summaryrefslogtreecommitdiffstats
path: root/src/corelib/concurrent/qtconcurrentmap.cpp
blob: 80baa8f22fed560f616203b57b95fc125bfc6681 (plain)
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
    /****************************************************************************
**
** Copyright (C) 2009 Nokia Corporation and/or its subsidiary(-ies).
** Contact: Nokia Corporation (qt-info@nokia.com)
**
** This file is part of the QtCore module of the Qt Toolkit.
**
** $QT_BEGIN_LICENSE:LGPL$
** No Commercial Usage
** This file contains pre-release code and may not be distributed.
** You may use this file in accordance with the terms and conditions
** contained in the either Technology Preview License Agreement or the
** Beta Release License Agreement.
**
** GNU Lesser General Public License Usage
** Alternatively, this file may be used under the terms of the GNU Lesser
** General Public License version 2.1 as published by the Free Software
** Foundation and appearing in the file LICENSE.LGPL included in the
** packaging of this file.  Please review the following information to
** ensure the GNU Lesser General Public License version 2.1 requirements
** will be met: http://www.gnu.org/licenses/old-licenses/lgpl-2.1.html.
**
** In addition, as a special exception, Nokia gives you certain
** additional rights. These rights are described in the Nokia Qt LGPL
** Exception version 1.0, included in the file LGPL_EXCEPTION.txt in this
** package.
**
** GNU General Public License Usage
** Alternatively, this file may be used under the terms of the GNU
** General Public License version 3.0 as published by the Free Software
** Foundation and appearing in the file LICENSE.GPL included in the
** packaging of this file.  Please review the following information to
** ensure the GNU General Public License version 3.0 requirements will be
** met: http://www.gnu.org/copyleft/gpl.html.
**
** If you are unsure which license is appropriate for your use, please
** contact the sales department at http://qt.nokia.com/contact.
** $QT_END_LICENSE$
**
****************************************************************************/

/*!
    \namespace QtConcurrent
    \inmodule QtCore
    \since 4.4
    \brief The QtConcurrent namespace provides high-level APIs that make it
    possible to write multi-threaded programs without using low-level
    threading primitives.

    See the \l {threads.html#qtconcurrent-intro}{Qt Concurrent} section in the \l{threads.html}{threading} documentation.
    \inheaderfile QtCore
    \ingroup thread
*/

/*!
    \namespace QtConcurrent::internal
    \internal

    \brief The QtConcurrent::internal namespace contains QtConcurrent
    implementation details.

    \ingroup thread
*/

/*!
    \enum QtConcurrent::ReduceOption
    This enum specifies the order of which results from the map or filter 
    function are passed to the reduce function.

    \value UnorderedReduce Reduction is done in an arbitrary order.
    \value OrderedReduce Reduction is done in the order of the
    original sequence.
    \value SequentialReduce Reduction is done sequentally: only one
    thread will enter the reduce function at a time. (Parallel reduction
    might be supported in a future version of Qt Concurrent.)
*/

/*!
    \headerfile <QtConcurrentMap>
    \title Concurrent Map and Map-Reduce
    \ingroup threading

    \brief The <QtConcurrentMap> header provides concurrent Map and MapReduce.

    These functions are a part of the \l {threads.html#qtconcurrent-intro}{Qt Concurrent} framework.

    The QtConcurrent::map(), QtConcurrent::mapped() and
    QtConcurrent::mappedReduced() functions run computations in parallel on
    the items in a sequence such as a QList or a QVector. QtConcurrent::map()
    modifies a sequence in-place, QtConcurrent::mapped() returns a new
    sequence containing the modified content, and QtConcurrent::mappedReduced()
    returns a single result.

    Each of the above functions has a blocking variant that returns
    the final result instead of a QFuture. You use them in the same
    way as the asynchronous variants.

    \snippet doc/src/snippets/code/src_corelib_concurrent_qtconcurrentmap.cpp 7

    Note that the result types above are not QFuture objects, but real result
    types (in this case, QList<QImage> and QImage).

    \section1 Concurrent Map

    QtConcurrent::mapped() takes an input sequence and a map function. This map
    function is then called for each item in the sequence, and a new sequence
    containing the return values from the map function is returned.

    The map function must be of the form:

    \snippet doc/src/snippets/code/src_corelib_concurrent_qtconcurrentmap.cpp 0

    T and U can be any type (and they can even be the same type), but T must
    match the type stored in the sequence. The function returns the modified
    or \e mapped content.

    This example shows how to apply a scale function to all the items
    in a sequence:

    \snippet doc/src/snippets/code/src_corelib_concurrent_qtconcurrentmap.cpp 1

    The results of the map are made available through QFuture.  See the
    QFuture and QFutureWatcher documentation for more information on how to
    use QFuture in your applications.

    If you want to modify a sequence in-place, use QtConcurrent::map(). The
    map function must then be of the form:

    \snippet doc/src/snippets/code/src_corelib_concurrent_qtconcurrentmap.cpp 2

    Note that the return value and return type of the map function are not
    used.

    Using QtConcurrent::map() is similar to using QtConcurrent::mapped():

    \snippet doc/src/snippets/code/src_corelib_concurrent_qtconcurrentmap.cpp 3

    Since the sequence is modified in place, QtConcurrent::map() does not
    return any results via QFuture. However, you can still use QFuture and
    QFutureWatcher to monitor the status of the map.

    \section1 Concurrent Map-Reduce

    QtConcurrent::mappedReduced() is similar to QtConcurrent::mapped(), but
    instead of returning a sequence with the new results, the results are
    combined into a single value using a reduce function.

    The reduce function must be of the form:

    \snippet doc/src/snippets/code/src_corelib_concurrent_qtconcurrentmap.cpp 4

    T is the type of the final result, U is the return type of the map
    function. Note that the return value and return type of the reduce
    function are not used.

    Call QtConcurrent::mappedReduced() like this:

    \snippet doc/src/snippets/code/src_corelib_concurrent_qtconcurrentmap.cpp 5

    The reduce function will be called once for each result returned by the map
    function, and should merge the \e{intermediate} into the \e{result}
    variable.  QtConcurrent::mappedReduced() guarantees that only one thread
    will call reduce at a time, so using a mutex to lock the result variable
    is not neccesary. The QtConcurrent::ReduceOptions enum provides a way to
    control the order in which the reduction is done. If
    QtConcurrent::UnorderedReduce is used (the default), the order is
    undefined, while QtConcurrent::OrderedReduce ensures that the reduction
    is done in the order of the original sequence.

    \section1 Additional API Features

    \section2 Using Iterators instead of Sequence

    Each of the above functions has a variant that takes an iterator range
    instead of a sequence. You use them in the same way as the sequence
    variants:

    \snippet doc/src/snippets/code/src_corelib_concurrent_qtconcurrentmap.cpp 6

    \section2 Blocking Variants

    Each of the above functions has a blocking variant that returns
    the final result instead of a QFuture. You use them in the same
    way as the asynchronous variants.

    \snippet doc/src/snippets/code/src_corelib_concurrent_qtconcurrentmap.cpp 7

    Note that the result types above are not QFuture objects, but real result
    types (in this case, QList<QImage> and QImage).

    \section2 Using Member Functions

    QtConcurrent::map(), QtConcurrent::mapped(), and
    QtConcurrent::mappedReduced() accept pointers to member functions.
    The member function class type must match the type stored in the sequence:

    \snippet doc/src/snippets/code/src_corelib_concurrent_qtconcurrentmap.cpp 8

    Note that when using QtConcurrent::mappedReduced(), you can mix the use of
    normal and member functions freely:

    \snippet doc/src/snippets/code/src_corelib_concurrent_qtconcurrentmap.cpp 9

    \section2 Using Function Objects

    QtConcurrent::map(), QtConcurrent::mapped(), and
    QtConcurrent::mappedReduced() accept function objects, which can be used to
    add state to a function call. The result_type typedef must define the 
    result type of the function call operator:

    \snippet doc/src/snippets/code/src_corelib_concurrent_qtconcurrentmap.cpp 14

    \section2 Using Bound Function Arguments

    Note that Qt does not provide support for bound functions. This is
    provided by 3rd party libraries like
    \l{http://www.boost.org/libs/bind/bind.html}{Boost} or
    \l{http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2005/n1836.pdf}{C++
    TR1 Library Extensions}.

    If you want to use a map function that takes more than one argument you can
    use boost::bind() or std::tr1::bind() to transform it onto a function that
    takes one argument.

    As an example, we'll use QImage::scaledToWidth():

    \snippet doc/src/snippets/code/src_corelib_concurrent_qtconcurrentmap.cpp 10

    scaledToWidth takes three arguments (including the "this" pointer) and
    can't be used with QtConcurrent::mapped() directly, because
    QtConcurrent::mapped() expects a function that takes one argument. To use
    QImage::scaledToWidth() with QtConcurrent::mapped() we have to provide a
    value for the \e{width} and the \e{transformation mode}:

    \snippet doc/src/snippets/code/src_corelib_concurrent_qtconcurrentmap.cpp 11

    The return value from boost::bind() is a function object (functor) with
    the following signature:

    \snippet doc/src/snippets/code/src_corelib_concurrent_qtconcurrentmap.cpp 12

    This matches what QtConcurrent::mapped() expects, and the complete example
    becomes:

    \snippet doc/src/snippets/code/src_corelib_concurrent_qtconcurrentmap.cpp 13
*/

/*!
    \fn QFuture<void> QtConcurrent::map(Sequence &sequence, MapFunction function)
    \relates <QtConcurrentMap>

    Calls \a function once for each item in \a sequence. The \a function is
    passed a reference to the item, so that any modifications done to the item
    will appear in \a sequence.
*/

/*!
    \fn QFuture<void> QtConcurrent::map(Iterator begin, Iterator end, MapFunction function)
    \relates <QtConcurrentMap>

    Calls \a function once for each item from \a begin to \a end. The
    \a function is passed a reference to the item, so that any modifications
    done to the item will appear in the sequence which the iterators belong to.
*/

/*!
    \fn QFuture<T> QtConcurrent::mapped(const Sequence &sequence, MapFunction function)
    \relates <QtConcurrentMap>

    Calls \a function once for each item in \a sequence and returns a future
    with each mapped item as a result. You can use QFuture::const_iterator or
    QFutureIterator to iterate through the results.
*/

/*!
    \fn QFuture<T> QtConcurrent::mapped(ConstIterator begin, ConstIterator end, MapFunction function)
    \relates <QtConcurrentMap>

    Calls \a function once for each item from \a begin to \a end and returns a
    future with each mapped item as a result. You can use
    QFuture::const_iterator or QFutureIterator to iterate through the results.
*/

/*!
    \fn QFuture<T> QtConcurrent::mappedReduced(const Sequence &sequence,
    MapFunction mapFunction, ReduceFunction reduceFunction,
    QtConcurrent::ReduceOptions reduceOptions)

    \relates <QtConcurrentMap>

    Calls \a mapFunction once for each item in \a sequence. The return value of
    each \a mapFunction is passed to \a reduceFunction.

    Note that while \a mapFunction is called concurrently, only one thread at a
    time will call \a reduceFunction. The order in which \a reduceFunction is
    called is determined by \a reduceOptions.
*/

/*!
    \fn QFuture<T> QtConcurrent::mappedReduced(ConstIterator begin,
    ConstIterator end, MapFunction mapFunction, ReduceFunction reduceFunction,
    QtConcurrent::ReduceOptions reduceOptions)

    \relates <QtConcurrentMap>

    Calls \a mapFunction once for each item from \a begin to \a end. The return
    value of each \a mapFunction is passed to \a reduceFunction.

    Note that while \a mapFunction is called concurrently, only one thread at a
    time will call \a reduceFunction. By default, the order in which
    \a reduceFunction is called is undefined.

    \note QtConcurrent::OrderedReduce results in the ordered reduction.
*/

/*!
  \fn void QtConcurrent::blockingMap(Sequence &sequence, MapFunction function)

  Calls \a function once for each item in \a sequence. The \a function is
  passed a reference to the item, so that any modifications done to the item
  will appear in \a sequence.

  \note This function will block until all items in the sequence have been processed.

  \sa map()
*/

/*!
  \fn void QtConcurrent::blockingMap(Iterator begin, Iterator end, MapFunction function)

  Calls \a function once for each item from \a begin to \a end. The
  \a function is passed a reference to the item, so that any modifications
  done to the item will appear in the sequence which the iterators belong to.

  \note This function will block until the iterator reaches the end of the
  sequence being processed.

  \sa map()
*/

/*!
  \fn T QtConcurrent::blockingMapped(const Sequence &sequence, MapFunction function)

  Calls \a function once for each item in \a sequence and returns a Sequence containing
  the results. The type of the results will match the type returned my the MapFunction.

  \note This function will block until all items in the sequence have been processed.

  \sa mapped()
*/

/*!
  \fn T QtConcurrent::blockingMapped(ConstIterator begin, ConstIterator end, MapFunction function)

  Calls \a function once for each item from \a begin to \a end and returns a
  container with the results. Specify the type of container as the a template
  argument, like this:
  
  \code
     QList<int> ints = QtConcurrent::blockingMapped<QList<int> >(beginIterator, endIterator, fn);
  \endcode

  \note This function will block until the iterator reaches the end of the
  sequence being processed.

  \sa mapped()
*/

/*!
  \fn T QtConcurrent::blockingMappedReduced(const Sequence &sequence, MapFunction mapFunction, ReduceFunction reduceFunction, QtConcurrent::ReduceOptions reduceOptions)

  \relates <QtConcurrentMap>

  Calls \a mapFunction once for each item in \a sequence. The return value of
  each \a mapFunction is passed to \a reduceFunction.

  Note that while \a mapFunction is called concurrently, only one thread at a
  time will call \a reduceFunction. The order in which \a reduceFunction is
  called is determined by \a reduceOptions.

  \note This function will block until all items in the sequence have been processed.

  \sa mapped()
*/

/*!
  \fn T QtConcurrent::blockingMappedReduced(ConstIterator begin, ConstIterator end, MapFunction mapFunction, ReduceFunction reduceFunction, QtConcurrent::ReduceOptions reduceOptions)

  \relates <QtConcurrentMap>

  Calls \a mapFunction once for each item from \a begin to \a end. The return
  value of each \a mapFunction is passed to \a reduceFunction.

  Note that while \a mapFunction is called concurrently, only one thread at a
  time will call \a reduceFunction. The order in which \a reduceFunction is
  called is undefined.

  \note This function will block until the iterator reaches the end of the
  sequence being processed.

  \sa blockingMappedReduced()
*/