4>
237 <p>Another application of the two-argument template forms is for mixed
238 arithmetics between a type
<code>T
</code> and a type
<code>U
</code> that
239 is convertible to
<code>T
</code>. In this case there are two ways where
240 the two-argument template forms are helpful: one is to provide the
241 respective signatures for operator overloading, the second is
244 <p>With respect to the operator overloading assume
<i>e.g.
</i> that
245 <code>U
</code> is
<code>int
</code>, that
<code>T
</code> is an
246 user-defined unlimited integer type, and that
<code>double
247 operator-(double, const T
&)
</code> exists. If one wants to compute
248 <code>int - T
</code> and does not provide
<code>T operator-(int, const
249 T
&)
</code>, the compiler will consider
<code>double operator-(double,
250 const T
&)
</code> to be a better match than
<code>T operator-(const
251 T
&, const T
&)
</code>, which will probably be different from the
252 user's intention. To define a complete set of operator signatures,
253 additional 'left' forms of the two-argument template forms are provided
254 (
<code><a href=
"#subtractable2_left">subtractable2_left
<T,
255 U
></a></code>,
<code><a href=
"#dividable2_left">dividable2_left
<T,
256 U
></a></code>,
<code><a href=
"#modable2_left">modable2_left
<T,
257 U
></a></code>) that define the signatures for non-commutative
258 operators where
<code>U
</code> appears on the left hand side
259 (
<code>operator-(const U
&, const T
&)
</code>,
260 <code>operator/(const U
&, const T
&)
</code>,
<code>operator%(const
261 U
&, const T
&)
</code>).
</p>
263 <p>With respect to the performance observe that when one uses the single
264 type binary operator for mixed type arithmetics, the type
<code>U
</code>
265 argument has to be converted to type
<code>T
</code>. In practice,
266 however, there are often more efficient implementations of, say
267 <code>T::operator-=(const U
&)
</code> that avoid unnecessary
268 conversions from
<code>U
</code> to
<code>T
</code>. The two-argument
269 template forms of the arithmetic operator create additional operator
270 interfaces that use these more efficient implementations. There is,
271 however, no performance gain in the 'left' forms: they still need a
272 conversion from
<code>U
</code> to
<code>T
</code> and have an
273 implementation equivalent to the code that would be automatically created
274 by the compiler if it considered the single type binary operator to be
277 <h3>Base Class
<a name=
"chaining">Chaining
</a> and Object Size
</h3>
279 <p>Every operator class template, except the
<a href=
280 "#ex_oprs">arithmetic examples
</a> and the
<a href=
"#iterator">iterator
281 helpers
</a>, has an additional, but optional, template type parameter
282 <code>B
</code>. This parameter will be a publicly-derived base class of
283 the instantiated template. This means it must be a class type. It can be
284 used to avoid the bloating of object sizes that is commonly associated
285 with multiple-inheritance from several empty base classes (see the
<a
286 href=
"#old_lib_note">note for users of older versions
</a> for more
287 details). To provide support for a group of operators, use the
288 <code>B
</code> parameter to chain operator templates into a single-base
289 class hierarchy, demostrated in the
<a href=
"#example">usage example
</a>.
290 The technique is also used by the composite operator templates to group
291 operator definitions. If a chain becomes too long for the compiler to
292 support, try replacing some of the operator templates with a single
293 grouped operator template that chains the old templates together; the
294 length limit only applies to the number of templates directly in the
295 chain, not those hidden in group templates.
</p>
297 <p><strong>Caveat:
</strong> to chain to a base class which is
298 <em>not
</em> a Boost operator template when using the
<a href=
299 "#two_arg">single-argument form
</a> of a Boost operator template, you
300 must specify the operator template with the trailing
<code>'
1'
</code> in
301 its name. Otherwise the library will assume you mean to define a binary
302 operation combining the class you intend to use as a base class and the
303 class you're deriving.
</p>
305 <h3>Separate,
<a name=
"explicit_instantiation">Explicit
306 Instantiation
</a></h3>
308 <p>On some compilers (
<i>e.g.
</i> Borland, GCC) even single-inheritance
309 seems to cause an increase in object size in some cases. If you are not
310 defining a class template, you may get better object-size performance by
311 avoiding derivation altogether, and instead explicitly instantiating the
312 operator template as follows:
</p>
316 class myclass // lose the inheritance...
321 // explicitly instantiate the operators I need.
322 template struct less_than_comparable
<myclass
>;
323 template struct equality_comparable
<myclass
>;
324 template struct incrementable
<myclass
>;
325 template struct decrementable
<myclass
>;
326 template struct addable
<myclass,long
>;
327 template struct subtractable
<myclass,long
>;
331 <p>Note that some operator templates cannot use this workaround and must
332 be a base class of their primary operand type. Those templates define
333 operators which must be member functions, and the workaround needs the
334 operators to be independent friend functions. The relevant templates
339 "#dereferenceable">dereferenceable
<></a></code></li>
341 <li><code><a href=
"#indexable">indexable
<></a></code></li>
343 <li>Any composite operator template that includes at least one of the
347 <p>As Daniel Kr
ügler pointed out, this technique violates
14.6.5/
2
348 and is thus non-portable. The reasoning is, that the operators injected
349 by the instantiation of e.g.
350 <code>less_than_comparable
<myclass
></code> can not be found
351 by ADL according to the rules given by
3.4.2/
2, since myclass is
352 not an associated class of
353 <code>less_than_comparable
<myclass
></code>.
354 Thus only use this technique if all else fails.
</p>
356 <h3>Requirement
<a name=
"portability">Portability
</a></h3>
358 <p>Many compilers (
<i>e.g.
</i> MSVC
6.3, GCC
2.95.2) will not enforce the
359 requirements in the operator template tables unless the operations which
360 depend on them are actually used. This is not standard-conforming
361 behavior. In particular, although it would be convenient to derive all
362 your classes which need binary operators from the
<code><a href=
363 "#operators1">operators
<></a></code> and
<code><a href=
364 "#operators2">operators2
<></a></code> templates, regardless of
365 whether they implement all the requirements of those templates, this
366 shortcut is not portable. Even if this currently works with your
367 compiler, it may not work later.
</p>
369 <h2><a name=
"example">Example
</a></h2>
371 <p>This example shows how some of the
<a href=
"#arithmetic">arithmetic
372 operator templates
</a> can be used with a geometric point class
375 template
<class T
>
376 class point // note: private inheritance is OK here!
377 : boost::addable
< point
<T
> // point + point
378 , boost::subtractable
< point
<T
> // point - point
379 , boost::dividable2
< point
<T
>, T // point / T
380 , boost::multipliable2
< point
<T
>, T // point * T, T * point
388 point operator+=(const point
&);
389 // point operator+(point, const point
&) automatically
390 // generated by addable.
392 point operator-=(const point
&);
393 // point operator-(point, const point
&) automatically
394 // generated by subtractable.
397 // point operator*(point, const T
&) and
398 // point operator*(const T
&, point) auto-generated
402 // point operator/(point, const T
&) auto-generated
409 // now use the point
<> class:
411 template
<class T
>
412 T length(const point
<T
> p)
414 return sqrt(p.x()*p.x() + p.y()*p.y());
417 const point
<float
> right(
0,
1);
418 const point
<float
> up(
1,
0);
419 const point
<float
> pi_over_4 = up + right;
420 const point
<float
> pi_over_4_normalized = pi_over_4 / length(pi_over_4);
423 <h2><a name=
"arithmetic">Arithmetic
</a> Operators
</h2>
425 <p>The arithmetic operator templates ease the task of creating a custom
426 numeric type. Given a core set of operators, the templates add related
427 operators to the numeric class. These operations are like the ones the
428 standard arithmetic types have, and may include comparisons, adding,
429 incrementing, logical and bitwise manipulations,
<i>etc
</i>. Further,
430 since most numeric types need more than one of these operators, some
431 templates are provided to combine several of the basic operator templates
432 in one declaration.
</p>
434 <p>The requirements for the types used to instantiate the simple operator
435 templates are specified in terms of expressions which must be valid and
436 the expression's return type. The composite operator templates only list
437 what other templates they use. The supplied operations and requirements
438 of the composite operator templates can be inferred from the operations
439 and requirements of the listed components.
</p>
441 <h3><a name=
"smpl_oprs">Simple Arithmetic Operators
</a></h3>
443 <p>These templates are
"simple" since they provide operators based on a
444 single operation the base type has to provide. They have an additional
445 optional template parameter
<code>B
</code>, which is not shown, for the
446 <a href=
"#chaining">base class chaining
</a> technique.
</p>
448 <p>The primary operand type
<code>T
</code> needs to be of class type,
449 built-in types are not supported.
</p>
451 <table cellpadding=
"5" border=
"1" align=
"center">
453 Simple Arithmetic Operator Template Classes
458 <table align=
"center" border=
"1">
464 <td><code>T
</code>: primary operand type
</td>
466 <td><code>U
</code>: alternate operand type
</td>
470 <td><code>t
</code>,
<code>t1
</code>: values of type
473 <td><code>u
</code>: value of type
<code>U
</code></td>
482 <th>Supplied Operations
</th>
484 <th>Requirements
</th>
489 "less_than_comparable1">less_than_comparable
<T
></a></code><br>
490 <code>less_than_comparable1
<T
></code></td>
492 <td><code>bool operator
>(const T
&, const T
&)
</code><br>
493 <code>bool operator
<=(const T
&, const T
&)
</code><br>
494 <code>bool operator
>=(const T
&, const T
&)
</code></td>
496 <td><code>t
< t1
</code>.
<br>
497 Return convertible to
<code>bool
</code>. See the
<a href=
498 "#ordering">Ordering Note
</a>.
</td>
502 <td><code><a name=
"less_than_comparable2">less_than_comparable
<T,
504 <code>less_than_comparable2
<T, U
></code></td>
506 <td><code>bool operator
<=(const T
&, const U
&)
</code><br>
507 <code>bool operator
>=(const T
&, const U
&)
</code><br>
508 <code>bool operator
>(const U
&, const T
&)
</code><br>
509 <code>bool operator
<(const U
&, const T
&)
</code><br>
510 <code>bool operator
<=(const U
&, const T
&)
</code><br>
511 <code>bool operator
>=(const U
&, const T
&)
</code></td>
513 <td><code>t
< u
</code>.
<code>t
> u
</code>.
<br>
514 Returns convertible to
<code>bool
</code>. See the
<a href=
515 "#ordering">Ordering Note
</a>.
</td>
520 "equality_comparable1">equality_comparable
<T
></a></code><br>
521 <code>equality_comparable1
<T
></code></td>
523 <td><code>bool operator!=(const T
&, const T
&)
</code></td>
525 <td><code>t == t1
</code>.
<br>
526 Return convertible to
<code>bool
</code>.
</td>
530 <td><code><a name=
"equality_comparable2">equality_comparable
<T,
532 <code>equality_comparable2
<T, U
></code></td>
534 <td><code>bool operator==(const U
&, const T
&)
</code><br>
535 <code>bool operator!=(const U
&, const T
&)
</code><br>
536 <code>bool operator!=(const T
&, const U
&)
</code></td>
538 <td><code>t == u
</code>.
<br>
539 Return convertible to
<code>bool
</code>.
</td>
543 <td><code><a name=
"addable1">addable
<T
></a></code><br>
544 <code>addable1
<T
></code></td>
546 <td><code>T operator+(const T
&, const T
&)
</code></td>
548 <td><code>T temp(t); temp += t1
</code>.
<br>
549 Return convertible to
<code>T
</code>. See the
<a href=
550 "#symmetry">Symmetry Note
</a>.
</td>
554 <td><code><a name=
"addable2">addable
<T, U
></a></code><br>
555 <code>addable2
<T, U
></code></td>
557 <td><code>T operator+(const T
&, const U
&)
</code><br>
558 <code>T operator+(const U
&, const T
& )
</code></td>
560 <td><code>T temp(t); temp += u
</code>.
<br>
561 Return convertible to
<code>T
</code>. See the
<a href=
562 "#symmetry">Symmetry Note
</a>.
</td>
567 "subtractable1">subtractable
<T
></a></code><br>
568 <code>subtractable1
<T
></code></td>
570 <td><code>T operator-(const T
&, const T
&)
</code></td>
572 <td><code>T temp(t); temp -= t1
</code>.
<br>
573 Return convertible to
<code>T
</code>. See the
<a href=
574 "#symmetry">Symmetry Note
</a>.
</td>
578 <td><code><a name=
"subtractable2">subtractable
<T,
580 <code>subtractable2
<T, U
></code></td>
582 <td><code>T operator-(const T
&, const U
&)
</code></td>
584 <td><code>T temp(t); temp -= u
</code>.
<br>
585 Return convertible to
<code>T
</code>. See the
<a href=
586 "#symmetry">Symmetry Note
</a>.
</td>
590 <td><code><a name=
"subtractable2_left">subtractable2_left
<T,
591 U
></a></code></td>
593 <td><code>T operator-(const U
&, const T
&)
</code></td>
595 <td><code>T temp(u); temp -= t
</code>.
<br>
596 Return convertible to
<code>T
</code>.
</td>
601 "multipliable1">multipliable
<T
></a></code><br>
602 <code>multipliable1
<T
></code></td>
604 <td><code>T operator*(const T
&, const T
&)
</code></td>
606 <td><code>T temp(t); temp *= t1
</code>.
<br>
607 Return convertible to
<code>T
</code>. See the
<a href=
608 "#symmetry">Symmetry Note
</a>.
</td>
612 <td><code><a name=
"multipliable2">multipliable
<T,
614 <code>multipliable2
<T, U
></code></td>
616 <td><code>T operator*(const T
&, const U
&)
</code><br>
617 <code>T operator*(const U
&, const T
&)
</code></td>
619 <td><code>T temp(t); temp *= u
</code>.
<br>
620 Return convertible to
<code>T
</code>. See the
<a href=
621 "#symmetry">Symmetry Note
</a>.
</td>
625 <td><code><a name=
"dividable1">dividable
<T
></a></code><br>
626 <code>dividable1
<T
></code></td>
628 <td><code>T operator/(const T
&, const T
&)
</code></td>
630 <td><code>T temp(t); temp /= t1
</code>.
<br>
631 Return convertible to
<code>T
</code>. See the
<a href=
632 "#symmetry">Symmetry Note
</a>.
</td>
636 <td><code><a name=
"dividable2">dividable
<T, U
></a></code><br>
637 <code>dividable2
<T, U
></code></td>
639 <td><code>T operator/(const T
&, const U
&)
</code></td>
641 <td><code>T temp(t); temp /= u
</code>.
<br>
642 Return convertible to
<code>T
</code>. See the
<a href=
643 "#symmetry">Symmetry Note
</a>.
</td>
647 <td><code><a name=
"dividable2_left">dividable2_left
<T,
648 U
></a></code></td>
650 <td><code>T operator/(const U
&, const T
&)
</code></td>
652 <td><code>T temp(u); temp /= t
</code>.
<br>
653 Return convertible to
<code>T
</code>.
</td>
657 <td><code><a name=
"modable1">modable
<T
></a></code><br>
658 <code>modable1
<T
></code></td>
660 <td><code>T operator%(const T
&, const T
&)
</code></td>
662 <td><code>T temp(t); temp %= t1
</code>.
<br>
663 Return convertible to
<code>T
</code>. See the
<a href=
664 "#symmetry">Symmetry Note
</a>.
</td>
668 <td><code><a name=
"modable2">modable
<T, U
></a></code><br>
669 <code>modable2
<T, U
></code></td>
671 <td><code>T operator%(const T
&, const U
&)
</code></td>
673 <td><code>T temp(t); temp %= u
</code>.
<br>
674 Return convertible to
<code>T
</code>. See the
<a href=
675 "#symmetry">Symmetry Note
</a>.
</td>
679 <td><code><a name=
"modable2_left">modable2_left
<T,
680 U
></a></code></td>
682 <td><code>T operator%(const U
&, const T
&)
</code></td>
684 <td><code>T temp(u); temp %= t
</code>.
<br>
685 Return convertible to
<code>T
</code>.
</td>
689 <td><code><a name=
"orable1">orable
<T
></a></code><br>
690 <code>orable1
<T
></code></td>
692 <td><code>T operator|(const T
&, const T
&)
</code></td>
694 <td><code>T temp(t); temp |= t1
</code>.
<br>
695 Return convertible to
<code>T
</code>. See the
<a href=
696 "#symmetry">Symmetry Note
</a>.
</td>
700 <td><code><a name=
"orable2">orable
<T, U
></a></code><br>
701 <code>orable2
<T, U
></code></td>
703 <td><code>T operator|(const T
&, const U
&)
</code><br>
704 <code>T operator|(const U
&, const T
&)
</code></td>
706 <td><code>T temp(t); temp |= u
</code>.
<br>
707 Return convertible to
<code>T
</code>. See the
<a href=
708 "#symmetry">Symmetry Note
</a>.
</td>
712 <td><code><a name=
"andable1">andable
<T
></a></code><br>
713 <code>andable1
<T
></code></td>
715 <td><code>T operator
&(const T
&, const T
&)
</code></td>
717 <td><code>T temp(t); temp
&= t1
</code>.
<br>
718 Return convertible to
<code>T
</code>. See the
<a href=
719 "#symmetry">Symmetry Note
</a>.
</td>
723 <td><code><a name=
"andable2">andable
<T, U
></a></code><br>
724 <code>andable2
<T, U
></code></td>
726 <td><code>T operator
&(const T
&, const U
&)
</code><br>
727 <code>T operator
&(const U
&, const T
&)
</code></td>
729 <td><code>T temp(t); temp
&= u
</code>.
<br>
730 Return convertible to
<code>T
</code>. See the
<a href=
731 "#symmetry">Symmetry Note
</a>.
</td>
735 <td><code><a name=
"xorable1">xorable
<T
></a></code><br>
736 <code>xorable1
<T
></code></td>
738 <td><code>T operator^(const T
&, const T
&)
</code></td>
740 <td><code>T temp(t); temp ^= t1
</code>.
<br>
741 Return convertible to
<code>T
</code>. See the
<a href=
742 "#symmetry">Symmetry Note
</a>.
</td>
746 <td><code><a name=
"xorable2">xorable
<T, U
></a></code><br>
747 <code>xorable2
<T, U
></code></td>
749 <td><code>T operator^(const T
&, const U
&)
</code><br>
750 <code>T operator^(const U
&, const T
&)
</code></td>
752 <td><code>T temp(t); temp ^= u
</code>.
<br>
753 Return convertible to
<code>T
</code>. See the
<a href=
754 "#symmetry">Symmetry Note
</a>.
</td>
759 "incrementable">incrementable
<T
></a></code></td>
761 <td><code>T operator++(T
&, int)
</code></td>
763 <td><code>T temp(t); ++t
</code><br>
764 Return convertible to
<code>T
</code>.
</td>
769 "decrementable">decrementable
<T
></a></code></td>
771 <td><code>T operator--(T
&, int)
</code></td>
773 <td><code>T temp(t); --t;
</code><br>
774 Return convertible to
<code>T
</code>.
</td>
779 "left_shiftable1">left_shiftable
<T
></a></code><br>
780 <code>left_shiftable1
<T
></code></td>
782 <td><code>T operator
<<(const T
&, const T
&)
</code></td>
784 <td><code>T temp(t); temp
<<= t1
</code>.
<br>
785 Return convertible to
<code>T
</code>. See the
<a href=
786 "#symmetry">Symmetry Note
</a>.
</td>
790 <td><code><a name=
"left_shiftable2">left_shiftable
<T,
792 <code>left_shiftable2
<T, U
></code></td>
794 <td><code>T operator
<<(const T
&, const U
&)
</code></td>
796 <td><code>T temp(t); temp
<<= u
</code>.
<br>
797 Return convertible to
<code>T
</code>. See the
<a href=
798 "#symmetry">Symmetry Note
</a>.
</td>
803 "right_shiftable1">right_shiftable
<T
></a></code><br>
804 <code>right_shiftable1
<T
></code></td>
806 <td><code>T operator
>>(const T
&, const T
&)
</code></td>
808 <td><code>T temp(t); temp
>>= t1
</code>.
<br>
809 Return convertible to
<code>T
</code>. See the
<a href=
810 "#symmetry">Symmetry Note
</a>.
</td>
814 <td><code><a name=
"right_shiftable2">right_shiftable
<T,
816 <code>right_shiftable2
<T, U
></code></td>
818 <td><code>T operator
>>(const T
&, const U
&)
</code></td>
820 <td><code>T temp(t); temp
>>= u
</code>.
<br>
821 Return convertible to
<code>T
</code>. See the
<a href=
822 "#symmetry">Symmetry Note
</a>.
</td>
826 <td><code><a name=
"equivalent1">equivalent
<T
></a></code><br>
827 <code>equivalent1
<T
></code></td>
829 <td><code>bool operator==(const T
&, const T
&)
</code></td>
831 <td><code>t
< t1
</code>.
<br>
832 Return convertible to
<code>bool
</code>. See the
<a href=
833 "#ordering">Ordering Note
</a>.
</td>
837 <td><code><a name=
"equivalent2">equivalent
<T, U
></a></code><br>
838 <code>equivalent2
<T, U
></code></td>
840 <td><code>bool operator==(const T
&, const U
&)
</code></td>
842 <td><code>t
< u
</code>.
<code>t
> u
</code>.
<br>
843 Returns convertible to
<code>bool
</code>. See the
<a href=
844 "#ordering">Ordering Note
</a>.
</td>
849 "partially_ordered1">partially_ordered
<T
></a></code><br>
850 <code>partially_ordered1
<T
></code></td>
852 <td><code>bool operator
>(const T
&, const T
&)
</code><br>
853 <code>bool operator
<=(const T
&, const T
&)
</code><br>
854 <code>bool operator
>=(const T
&, const T
&)
</code></td>
856 <td><code>t
< t1
</code>.
<code>t == t1
</code>.
<br>
857 Returns convertible to
<code>bool
</code>. See the
<a href=
858 "#ordering">Ordering Note
</a>.
</td>
862 <td><code><a name=
"partially_ordered2">partially_ordered
<T,
864 <code>partially_ordered2
<T, U
></code></td>
866 <td><code>bool operator
<=(const T
&, const U
&)
</code><br>
867 <code>bool operator
>=(const T
&, const U
&)
</code><br>
868 <code>bool operator
>(const U
&, const T
&)
</code><br>
869 <code>bool operator
<(const U
&, const T
&)
</code><br>
870 <code>bool operator
<=(const U
&, const T
&)
</code><br>
871 <code>bool operator
>=(const U
&, const T
&)
</code></td>
873 <td><code>t
< u
</code>.
<code>t
> u
</code>.
<code>t ==
875 Returns convertible to
<code>bool
</code>. See the
<a href=
876 "#ordering">Ordering Note
</a>.
</td>
880 <h4><a name=
"ordering">Ordering
</a> Note
</h4>
882 <p>The
<code><a href=
883 "#less_than_comparable1">less_than_comparable
<T
></a></code> and
884 <code><a href=
"#partially_ordered1">partially_ordered
<T
></a></code>
885 templates provide the same set of operations. However, the workings of
887 "#less_than_comparable1">less_than_comparable
<T
></a></code> assume
888 that all values of type
<code>T
</code> can be placed in a total order. If
889 that is not true (
<i>e.g.
</i> Not-a-Number values in IEEE floating point
890 arithmetic), then
<code><a href=
891 "#partially_ordered1">partially_ordered
<T
></a></code> should be
892 used. The
<code><a href=
893 "#partially_ordered1">partially_ordered
<T
></a></code> template can
894 be used for a totally-ordered type, but it is not as efficient as
896 "#less_than_comparable1">less_than_comparable
<T
></a></code>. This
897 rule also applies for
<code><a href=
898 "#less_than_comparable2">less_than_comparable
<T, U
></a></code> and
899 <code><a href=
"#partially_ordered2">partially_ordered
<T,
900 U
></a></code> with respect to the ordering of all
<code>T
</code> and
901 <code>U
</code> values, and for both versions of
<code><a href=
902 "#equivalent1">equivalent
<></a></code>. The solution for
<code><a
903 href=
"#equivalent1">equivalent
<></a></code> is to write a custom
904 <code>operator==
</code> for the target class.
</p>
906 <h4><a name=
"symmetry">Symmetry
</a> Note
</h4>
908 <p>Before talking about symmetry, we need to talk about optimizations to
909 understand the reasons for the different implementation styles of
910 operators. Let's have a look at
<code>operator+
</code> for a class
911 <code>T
</code> as an example:
</p>
913 T operator+( const T
& lhs, const T
& rhs )
915 return T( lhs ) += rhs;
918 This would be a normal implementation of
<code>operator+
</code>, but it
919 is not an efficient one. An unnamed local copy of
<code>lhs
</code> is
920 created,
<code>operator+=
</code> is called on it and it is copied to the
921 function return value (which is another unnamed object of type
922 <code>T
</code>). The standard doesn't generally allow the intermediate
923 object to be optimized away:
926 3.7.2/
2: Automatic storage duration
<br>
928 If a named automatic object has initialization or a destructor with
929 side effects, it shall not be destroyed before the end of its block,
930 nor shall it be eliminated as an optimization even if it appears to be
931 unused, except that a class object or its copy may be eliminated as
934 The reference to
12.8 is important for us:
937 12.8/
15: Copying class objects
<br>
939 For a function with a class return type, if the expression in the
940 return statement is the name of a local object, and the cv-unqualified
941 type of the local object is the same as the function return type, an
942 implementation is permitted to omit creating the temporary object to
943 hold the function return value, even if the class copy constructor or
944 destructor has side effects.
946 This optimization is known as the named return value optimization (NRVO),
947 which leads us to the following implementation for
948 <code>operator+
</code>:
950 T operator+( const T
& lhs, const T
& rhs )
957 Given this implementation, the compiler is allowed to remove the
958 intermediate object. Sadly, not all compiler implement the NRVO, some
959 even implement it in an incorrect way which makes it useless here.
960 Without the NRVO, the NRVO-friendly code is no worse than the original
961 code showed above, but there is another possible implementation, which
962 has some very special properties:
964 T operator+( T lhs, const T
& rhs )
969 The difference to the first implementation is that
<code>lhs
</code> is
970 not taken as a constant reference used to create a copy; instead,
971 <code>lhs
</code> is a by-value parameter, thus it is already the copy
972 needed. This allows another optimization (
12.2/
2) for some cases.
973 Consider
<code>a
+
b
+
c
</code> where the result of
974 <code>a
+
b
</code> is not copied when used as
<code>lhs
</code>
975 when adding
<code>c
</code>. This is more efficient than the original
976 code, but not as efficient as a compiler using the NRVO. For most people,
977 it is still preferable for compilers that don't implement the NRVO, but
978 the
<code>operator+
</code> now has a different function signature. Also,
979 the number of objects created differs for
980 <code>(a
+
b
)
+
c
</code> and
981 <code>a
+
(
b
+
c
)
</code>. Most probably,
982 this won't be a problem for you, but if your code relies on the function
983 signature or a strict symmetric behaviour, you should set
984 <code>BOOST_FORCE_SYMMETRIC_OPERATORS
</code> in your user-config. This
985 will force the NRVO-friendly implementation to be used even for compilers
986 that don't implement the NRVO.
<br>
989 <h3><a name=
"grpd_oprs">Grouped Arithmetic Operators
</a></h3>
991 <p>The following templates provide common groups of related operations.
992 For example, since a type which is addable is usually also subractable,
993 the
<code><a href=
"#additive1">additive
</a></code> template provides the
994 combined operators of both. The grouped operator templates have an
995 additional optional template parameter
<code>B
</code>, which is not
996 shown, for the
<a href=
"#chaining">base class chaining
</a> technique.
</p>
998 <table cellpadding=
"5" border=
"1" align=
"center">
1000 Grouped Arithmetic Operator Template Classes
1005 <table align=
"center" border=
"1">
1011 <td><code>T
</code>: primary operand type
</td>
1013 <td><code>U
</code>: alternate operand type
</td>
1022 <th>Component Operator Templates
</th>
1027 "totally_ordered1">totally_ordered
<T
></a></code><br>
1028 <code>totally_ordered1
<T
></code></td>
1033 "#less_than_comparable1">less_than_comparable
<T
></a></code></li>
1036 "#equality_comparable1">equality_comparable
<T
></a></code></li>
1042 <td><code><a name=
"totally_ordered2">totally_ordered
<T,
1043 U
></a></code><br>
1044 <code>totally_ordered2
<T, U
></code></td>
1049 "#less_than_comparable2">less_than_comparable
<T,
1050 U
></a></code></li>
1053 "#equality_comparable2">equality_comparable
<T,
1054 U
></a></code></li>
1060 <td><code><a name=
"additive1">additive
<T
></a></code><br>
1061 <code>additive1
<T
></code></td>
1065 <li><code><a href=
"#addable1">addable
<T
></a></code></li>
1068 "#subtractable1">subtractable
<T
></a></code></li>
1074 <td><code><a name=
"additive2">additive
<T, U
></a></code><br>
1075 <code>additive2
<T, U
></code></td>
1079 <li><code><a href=
"#addable2">addable
<T, U
></a></code></li>
1081 <li><code><a href=
"#subtractable2">subtractable
<T,
1082 U
></a></code></li>
1089 "multiplicative1">multiplicative
<T
></a></code><br>
1090 <code>multiplicative1
<T
></code></td>
1095 "#multipliable1">multipliable
<T
></a></code></li>
1098 "#dividable1">dividable
<T
></a></code></li>
1104 <td><code><a name=
"multiplicative2">multiplicative
<T,
1105 U
></a></code><br>
1106 <code>multiplicative2
<T, U
></code></td>
1110 <li><code><a href=
"#multipliable2">multipliable
<T,
1111 U
></a></code></li>
1113 <li><code><a href=
"#dividable2">dividable
<T,
1114 U
></a></code></li>
1121 "integer_multiplicative1">integer_multiplicative
<T
></a></code><br>
1123 <code>integer_multiplicative1
<T
></code></td>
1128 "#multiplicative1">multiplicative
<T
></a></code></li>
1130 <li><code><a href=
"#modable1">modable
<T
></a></code></li>
1137 "integer_multiplicative2">integer_multiplicative
<T,
1138 U
></a></code><br>
1139 <code>integer_multiplicative2
<T, U
></code></td>
1143 <li><code><a href=
"#multiplicative2">multiplicative
<T,
1144 U
></a></code></li>
1146 <li><code><a href=
"#modable2">modable
<T, U
></a></code></li>
1152 <td><code><a name=
"arithmetic1">arithmetic
<T
></a></code><br>
1153 <code>arithmetic1
<T
></code></td>
1157 <li><code><a href=
"#additive1">additive
<T
></a></code></li>
1160 "#multiplicative1">multiplicative
<T
></a></code></li>
1166 <td><code><a name=
"arithmetic2">arithmetic
<T, U
></a></code><br>
1167 <code>arithmetic2
<T, U
></code></td>
1171 <li><code><a href=
"#additive2">additive
<T,
1172 U
></a></code></li>
1174 <li><code><a href=
"#multiplicative2">multiplicative
<T,
1175 U
></a></code></li>
1182 "integer_arithmetic1">integer_arithmetic
<T
></a></code><br>
1183 <code>integer_arithmetic1
<T
></code></td>
1187 <li><code><a href=
"#additive1">additive
<T
></a></code></li>
1190 "#integer_multiplicative1">integer_multiplicative
<T
></a></code></li>
1196 <td><code><a name=
"integer_arithmetic2">integer_arithmetic
<T,
1197 U
></a></code><br>
1198 <code>integer_arithmetic2
<T, U
></code></td>
1202 <li><code><a href=
"#additive2">additive
<T,
1203 U
></a></code></li>
1206 "#integer_multiplicative2">integer_multiplicative
<T,
1207 U
></a></code></li>
1213 <td><code><a name=
"bitwise1">bitwise
<T
></a></code><br>
1214 <code>bitwise1
<T
></code></td>
1218 <li><code><a href=
"#xorable1">xorable
<T
></a></code></li>
1220 <li><code><a href=
"#andable1">andable
<T
></a></code></li>
1222 <li><code><a href=
"#orable1">orable
<T
></a></code></li>
1228 <td><code><a name=
"bitwise2">bitwise
<T, U
></a></code><br>
1229 <code>bitwise2
<T, U
></code></td>
1233 <li><code><a href=
"#xorable2">xorable
<T, U
></a></code></li>
1235 <li><code><a href=
"#andable2">andable
<T, U
></a></code></li>
1237 <li><code><a href=
"#orable2">orable
<T, U
></a></code></li>
1244 "unit_steppable">unit_steppable
<T
></a></code></td>
1249 "#incrementable">incrementable
<T
></a></code></li>
1252 "#decrementable">decrementable
<T
></a></code></li>
1258 <td><code><a name=
"shiftable1">shiftable
<T
></a></code><br>
1259 <code>shiftable1
<T
></code></td>
1264 "#left_shiftable1">left_shiftable
<T
></a></code></li>
1267 "#right_shiftable1">right_shiftable
<T
></a></code></li>
1273 <td><code><a name=
"shiftable2">shiftable
<T, U
></a></code><br>
1274 <code>shiftable2
<T, U
></code></td>
1278 <li><code><a href=
"#left_shiftable2">left_shiftable
<T,
1279 U
></a></code></li>
1281 <li><code><a href=
"#right_shiftable2">right_shiftable
<T,
1282 U
></a></code></li>
1289 "ring_operators1">ring_operators
<T
></a></code><br>
1290 <code>ring_operators1
<T
></code></td>
1294 <li><code><a href=
"#additive1">additive
<T
></a></code></li>
1297 "#multipliable1">multipliable
<T
></a></code></li>
1303 <td><code><a name=
"ring_operators2">ring_operators
<T,
1304 U
></a></code><br>
1305 <code>ring_operators2
<T, U
></code></td>
1309 <li><code><a href=
"#additive2">additive
<T,
1310 U
></a></code></li>
1312 <li><code><a href=
"#subtractable2_left">subtractable2_left
<T,
1313 U
></a></code></li>
1315 <li><code><a href=
"#multipliable2">multipliable
<T,
1316 U
></a></code></li>
1323 "ordered_ring_operators1">ordered_ring_operators
<T
></a></code><br>
1325 <code>ordered_ring_operators1
<T
></code></td>
1330 "#ring_operators1">ring_operators
<T
></a></code></li>
1333 "#totally_ordered1">totally_ordered
<T
></a></code></li>
1340 "ordered_ring_operators2">ordered_ring_operators
<T,
1341 U
></a></code><br>
1342 <code>ordered_ring_operators2
<T, U
></code></td>
1346 <li><code><a href=
"#ring_operators2">ring_operators
<T,
1347 U
></a></code></li>
1349 <li><code><a href=
"#totally_ordered2">totally_ordered
<T,
1350 U
></a></code></li>
1357 "field_operators1">field_operators
<T
></a></code><br>
1358 <code>field_operators1
<T
></code></td>
1363 "#ring_operators1">ring_operators
<T
></a></code></li>
1366 "#dividable1">dividable
<T
></a></code></li>
1372 <td><code><a name=
"field_operators2">field_operators
<T,
1373 U
></a></code><br>
1374 <code>field_operators2
<T, U
></code></td>
1378 <li><code><a href=
"#ring_operators2">ring_operators
<T,
1379 U
></a></code></li>
1381 <li><code><a href=
"#dividable2">dividable
<T,
1382 U
></a></code></li>
1384 <li><code><a href=
"#dividable2_left">dividable2_left
<T,
1385 U
></a></code></li>
1392 "ordered_field_operators1">ordered_field_operators
<T
></a></code><br>
1394 <code>ordered_field_operators1
<T
></code></td>
1399 "#field_operators1">field_operators
<T
></a></code></li>
1402 "#totally_ordered1">totally_ordered
<T
></a></code></li>
1409 "ordered_field_operators2">ordered_field_operators
<T,
1410 U
></a></code><br>
1411 <code>ordered_field_operators2
<T, U
></code></td>
1415 <li><code><a href=
"#field_operators2">field_operators
<T,
1416 U
></a></code></li>
1418 <li><code><a href=
"#totally_ordered2">totally_ordered
<T,
1419 U
></a></code></li>
1426 "euclidean_ring_operators1">euclidean_ring_operators
<T
></a></code><br>
1428 <code>euclidean_ring_operators1
<T
></code></td>
1433 "#ring_operators1">ring_operators
<T
></a></code></li>
1436 "#dividable1">dividable
<T
></a></code></li>
1438 <li><code><a href=
"#modable1">modable
<T
></a></code></li>
1445 "euclidean_ring_operators2">euclidean_ring_operators
<T,
1446 U
></a></code><br>
1447 <code>euclidean_ring_operators2
<T, U
></code></td>
1451 <li><code><a href=
"#ring_operators2">ring_operators
<T,
1452 U
></a></code></li>
1454 <li><code><a href=
"#dividable2">dividable
<T,
1455 U
></a></code></li>
1457 <li><code><a href=
"#dividable2_left">dividable2_left
<T,
1458 U
></a></code></li>
1460 <li><code><a href=
"#modable2">modable
<T, U
></a></code></li>
1462 <li><code><a href=
"#modable2_left">modable2_left
<T,
1463 U
></a></code></li>
1470 "ordered_euclidean_ring_operators1">ordered_euclidean_ring_operators
<T
></a></code><br>
1472 <code>ordered_euclidean_ring_operators1
<T
></code></td>
1477 "#euclidean_ring_operators1">euclidean_ring_operators
<T
></a></code></li>
1480 "#totally_ordered1">totally_ordered
<T
></a></code></li>
1487 "ordered_euclidean_ring_operators2">ordered_euclidean_ring_operators
<T,
1488 U
></a></code><br>
1489 <code>ordered_euclidean_ring_operators2
<T, U
></code></td>
1494 "#euclidean_ring_operators2">euclidean_ring_operators
<T,
1495 U
></a></code></li>
1497 <li><code><a href=
"#totally_ordered2">totally_ordered
<T,
1498 U
></a></code></li>
1504 <h4>Spelling: euclidean vs. euclidian
</h4>
1506 <p>Older versions of the Boost.Operators library used
1507 "<code>euclidian
</code>", but it was pointed out that
1508 "<code>euclidean
</code>" is the more common spelling.
1509 To be compatible with older version, the library now supports
1513 <h3><a name=
"ex_oprs">Example
</a> Templates
</h3>
1515 <p>The arithmetic operator class templates
<code><a href=
1516 "#operators1">operators
<></a></code> and
<code><a href=
1517 "#operators2">operators2
<></a></code> are examples of
1518 non-extensible operator grouping classes. These legacy class templates,
1519 from previous versions of the header, cannot be used for
<a href=
1520 "#chaining">base class chaining
</a>.
</p>
1522 <table cellpadding=
"5" border=
"1" align=
"center">
1524 Final Arithmetic Operator Template Classes
1529 <table align=
"center" border=
"1">
1535 <td><code>T
</code>: primary operand type
</td>
1537 <td><code>U
</code>: alternate operand type
</td>
1546 <th>Component Operator Templates
</th>
1550 <td><code><a name=
"operators1">operators
<T
></a></code></td>
1555 "#totally_ordered1">totally_ordered
<T
></a></code></li>
1558 "#integer_arithmetic1">integer_arithmetic
<T
></a></code></li>
1560 <li><code><a href=
"#bitwise1">bitwise
<T
></a></code></li>
1563 "#unit_steppable">unit_steppable
<T
></a></code></li>
1569 <td><code><a name=
"operators2">operators
<T, U
></a></code><br>
1570 <code>operators2
<T, U
></code></td>
1574 <li><code><a href=
"#totally_ordered2">totally_ordered
<T,
1575 U
></a></code></li>
1577 <li><code><a href=
"#integer_arithmetic2">integer_arithmetic
<T,
1578 U
></a></code></li>
1580 <li><code><a href=
"#bitwise2">bitwise
<T, U
></a></code></li>
1586 <h3><a name=
"a_demo">Arithmetic Operators Demonstration
</a> and Test
1589 <p>The
<cite><a href=
"operators_test.cpp">operators_test.cpp
</a></cite>
1590 program demonstrates the use of the arithmetic operator templates, and
1591 can also be used to verify correct operation. Check the compiler status
1592 report for the test results with selected platforms.
</p>
1594 <h2><a name=
"deref">Dereference
</a> Operators and Iterator Helpers
</h2>
1596 <p>The
<a href=
"#iterator">iterator helper
</a> templates ease the task of
1597 creating a custom iterator. Similar to arithmetic types, a complete
1598 iterator has many operators that are
"redundant" and can be implemented
1599 in terms of the core set of operators.
</p>
1601 <p>The
<a href=
"#dereference">dereference operators
</a> were motivated by
1602 the
<a href=
"#iterator">iterator helpers
</a>, but are often useful in
1603 non-iterator contexts as well. Many of the redundant iterator operators
1604 are also arithmetic operators, so the iterator helper classes borrow many
1605 of the operators defined above. In fact, only two new operators need to
1606 be defined (the pointer-to-member
<code>operator-
></code> and the
1607 subscript
<code>operator[]
</code>)!
</p>
1609 <p>The requirements for the types used to instantiate the dereference
1610 operators are specified in terms of expressions which must be valid and
1611 their return type. The composite operator templates list their component
1612 templates, which the instantiating type must support, and possibly other
1615 <h3><a name=
"dereference">Dereference
</a> Operators
</h3>
1617 <p>All the dereference operator templates in this table accept an
1618 optional template parameter (not shown) to be used for
<a href=
1619 "#chaining">base class chaining
</a>.
</p>
1621 <table cellpadding=
"5" border=
"1" align=
"center">
1623 Dereference Operator Template Classes
1628 <table align=
"center" border=
"1">
1634 <td><code>T
</code>: operand type
</td>
1636 <td><code>P
</code>:
<code>pointer
</code> type
</td>
1640 <td><code>D
</code>:
<code>difference_type
</code></td>
1642 <td><code>R
</code>:
<code>reference
</code> type
</td>
1646 <td><code>i
</code>: object of type
<code>T
</code> (an
1649 <td><code>n
</code>: object of type
<code>D
</code> (an
1659 <th>Supplied Operations
</th>
1661 <th>Requirements
</th>
1665 <td><code><a name=
"dereferenceable">dereferenceable
<T,
1666 P
></a></code></td>
1668 <td><code>P operator-
>() const
</code></td>
1670 <td><code>(
&*i)
</code>. Return convertible to
1671 <code>P
</code>.
</td>
1675 <td><code><a name=
"indexable">indexable
<T, D,
1676 R
></a></code></td>
1678 <td><code>R operator[](D n) const
</code></td>
1680 <td><code>*(i
+
n)
</code>. Return of type
1681 <code>R
</code>.
</td>
1685 <h3><a name=
"grpd_iter_oprs">Grouped Iterator Operators
</a></h3>
1687 <p>There are five iterator operator class templates, each for a different
1688 category of iterator. The following table shows the operator groups for
1689 any category that a custom iterator could define. These class templates
1690 have an additional optional template parameter
<code>B
</code>, which is
1691 not shown, to support
<a href=
"#chaining">base class chaining
</a>.
</p>
1693 <table cellpadding=
"5" border=
"1" align=
"center">
1695 Iterator Operator Class Templates
1700 <table align=
"center" border=
"1">
1706 <td><code>T
</code>: operand type
</td>
1708 <td><code>P
</code>:
<code>pointer
</code> type
</td>
1712 <td><code>D
</code>:
<code>difference_type
</code></td>
1714 <td><code>R
</code>:
<code>reference
</code> type
</td>
1718 <td><code>V
</code>:
<code>value_type
</code></td>
1730 <th>Component Operator Templates
</th>
1734 <td><code><a name=
"input_iteratable">input_iteratable
<T,
1735 P
></a></code></td>
1740 "#equality_comparable1">equality_comparable
<T
></a></code></li>
1743 "#incrementable">incrementable
<T
></a></code></li>
1745 <li><code><a href=
"#dereferenceable">dereferenceable
<T,
1746 P
></a></code></li>
1753 "output_iteratable">output_iteratable
<T
></a></code></td>
1758 "#incrementable">incrementable
<T
></a></code></li>
1764 <td><code><a name=
"forward_iteratable">forward_iteratable
<T,
1765 P
></a></code></td>
1769 <li><code><a href=
"#input_iteratable">input_iteratable
<T,
1770 P
></a></code></li>
1777 "bidirectional_iteratable">bidirectional_iteratable
<T,
1778 P
></a></code></td>
1782 <li><code><a href=
"#forward_iteratable">forward_iteratable
<T,
1783 P
></a></code></li>
1786 "#decrementable">decrementable
<T
></a></code></li>
1793 "random_access_iteratable">random_access_iteratable
<T, P, D,
1794 R
></a></code></td>
1799 "#bidirectional_iteratable">bidirectional_iteratable
<T,
1800 P
></a></code></li>
1803 "#totally_ordered1">totally_ordered
<T
></a></code></li>
1805 <li><code><a href=
"#additive2">additive
<T,
1806 D
></a></code></li>
1808 <li><code><a href=
"#indexable">indexable
<T, D,
1809 R
></a></code></li>
1815 <h3><a name=
"iterator">Iterator
</a> Helpers
</h3>
1817 <p>There are also five iterator helper class templates, each
1818 corresponding to a different iterator category. These classes cannot be
1819 used for
<a href=
"#chaining">base class chaining
</a>. The following
1820 summaries show that these class templates supply both the iterator
1821 operators from the
<a href=
"#grpd_iter_oprs">iterator operator class
1822 templates
</a> and the iterator typedef's required by the C++ standard
1823 (
<code>iterator_category
</code>,
<code>value_type
</code>,
1826 <table cellpadding=
"5" border=
"1" align=
"center">
1828 Iterator Helper Class Templates
1833 <table align=
"center" border=
"1">
1839 <td><code>T
</code>: operand type
</td>
1841 <td><code>P
</code>:
<code>pointer
</code> type
</td>
1845 <td><code>D
</code>:
<code>difference_type
</code></td>
1847 <td><code>R
</code>:
<code>reference
</code> type
</td>
1851 <td><code>V
</code>:
<code>value_type
</code></td>
1853 <td><code>x1, x2
</code>: objects of type
<code>T
</code></td>
1862 <th>Operations
& Requirements
</th>
1865 <tr valign=
"baseline">
1866 <td><code><a name=
"input_iterator_helper">input_iterator_helper
<T,
1867 V, D, P, R
></a></code></td>
1870 Supports the operations and has the requirements of
1873 <li><code><a href=
"#input_iteratable">input_iteratable
<T,
1874 P
></a></code></li>
1879 <tr valign=
"baseline">
1881 "output_iterator_helper">output_iterator_helper
<T
></a></code></td>
1884 Supports the operations and has the requirements of
1888 "#output_iteratable">output_iteratable
<T
></a></code></li>
1890 See also [
<a href=
"#1">1</a>], [
<a href=
"#2">2</a>].
1894 <tr valign=
"baseline">
1896 "forward_iterator_helper">forward_iterator_helper
<T, V, D, P,
1897 R
></a></code></td>
1900 Supports the operations and has the requirements of
1903 <li><code><a href=
"#forward_iteratable">forward_iteratable
<T,
1904 P
></a></code></li>
1909 <tr valign=
"baseline">
1911 "bidirectional_iterator_helper">bidirectional_iterator_helper
<T,
1912 V, D, P, R
></a></code></td>
1915 Supports the operations and has the requirements of
1919 "#bidirectional_iteratable">bidirectional_iteratable
<T,
1920 P
></a></code></li>
1925 <tr valign=
"baseline">
1927 "random_access_iterator_helper">random_access_iterator_helper
<T,
1928 V, D, P, R
></a></code></td>
1931 Supports the operations and has the requirements of
1935 "#random_access_iteratable">random_access_iteratable
<T, P, D,
1936 R
></a></code></li>
1938 To satisfy
<cite><a href=
1939 "http://www.sgi.com/tech/stl/RandomAccessIterator.html">RandomAccessIterator
</a></cite>,
1940 <code>x1 - x2
</code> with return convertible to
<code>D
</code> is
1946 <h4><a name=
"iterator_helpers_notes">Iterator Helper Notes
</a></h4>
1948 <p><a name=
"1">[
1]
</a> Unlike other iterator helpers templates,
1949 <code>output_iterator_helper
</code> takes only one template parameter -
1950 the type of its target class. Although to some it might seem like an
1951 unnecessary restriction, the standard requires
1952 <code>difference_type
</code> and
<code>value_type
</code> of any output
1953 iterator to be
<code>void
</code> (
24.3.1 [lib.iterator.traits]), and
1954 <code>output_iterator_helper
</code> template respects this requirement.
1955 Also, output iterators in the standard have void
<code>pointer
</code> and
1956 <code>reference
</code> types, so the
<code>output_iterator_helper
</code>
1959 <p><a name=
"2">[
2]
</a> As self-proxying is the easiest and most common
1960 way to implement output iterators (see, for example, insert [
24.4.2] and
1961 stream iterators [
24.5] in the standard library),
1962 <code>output_iterator_helper
</code> supports the idiom by defining
1963 <code>operator*
</code> and
<code>operator++
</code> member functions which
1964 just return a non-const reference to the iterator itself. Support for
1965 self-proxying allows us, in many cases, to reduce the task of writing an
1966 output iterator to writing just two member functions - an appropriate
1967 constructor and a copy-assignment operator. For example, here is a
1968 possible implementation of
<code><a href=
1969 "../iterator/doc/function_output_iterator.html">boost::function_output_iterator
</a></code>
1972 template
<class UnaryFunction
>
1973 struct function_output_iterator
1974 : boost::output_iterator_helper
< function_output_iterator
<UnaryFunction
> >
1976 explicit function_output_iterator(UnaryFunction const
& f = UnaryFunction())
1979 template
<typename T
>
1980 function_output_iterator
& operator=(T const
& value)
1982 this-
>func(value);
1991 <p>Note that support for self-proxying does not prevent you from using
1992 <code>output_iterator_helper
</code> to ease any other, different kind of
1993 output iterator's implementation. If
1994 <code>output_iterator_helper
</code>'s target type provides its own
1995 definition of
<code>operator*
</code> or/and
<code>operator++
</code>, then
1996 these operators will get used and the ones supplied by
1997 <code>output_iterator_helper
</code> will never be instantiated.
</p>
1999 <h3><a name=
"i_demo">Iterator Demonstration
</a> and Test Program
</h3>
2001 <p>The
<cite><a href=
"iterators_test.cpp">iterators_test.cpp
</a></cite>
2002 program demonstrates the use of the iterator templates, and can also be
2003 used to verify correct operation. The following is the custom iterator
2004 defined in the test program. It demonstrates a correct (though trivial)
2005 implementation of the core operations that must be defined in order for
2006 the iterator helpers to
"fill in" the rest of the iterator
2011 template
<class T, class R, class P
>
2013 : public boost::random_access_iterator_helper
<
2014 test_iter
<T,R,P
>, T, std::ptrdiff_t, P, R
>
2016 typedef test_iter self;
2017 typedef R Reference;
2018 typedef std::ptrdiff_t Distance;
2021 explicit test_iter(T* i =
0);
2022 test_iter(const self
& x);
2023 self
& operator=(const self
& x);
2024 Reference operator*() const;
2025 self
& operator++();
2026 self
& operator--();
2027 self
& operator+=(Distance n);
2028 self
& operator-=(Distance n);
2029 bool operator==(const self
& x) const;
2030 bool operator
<(const self
& x) const;
2031 friend Distance operator-(const self
& x, const self
& y);
2036 <p>Check the
<a href=
"http://www.boost.org/development/testing.html">compiler status
2037 report
</a> for the test results with selected platforms.
</p>
2040 <h2><a name=
"contributors">Contributors
</a></h2>
2043 <dt><a href=
"http://www.boost.org/people/dave_abrahams.htm">Dave Abrahams
</a></dt>
2045 <dd>Started the library and contributed the arithmetic operators in
2047 "../../boost/operators.hpp">boost/operators.hpp
</a></cite>.
</dd>
2049 <dt><a href=
"http://www.boost.org/people/jeremy_siek.htm">Jeremy Siek
</a></dt>
2051 <dd>Contributed the
<a href=
"#deref">dereference operators and iterator
2052 helpers
</a> in
<cite><a href=
2053 "../../boost/operators.hpp">boost/operators.hpp
</a></cite>. Also
2054 contributed
<cite><a href=
2055 "iterators_test.cpp">iterators_test.cpp
</a></cite>.
</dd>
2057 <dt><a href=
"http://www.boost.org/people/aleksey_gurtovoy.htm">Aleksey
2060 <dd>Contributed the code to support
<a href=
"#chaining">base class
2061 chaining
</a> while remaining backward-compatible with old versions of
2064 <dt><a href=
"http://www.boost.org/people/beman_dawes.html">Beman Dawes
</a></dt>
2066 <dd>Contributed
<cite><a href=
2067 "operators_test.cpp">operators_test.cpp
</a></cite>.
</dd>
2069 <dt><a href=
"http://www.boost.org/people/daryle_walker.html">Daryle Walker
</a></dt>
2071 <dd>Contributed classes for the shift operators, equivalence, partial
2072 ordering, and arithmetic conversions. Added the grouped operator
2073 classes. Added helper classes for input and output iterators.
</dd>
2075 <dt>Helmut Zeisel
</dt>
2077 <dd>Contributed the 'left' operators and added some grouped operator
2080 <dt>Daniel Frey
</dt>
2082 <dd>Contributed the NRVO-friendly and symmetric implementation of
2083 arithmetic operators.
</dd>
2087 <h2>Note for Users of
<a name=
"old_lib_note">Older Versions
</a></h2>
2089 <p>The
<a href=
"#chaining">changes in the library interface and
2090 recommended usage
</a> were motivated by some practical issues described
2091 below. The new version of the library is still backward-compatible with
2092 the former one (so you're not
<em>forced
</em> change any existing code),
2093 but the old usage is deprecated. Though it was arguably simpler and more
2094 intuitive than using
<a href=
"#chaining">base class chaining
</a>, it has
2095 been discovered that the old practice of deriving from multiple operator
2096 templates can cause the resulting classes to be much larger than they
2097 should be. Most modern C++ compilers significantly bloat the size of
2098 classes derived from multiple empty base classes, even though the base
2099 classes themselves have no state. For instance, the size of
2100 <code>point
<int
></code> from the
<a href=
"#example">example
</a>
2101 above was
12-
24 bytes on various compilers for the Win32 platform,
2102 instead of the expected
8 bytes.
</p>
2104 <p>Strictly speaking, it was not the library's fault--the language rules
2105 allow the compiler to apply the empty base class optimization in that
2106 situation. In principle an arbitrary number of empty base classes can be
2107 allocated at the same offset, provided that none of them have a common
2108 ancestor (see section
10.5 [class.derived] paragraph
5 of the standard).
2109 But the language definition also doesn't
<em>require
</em> implementations
2110 to do the optimization, and few if any of today's compilers implement it
2111 when multiple inheritance is involved. What's worse, it is very unlikely
2112 that implementors will adopt it as a future enhancement to existing
2113 compilers, because it would break binary compatibility between code
2114 generated by two different versions of the same compiler. As Matt Austern
2115 said,
"One of the few times when you have the freedom to do this sort of
2116 thing is when you're targeting a new architecture...". On the other hand,
2117 many common compilers will use the empty base optimization for single
2118 inheritance hierarchies.
</p>
2120 <p>Given the importance of the issue for the users of the library (which
2121 aims to be useful for writing light-weight classes like
2122 <code>MyInt
</code> or
<code>point
<></code>), and the forces
2123 described above, we decided to change the library interface so that the
2124 object size bloat could be eliminated even on compilers that support only
2125 the simplest form of the empty base class optimization. The current
2126 library interface is the result of those changes. Though the new usage is
2127 a bit more complicated than the old one, we think it's worth it to make
2128 the library more useful in real world. Alexy Gurtovoy contributed the
2129 code which supports the new usage idiom while allowing the library remain
2130 backward-compatible.
</p>
2133 <p>Revised:
7 Aug
2008</p>
2135 <p>Copyright
© Beman Dawes, David Abrahams,
1999-
2001.
</p>
2136 <p>Copyright
© Daniel Frey,
2002-
2009.
</p>
2137 <p>Use, modification, and distribution is subject to the Boost Software
2138 License, Version
1.0. (See accompanying file
2139 <a href=
"../../LICENSE_1_0.txt">LICENSE_1_0.txt
</a> or copy at
2140 <a href=
"http://www.boost.org/LICENSE_1_0.txt">
2141 www.boost.org/LICENSE_1_0.txt
</a>)
</p>
projects / ceph.git / blob