forked from boostorg/utility
-
Notifications
You must be signed in to change notification settings - Fork 0
/
Copy pathutility.htm
583 lines (494 loc) · 24.3 KB
/
utility.htm
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1">
<title>Header boost/utility.hpp Documentation</title>
</head>
<body bgcolor="#FFFFFF" text="#000000">
<h1><img src="../../boost.png" alt="boost.png (6897 bytes)" align="center" WIDTH="277" HEIGHT="86">Header
<a href="../../boost/utility.hpp">boost/utility.hpp</a></h1>
<p>The entire contents of the header <code><a href="../../boost/utility.hpp"><boost/utility.hpp></a></code>
are in <code>namespace boost</code>.</p>
<h2>Contents</h2>
<ul>
<li>
Class templates supporting the <a href="doc/html/base_from_member.html">
base-from-member idiom</a></li>
<li>
Function templates <a href="../core/doc/html/core/checked_delete.html">checked_delete() and
checked_array_delete()</a> (moved to the Boost.Core library)</li>
<li>
Function templates <a href="../iterator/doc/html/iterator/algorithms/next_prior.html">next() and prior()</a> (moved to the Boost.Iterator library)</li>
<li>
Class <a href="../core/doc/html/core/noncopyable.html">noncopyable</a> (moved to the Boost.Core library)</li>
<li>
Function template <a href="../core/doc/html/core/addressof.html">addressof()</a> (moved to the Boost.Core library)</li>
<li>Class template <a href="#result_of">result_of</a></li>
<li>
Macro <a href="#BOOST_BINARY">BOOST_BINARY</a></li>
<li><a href="index.html">Other utilities not part of <code>utility.hpp</code></a></li>
</ul>
<h2>
<h2><a name="result_of">Class template
result_of</a></h2> <p>The class template
<code>result_of</code> helps determine the type of a
call expression. For example, given an lvalue <code>f</code> of
type <code>F</code> and lvalues <code>t1</code>,
<code>t2</code>, ..., <code>t<em>N</em></code> of
types <code>T1</code>, <code>T2</code>, ...,
<code>T<em>N</em></code>, respectively, the type
<code>result_of<F(T1, T2, ...,
T<em>N</em>)>::type</code> defines the result type
of the expression <code>f(t1, t2,
...,t<em>N</em>)</code>. This implementation permits
the type <code>F</code> to be a function pointer,
function reference, member function pointer, or class
type. By default, <em>N</em> may be any value between 0 and
16. To change the upper limit, define the macro
<code>BOOST_RESULT_OF_NUM_ARGS</code> to the maximum
value for <em>N</em>. Class template <code>result_of</code>
resides in the header <code><<a
href="../../boost/utility/result_of.hpp">boost/utility/result_of.hpp</a>></code>.</p>
<p>If your compiler's support for <code>decltype</code> is
adequate, <code>result_of</code> automatically uses it to
deduce the type of the call expression, in which case
<code>result_of<F(T1, T2, ...,
T<em>N</em>)>::type</code> names the type
<code>decltype(boost::declval<F>()(boost::declval<T1>(),
boost::declval<T2>(), ...,
boost::declval<T<em>N</em>>()))</code>, as in the
following example.</p>
<blockquote>
<pre>struct functor {
template<class T>
T operator()(T x)
{
return x;
}
};
typedef boost::result_of<
functor(int)
>::type type; // type is int</pre>
</blockquote>
<p>You can test whether <code>result_of</code> is using
<code>decltype</code> by checking if the macro
<code>BOOST_RESULT_OF_USE_DECLTYPE</code> is defined after
including <code>result_of.hpp</code>. You can also force
<code>result_of</code> to use <code>decltype</code> by
defining <code>BOOST_RESULT_OF_USE_DECLTYPE</code> prior
to including <code>result_of.hpp</code>.</p>
<p>If <code>decltype</code> is not used,
then automatic result type deduction of function
objects is not possible. Instead, <code>result_of</code>
uses the following protocol to allow the programmer to
specify a type. When <code>F</code> is a class type with a
member type <code>result_type</code>,
<code>result_of<F(T1, T2, ...,
T<em>N</em>)>::type</code> is
<code>F::result_type</code>. When <code>F</code> does
not contain <code>result_type</code>,
<code>result_of<F(T1, T2, ...,
T<em>N</em>)>::type</code> is <code>F::result<F(T1,
T2, ..., T<em>N</em>)>::type</code> when
<code><em>N</em> > 0</code> or <code>void</code>
when <code><em>N</em> = 0</code>. Note that it is the
responsibility of the programmer to ensure that
function objects accurately advertise their result
type via this protocol, as in the following
example.</p>
<blockquote>
<pre>struct functor {
template<class> struct result;
template<class F, class T>
struct result<F(T)> {
typedef T type;
};
template<class T>
T operator()(T x)
{
return x;
}
};
typedef boost::result_of<
functor(int)
>::type type; // type is int</pre>
</blockquote>
<p>Since <code>decltype</code> is a new language
feature recently standardized in C++11,
if you are writing a function object
to be used with <code>result_of</code>, for
maximum portability, you might consider following
the above protocol even if your compiler has
proper <code>decltype</code> support. If you wish to continue to
use the protocol on compilers that
support <code>decltype</code>, there are two options:
You can use <code>boost::tr1_result_of</code>, which is also
defined in <code><<a href="../../boost/utility/result_of.hpp">boost/utility/result_of.hpp</a>></code>.
Alternatively, you can define the macro
<code>BOOST_RESULT_OF_USE_TR1</code>, which causes
<code>result_of</code> to use the protocol described
above instead of <code>decltype</code>. If you choose to
follow the protocol, take care to ensure that the
<code>result_type</code> and
<code>result<></code> members accurately
represent the return type of
<code>operator()</code> given a call expression.</p>
<p>Additionally, <code>boost::result_of</code>
provides a third mode of operation, which some users
may find convenient. When
<code>BOOST_RESULT_OF_USE_TR1_WITH_DECLTYPE_FALLBACK</code>
is defined, <code>boost::result_of</code> behaves as
follows. If the function object has a member
type <code>result_type</code> or member
template <code>result<></code>, then
<code>boost::result_of</code> will use the TR1
protocol. Otherwise,
<code>boost::result_of</code> will
use <code>decltype</code>. Using TR1 with
a <code>declytpe</code> fallback may workaround
certain problems at the cost of portability. For
example:
<ul>
<li>Deficient compiler: If your code
requires <code>boost::result_of</code> to work
with incomplete return types but your
compiler's <code>decltype</code> implementation
does not support incomplete return types, then you
can use the TR1 protocol as a workaround. Support
for incomplete return types was added late in the
C++11 standardization process
(see <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2011/n3276.pdf">N3276</a>)
and is not implemented by some compilers.</li>
<li>Deficient legacy code: If your existing TR1
function object advertises a different type than
the actual result type deduced
by <code>decltype</code>, then using TR1 with a
<code>decltype</code> fallback will allow you to
work with both your existing TR1 function objects
and new C++11 function object. This situation
could occur if your legacy function objects
misused the TR1 protocol. See the documentation on
known <a href="#result_of_tr1_diff">differences</a>
between <code>boost::result_of</code> and TR1.</li>
</ul>
<a name="BOOST_NO_RESULT_OF"></a>
<p>This implementation of <code>result_of</code>
requires class template partial specialization, the
ability to parse function types properly, and support
for SFINAE. If <code>result_of</code> is not supported
by your compiler, including the header
<code>boost/utility/result_of.hpp</code> will
define the macro <code>BOOST_NO_RESULT_OF</code>.</p>
<p>For additional information
about <code>result_of</code>, see the C++ Library
Technical Report,
<a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2005/n1836.pdf">N1836</a>,
or, for motivation and design rationale,
the <code>result_of</code> <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2003/n1454.html">proposal</a>.</p>
<a name="result_of_guidelines">
<h3>Usage guidelines for boost::result_of</h3>
</a>
<p>The following are general suggestions about when
and how to use <code>boost::result_of</code>.</p>
<ol>
<li> If you are targeting C++11 and are not concerned
about portability to non-compliant compilers or
previous versions of the standard, then use
<code>std::result_of</code>. If <code>std::result_of</code>
meets your needs, then there's no reason to stop using
it.</li>
<li> If you are targeting C++11 but may port your code
to legacy compilers at some time in the future, then
use <code>boost::result_of</code> with
<code>decltype</code>. When <code>decltype</code> is
used <code>boost::result_of</code>
and <code>std::result_of</code> are usually
interchangeable. See the documentation on
known <a href="#result_of_cxx11_diff">differences</a>
between boost::result_of and C++11 result_of.</li>
<li> If compiler portability is required,
use <code>boost::result_of</code> with the TR1 protocol.</li>
</ol>
<p>Regardless of how you
configure <code>boost::result_of</code>, it is
important to bear in mind that the return type of a
function may change depending on its arguments, and
additionally, the return type of a member function may
change depending on the cv-qualification of the
object. <code>boost::result_of</code> must be passed
the appropriately cv-qualified types in order to
deduce the corresponding return type. For example:
<blockquote>
<pre>struct functor {
int& operator()(int);
int const& operator()(int) const;
float& operator()(float&);
float const& operator()(float const&);
};
typedef boost::result_of<
functor(int)
>::type type1; // type1 is int &
typedef boost::result_of<
const functor(int)
>::type type2; // type2 is int const &
typedef boost::result_of<
functor(float&)
>::type type3; // type3 is float &
typedef boost::result_of<
functor(float const&)
>::type type4; // type4 is float const &</pre>
</blockquote>
<a name="result_of_tr1_protocol_guidelines">
<h3>Usage guidelines for the TR1 result_of protocol</h3>
</a>
<p>On compliant C++11
compilers, <code>boost::result_of</code> can
use <code>decltype</code> to deduce the type of any
call expression, including calls to function
objects. However, on pre-C++11 compilers or on
compilers without adequate decltype support,
additional scaffolding is needed from function
objects as described above. The following are
suggestions about how to use the TR1 protocol.</p>
<ul>
<li>When the return type does not depend on the
argument types or the cv-qualification of the
function object, simply
define <code>result_type</code>. There is no need
to use the <code>result</code> template unless the
return type varies.</li>
<li>Use the protocol specified type when defining
function prototypes. This can help ensure the
actual return type does not get out of sync with
the protocol specification. For example:
<blockquote>
<pre>struct functor {
typedef int result_type;
result_type operator()(int);
};</pre>
</blockquote> </li>
<li>Always specify the <code>result</code>
specialization near the corresponding
<code>operator()</code> overload. This can make it
easier to keep the specializations in sync with the
overloads. For example:
<blockquote>
<pre>struct functor {
template<class> struct result;
template<class F>
struct result<F(int)> {
typedef int& type;
};
result<functor(int)>::type operator()(int);
template<class F>
struct result<const F(int)> {
typedef int const& type;
};
result<const functor(int)>::type operator()(int) const;
};</pre>
</blockquote> </li>
<li>Use type transformations to simplify
the <code>result</code> template specialization. For
example, the following uses
<a href="../type_traits/doc/html/index.html">Boost.TypeTraits</a>
to specialize the <code>result</code> template for
a single <code>operator()</code> that can be called on
both a const and non-const function object with
either an lvalue or rvalue argument.
<blockquote>
<pre>struct functor {
template<class> struct result;
template<class F, class T>
struct result<F(T)>
: boost::remove_cv<
typename boost::remove_reference<T>::type
>
{};
template<class T>
T operator()(T const& x) const;
};</pre>
</blockquote></li>
</ul>
<a name="result_of_tr1_diff">
<h3>Known differences between boost::result_of and TR1 result_of</h3>
</a>
When using <code>decltype</code>, <code>boost::result_of</code>
ignores the TR1 protocol and instead deduces the
return type of function objects directly
via <code>decltype</code>. In most situations, users
will not notice a difference, so long as they use the
protocol correctly. The following are situations in
which the type deduced
by <code>boost::result_of</code> is known to differ depending on
whether <code>decltype</code> or the TR1 protocol is
used.
<ul>
<li> TR1 protocol misusage
<p>When using the TR1
protocol, <code>boost::result_of</code> cannot
detect whether the actual type of a call to a
function object is the same as the type specified
by the protocol, which allows for the possibility
of inadvertent mismatches between the specified
type and the actual type. When
using <code>decltype</code>, these subtle bugs
may result in compilation errors. For example:</p>
<blockquote>
<pre>struct functor {
typedef short result_type;
int operator()(short);
};
#ifdef BOOST_RESULT_OF_USE_DECLTYPE
BOOST_STATIC_ASSERT((
boost::is_same<boost::result_of<functor(short)>::type, int>::value
));
#else
BOOST_STATIC_ASSERT((
boost::is_same<boost::result_of<functor(short)>::type, short>::value
));
#endif</pre>
</blockquote>
<p>Note that the user can
force <code>boost::result_of</code> to use the TR1
protocol even on platforms that
support <code>decltype</code> by
defining <code>BOOST_RESULT_OF_USE_TR1</code>.</p></li>
<li> Nullary function objects
<p>When using the TR1 protocol, <code>boost::result_of</code>
cannot always deduce the type of calls to
nullary function objects, in which case the
type defaults to void. When using <code>decltype</code>,
<code>boost::result_of</code> always gives the actual type of the
call expression. For example:</p>
<blockquote>
<pre>struct functor {
template<class> struct result {
typedef int type;
};
int operator()();
};
#ifdef BOOST_RESULT_OF_USE_DECLTYPE
BOOST_STATIC_ASSERT((
boost::is_same<boost::result_of<functor()>::type, int>::value
));
#else
BOOST_STATIC_ASSERT((
boost::is_same<boost::result_of<functor()>::type, void>::value
));
#endif</pre>
</blockquote>
<p>Note that there are some workarounds for the
nullary function problem. So long as the return
type does not vary,
<code>result_type</code> can always be used to
specify the return type regardless of arity. If the
return type does vary, then the user can
specialize <code>boost::result_of</code> itself for
nullary calls.</p></li>
<li> Non-class prvalues and cv-qualification
<p>When using the TR1
protocol, <code>boost::result_of</code> will
report the cv-qualified type specified
by <code>result_type</code> or
the <code>result</code> template regardless of
the actual cv-qualification of the call
expression. When using
<code>decltype</code>, <code>boost::result_of</code>
will report the actual type of the call expression,
which is not cv-qualified when the expression is a
non-class prvalue. For example:</p>
<blockquote>
<pre>struct functor {
template<class> struct result;
template<class F, class T> struct result<F(const T)> {
typedef const T type;
};
const short operator()(const short);
int const & operator()(int const &);
};
// Non-prvalue call expressions work the same with or without decltype.
BOOST_STATIC_ASSERT((
boost::is_same<
boost::result_of<functor(int const &)>::type,
int const &
::value
));
// Non-class prvalue call expressions are not actually cv-qualified,
// but only the decltype-based result_of reports this accurately.
#ifdef BOOST_RESULT_OF_USE_DECLTYPE
BOOST_STATIC_ASSERT((
boost::is_same<
boost::result_of<functor(const short)>::type,
short
::value
));
#else
BOOST_STATIC_ASSERT((
boost::is_same<
boost::result_of<functor(const short)>::type,
const short
::value
));
#endif</pre>
</blockquote></li>
</ul>
<a name="result_of_cxx11_diff">
<h3>Known differences between boost::result_of and C++11 result_of</h3>
</a>
<p>When using <code>decltype</code>, <code>boost::result_of</code>
implements most of the C++11 result_of
specification. One known exception is that
<code>boost::result_of</code> does not implement the
requirements regarding pointers to member data.</p>
<p>Created by Doug Gregor. Contributions from Daniel Walker, Eric Niebler, Michel Morin and others</p>
<h2><a name="BOOST_BINARY">Macro BOOST_BINARY</a></h2>
<p>The macro <code>BOOST_BINARY</code> is used for the
representation of binary literals. It takes as an argument
a binary number arranged as an arbitrary amount of 1s and 0s in
groupings of length 1 to 8, with groups separated
by spaces. The type of the literal yielded is determined by
the same rules as those of hex and octal
literals (<i>2.13.1p1</i>). By implementation, this macro
expands directly to an octal literal during preprocessing, so
there is no overhead at runtime and the result is useable in
any place that an octal literal would be.</p>
<p>In order to directly support binary literals with suffixes,
additional macros of the form BOOST_BINARY_XXX are also
provided, where XXX is a standard integer suffix in all capital
letters. In addition, LL and ULL suffixes may be used for representing
long long and unsigned long long types in compilers which provide
them as an extension.</p>
<p>The BOOST_BINARY family of macros resides in the header
<a
href="../../boost/utility/binary.hpp"><boost/utility/binary.hpp></a>
which is automatically included by
<a
href="../../boost/utility.hpp"><boost/utility.hpp></a>.
<p>Contributed by Matt Calabrese.</p><p>
</p><h3>Example</h3>
<blockquote>
<pre>
void foo( int );
void foo( unsigned long );
void bar()
{
int value1 = BOOST_BINARY( 100 111000 01 1 110 );
unsigned long value2 = BOOST_BINARY_UL( 100 001 ); // unsigned long
long long value3 = BOOST_BINARY_LL( 11 000 ); // long long if supported
assert( BOOST_BINARY( 10010 )
& BOOST_BINARY( 11000 )
== BOOST_BINARY( 10000 )
);
foo( BOOST_BINARY( 1010 ) ); // calls the first foo
foo( BOOST_BINARY_LU( 1010 ) ); // calls the second foo
}
</pre></blockquote>
<hr>
<p>Revised <!--webbot bot="Timestamp" S-Type="EDITED" S-Format="%d %B, %Y" startspan
-->04 September, 2008<!--webbot bot="Timestamp" endspan i-checksum="39369"
-->
</p>
<p>© Copyright Beman Dawes 1999-2003.</p>
<p>Distributed under the Boost Software License, Version 1.0. See
<a href="http://www.boost.org/LICENSE_1_0.txt">www.boost.org/LICENSE_1_0.txt</a></p>
</body>
</html>